Mechano-optically Co-designed Highly-scalable Silicon Photonic MEMS Switches with Buckling-free 2×2 Horizontal Adiabatic Directional Couplers

Mechano-optically Co-designed Highly-scalable Silicon Photonic MEMS Switches with Buckling-free 2×2 Horizontal Adiabatic Directional Couplers | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Mechano-optically Co-designed Highly-scalable Silicon Photonic MEMS Switches with Buckling-free 2×2 Horizontal Adiabatic Directional Couplers Daoxin Dai, Yinpeng Hu, Jiayue Zhu, Yunzhi Liu, Linyan Lyu, Ye Lu, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7519103/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 11 You are reading this latest preprint version Abstract Micro-electromechanical system (MEMS) photonic switches based on adiabatic directional couplers (ADCs) potentially offer the advantages including large fabrication tolerance and broad bandwidth, which is essential for large-scale reconfigurable photonic integrated circuits in optical networks and artificial intelligence computing infrastructures. However, the scalability of previous ADC-based MEMS switches is challenging due to the complicated fabrication of vertical ADC switches and buckling-induced performance deterioration of horizontal ADC switches. Here we propose a mechano-optically co-designed 2×2 MEMS switch based on a unique buckling-free horizontal ADC by introducing residual-strain management structures including slab anchors and extra S-bends. For the fabricated device, the waveguide buckling is alleviated significantly and high optical performance is achieved with low loss and low crosstalk over a broad bandwidth of 180 nm. It also features fast switching speed of ~ 2 µs, reliable durability with > 7.2 billion switching cycles, and exceptional scalability with the realization of a 64×64 Benes switch array. The present 2×2 horizontal ADC switches are compatible with all mainstream array topologies and can be fabricated using simple standard silicon photonic foundry processes, which are not accessible for those 1×2 ADC switches reported previously. With these advantages, the present design provides a highly-scalable solution with great potential for MEMS/NEMS photonic devices used in versatile applications. Physical sciences/Optics and photonics/Other photonics/Micro-optics Physical sciences/Nanoscience and technology/Nanoscale devices/NEMS Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction The rapid development of artificial intelligence, 5G/6G networks, and Internet of Things (IoT) has generated substantial demands for data transmission and processing, making photonic information transmission and processing an attractive solution due to its ultrahigh parallelism, large bandwidth, and low latency. Consequently, various large-scale photonic integrated circuits (PICs) have been demonstrated, including on-chip photonic networks 1 , programmable photonic chips 2 , photonic field-programmable gate arrays (FPGAs) 3 , optical neural networks (ONNs) 4 , 5 , 6 , as well as quantum information processors 7 , 8 , where highly-scalable photonic switches are essential components to enable flexible circuit reconfigurations. The scaling of conventional photonic switches 9 , 10 , 11 , 12 , 13 , 14 utilizing Mach-Zehnder interferometers or micro-ring resonators is challenging due to the issues including high power consumption for thermo-optic devices, high carrier absorption loss for electro-optic devices, and sophisticated calibration processes due to the analog nature of the phase shifters. In contrast, micro-electromechanical system (MEMS) photonic switches 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , which operate by modulating the mode coupling between waveguides in directional couplers (DCs) or adiabatic directional couplers (ADCs), offer a promising alternative. These switches potentially feature (1) low switching energy consumption of pJ-level and near-zero static power consumption 24 for electrostatic actuation and (2) no carrier absorption, making them ideal for large-scale switch arrays. The pioneering works on MEMS switches utilizing DCs have been demonstrated with a large scale up to 50×50 15 . However, their practical applications are compromised due to the narrow bandwidth of DCs. Also, due to the analog nature of their operation principle, the voltage for electrostatic actuation must be accurately controlled for each switch cell, which requires cumbersome electronic circuits for control and drive. More recently, ADC-based MEMS switches 19 , 20 , 21 , 22 , 23 emerged as a promising solution due to their advantages of low excess losses, large bandwidths, digital operation, and exceptional fabrication tolerance. Particularly, vertical ADCs (VADCs) 19 , 20 , 21 and horizontal ADCs (HADCs) 22 , 23 are employed successfully for MEMS switches. Unfortunately, both these previously reported VADC- and HADC-based elementary switches are limited to a 1×2 configuration, which are incompatible with widely-used topologies such as Benes, PI-Loss, Switch-and-Select, etc. 25 , 26 , while each of these array topologies has its own uniqueness in scalability and functionality. In contrast, this paper focuses on the 2×2 elementary switch as the preference for flexible compatibility with various topologies available for diverse switching scales and applications. Besides, it is crucial to achieve high robustness and high performances with simple fabrication processes for elementary switches, especially when scaled to an N×N optical switch array with a large port count. For the VADC-based MEMS switches demonstrated previously 19 , 20 , 21 , impressive performances were achieved with low excess losses of 70 dB, large bandwidths of 300 nm and fast ON/OFF switching speed of 400/300 ns. Leveraging the elementary switch with such high performance, a 240×240 switch array with Cross-Bar topology 21 and a 128×128 LiDAR system incorporating a focal plane switch array (FPSA) 27 have been demonstrated. However, two silicon layers are required for the VADC design, which are not available in most silicon photonic foundry processes. Consequently, complicated back-end processes are required, which may limit the scalability due to high fabrication cost and compromised yield. Furthermore, the driving voltage of the elementary switch is as large as 60 V or beyond, which complicates the driving circuits due to the difficulties in implementing a multi-channel (100s or even 10,000s channels) voltage source with such a high voltage. In contrast, developing HADC-based MEMS switches is recognized as another attractive scheme because it only needs a single layer of silicon and is fully compatible with standard silicon photonic foundry processes, simplifying fabrication processes compared to VADC switches 19 , 20 , 21 . To date, 1×2 elementary switch 23 and single-pole double-throw (SPDT) elementary switch 22 have been proposed based on HADCs. For the pioneering HADC-based 1×2 elementary switch 23 , a three-comb-drive device design and 8×8 Cross-Bar switch array have been implemented. Unfortunately, the fabricated switch exhibits high excess losses of ~ 4 dB, high crosstalk of ~ − 10 dB over the bandwidth of 108 nm, and ON/OFF switching speed of 36.7/21.4 µs in experiment. Furthermore, the measured excess loss in the ON state varies up to ~ 1.7 dB, depending on the ADC gap, which indicates slightly nonadiabatic coupling in the fabricated device. For the HADC-based SPDT elementary switch 22 , a unique switch design and fast ON/OFF switching speed of 0.82/0.73 µs have been achieved. However, the excess-loss difference between the two output ports is as high as ~ 5 dB and the bandwidth is only ~ 25 nm in experiment. In addition, each SPDT switch requires two, instead of one, electrical pads, which doubles the number of driving circuits and compromises the scalability to the array. These limitations highlight the need for significant performance improvements in HADC-based MEMS switches. It is important to note that suspended waveguide structures used in MEMS switches are susceptible to significant inadvertent structural deformation, such as beam buckling 28 , 29 , due to the release of the residual strain in the silicon-on-insulator (SOI) wafer used dominantly for PICs. The resultant waveguide deformation and/or misalignment often leads to significant and random switch performance deterioration, which greatly compromises the scalability. This issue is especially pronounced for digital photonic switches, for which fine-tuning to compensate the coupling change due to the waveguide deformation and/or misalignment is often undesired or infeasible. Therefore, systematic residue strain management is extremely critical for MEMS photonic switches by co-designing the photonic/mechanical structures, which has not been thoroughly investigated previously and will be the key focus here. In this paper, we propose and experimentally demonstrate a highly-scalable 2×2 silicon photonic MEMS switch based on a mechano-optically co-designed buckling-free HADC by introducing extra S-bends and partially-etched taper waveguides with slab anchors. All these structures of the HADC are designed optimally to achieve high photonic switching performances with low excess losses and high extinction ratios over a broad bandwidth. Meanwhile, these structures are also designed mechanically to effectively suppress the waveguide buckling caused by the release of the residual strain in the SOI wafer, which often causes significant misalignment between the two waveguides in conventional HADCs, leading to photonic performance deterioration or even failure. With such a novel structural design, the present HADC switch developed with a single layer of silicon with standard silicon photonic foundry processes is buckling-free and achieves unprecedented photonic performance for HADC switches, including ultra-low excess losses, high extinction ratios and broad bandwidth, as shown in Table 1 . Fast ON/OFF switching speed of 2.2/1.9 µs and reliable durability with > 7.2 billion switching-cycles are also demonstrated. Besides, the present MEMS switch features 2×2 design instead of the previous 1×2 design (see Supplementary Section 1), making it compatible with all widely used array topologies including Benes, Cross-Bar, PI-Loss and Switch-and-Select etc. Furthermore, a 64×64 Benes switch array is realized as a proof of concept with high performances shown in Table 1 . The switch array can be scaled to as large as 128×128 when the waveguide loss is lowered further by optimizing the fabrication processes, showing great potential for photonic interconnects, programmable PICs, photonic FPGAs, ONNs, quantum information processors and FPSA LiDAR. Table 1 Theoretical and experimental photonic performance of the devices Devices Data type State Excess loss (dB) Crosstalk (dB) Bandwidth (nm) Elementary switch simulated OFF 0–0.07 –52.5 – − 28.5 1400–1700 ON 0–0.06 –47.7 – − 21.1 measured OFF 0.11–0.43 –61.8 – − 42.0 1420–1600 (Limited by the used grating coupler) ON 0.05–0.99 –24.9 – − 10.3 64×64 Benes switch array measured All-OFF 4.1–9.7 ≤ − 28.3 – − 36.7 1530–1590 (Limited by the used grating coupler) Single-ON 5.3–8.8 ≥ 38.5 (ON/OFF extinction ratio) Results Mechano-optical co-design of the buckling-free HADC MEMS switch. Figure 1 a shows the schematic configuration of the present buckling-free MEMS switch based on a gap-adjustable HADC, which includes a pair of centrosymmetric taper waveguides with a constant gap. Especially, the length of the linear taper waveguides used for ADCs is usually tens or even hundreds of micrometers to ensure adiabatic mode evolution over a broad bandwidth. However, such long suspended waveguides often buckle strongly due to the release of the residual strain in the used SOI wafer, which may cause significant misalignment between the two waveguides in the coupling region, leading to photonic performance degradation or even failure. To address this issue, the taper parts used here for the HADC is designed to be partially etched with the slabs, which then function as the mechanical anchors. In particular, the slab anchors are tapered as well to avoid scattering losses due to the abrupt change of the waveguide cross-section, as shown in Fig. 1 a. Additionally, extra S-bends are introduced to further reduce the waveguide buckling (see Fig. 1 a). For the suspended waveguides of the HADC, one is connected to the stationary silicon core-layer by a slab anchor, while the other one is connected through another slab anchor to the MEMS actuator consisting of a mechanical stopper, a perforated shuttle beam, a structural aligner, folded springs, and electrostatic combs, which enables the movement in the y direction by the electrostatic actuation 30 . In the OFF state, the two waveguides are separated with a sufficiently large gap to be decoupled and thus the incident light eventually outputs at the bar port on the same side, as shown in Fig. 1 b. In the ON state, the gap between the two waveguides is narrowed electrostatically and the input light is then coupled adiabatically to the adjacent waveguide and output to the cross port on the other side, as shown in Fig. 1 c. To achieve a buckling-free HADC with precise residue strain management, it is essential to first accurately estimate the residual strain in the SOI wafer used. To this end, we have designed, fabricated, and characterized a group of doubly-clamped suspended beams along and crystallographic directions of silicon. The length and width of the beams are 50 µm and 4 µm, respectively. The optical microscope image of the doubly-clamped suspended beams is shown in Fig. 2 a. Laser confocal microscope (LCM) with 20 nm z -axis step size is used to measure the buckling of the suspended beams, as shown in Fig. 2 b. All beams along different directions have equivalent buckling in the z direction, either up or down. Therefore, we show the measured results along A–A’ in Fig. 2 a to represent the buckling behavior, as the black circles show in Fig. 2 c. Finally, the buckling with different residual strain is simulated with the finite element method (FEM) and the results with isotropic residual strain of 5 × 10 –4 agree well with the measured results, as shown by the red curve in Fig. 2 c. This confirms the accuracy of the residual strain estimation and provides a foundation for mechano-optical co-design of the buckling-free HADC structures with effective strain management. The three-dimensional (3D) finite-difference time-domain (FDTD) and finite element method (FEM) are used to mechano-optically co-design the photonic and mechanical structures of the HADC systematically. The schematic diagram of the HADC is shown in Fig. 3 a, b and the parameters are chosen preliminarily as listed in Table 2 to achieve high-performance adiabatic coupling. Here the total length of the coupling region is chosen as 60 µm, i.e., L c = L a + 2 L t = 60 µm, where L t and L a are respectively the lengths of the taper and the slab anchors. Initially, the lengths L t and L a are optimized without including the extra S-bends for achieving sufficiently low excess losses and avoid notable waveguide-buckling. Figure 3 c gives the simulated excess losses of the HADC designed with L t = 25, 20, 15, 10, and 5 µm, respectively. Definitely, in order to minimize the scattering loss due to the introduction of the slab anchors, the length L t should be sufficiently long. For example, here we choose L t ≥ 15 µm and obtain low scattering losses of 0–0.07 dB over the broad bandwidth of 1.4–1.7 µm. On the other hand, larger L t results in a shorter anchor length L a , which reduces the stability of the waveguides and might lead to stronger buckling both in the y and z directions. Note that the buckling of the stationary waveguide is stronger than the movable waveguide because the former is directly doubly-clamped by a pair of mode converters while the latter has many other waveguide structures including S-bends and meandering waveguides to help release the residual strain. Consequently, only the buckling of the stationary waveguide is analyzed, as shown in Fig. 3 d, e. The residual strain of the used SOI wafer is estimated to be 5×10 –4 according to our measurement described above. Evidently, the waveguide buckling increases with decreased L a . Therefore, the length L a is chosen to be the maximum of 30 µm for weakening the buckling ( d y ≤ 97 nm in the y direction, and d z ≤ 560 nm in the z direction) regarding that the condition of L t ≥15 µm for achieving low scattering losses. Nevertheless, the waveguide buckling offsets d y and d z are still too large for the HADC to work. Table 2 Parameters of the designed HADC Parameters Values Preliminary parameters w 1 450 nm w 2 350 nm L s 17.5 µm W s 1 µm L c 60 µm w t 1 µm t 1 70 nm t 2 150 nm g ON 25 nm Optimized parameters L a 30 µm L t 15 µm, ( L c – L a )/2 g OFF 900 nm Therefore, extra S-bends are also introduced at the both ends of the stationary waveguide to further decrease the buckling. Theoretically, a larger S-bends enables lower bending loss and smaller waveguide buckling, but introduce larger footprints and higher propagation losses. Therefore, the extra S-bends are designed to achieve low excess loss and compact footprints (see Supplementary Section 2). As a result, the waveguide buckling is minimized to be < 42 nm as shown in Fig. 3 f. Thus, residual strain management structures have been implemented with the mechano-optically co-design, so that the misalignment between the stationary and movable waveguides is effectively minimized in the ON state, as shown in Fig. 3 g, h. Here, a direct comparison is given by simulating a conventional MEMS switch without any mechano-optically co-designed structure consisting of the slab anchors and the extra S-bends, as shown in Fig. 3 i, j, indicating that there occurs a significant buckling offset of 1.3 µm, which seriously prohibits the desired coupling between the waveguides of the HADC in the ON state. To ensure low crosstalk in the OFF state and acceptably low driving voltages for switching on, the initial width g OFF of the gap between the two waveguides in the OFF state is designed to be 900 nm. The simulated transmission spectra and light propagation at 1550 nm of the HADC in the OFF state are shown in Fig. 3 k, m, which shows low excess losses of 0–0.07 dB and low crosstalk of − 52.5 – − 28.5 dB over the broad bandwidth of 1400–1700 nm. Higher crosstalk at longer wavelengths is attributed to the weaker optical mode confinement and thus stronger evanescent field coupling. In the ON state, the width g ON of the gap between the two waveguides is designed to be 25 nm to ensure adiabatic mode evolution within a short coupling region over a broad bandwidth. The simulated transmission spectra and light propagation at 1550 nm in the ON state are shown in Fig. 3 l, n, showing low excess losses of 0–0.06 dB and low crosstalk of − 47.7 – − 21.1 dB over a large bandwidth. Furthermore, the entire MEMS structure in the ON state is simulated numerically as shown in Fig. 3 o, which indicates ~ 2.5×10 –3 maximum principal strain at the structural aligners, well below the damage threshold of silicon. The actuation voltage of the device is simulated as shown in Fig. 3 p. Here the red line shows the quadratic relationship between the driving force generated by the electrostatic combs and the applied voltage, while the blue line shows the linear relationship between the driving force and the displacement. The elastic deformation of the mechanical structures follows Hooke's law, with a spring constant of ~ 0.36 N/m. According to the electromechanical simulation results, the threshold voltage for switching on is ~ 24 V for the designed switch with an initial 900-nm gap. Furthermore, the energy consumption of the elementary switch is estimated to be ~ 0.64 pJ (see Supplementary Section 7). Experiment results The designed devices were fabricated by electron beam lithography (EBL) processes. The perspective scanning electron microscope (SEM) image of the elementary switch is shown in Fig. 4 a, showing that the footprint of the device is approximately 100×100 µm 2 . The stationary waveguide, the movable waveguide and the S-bends are all suspended as shown in the stitched close-up view in Fig. 4 b. The width of the taper waveguides and the gap between the waveguides are measured to be w 1 = 440 nm, w 2 = 340 nm, and g OFF = 905 nm, respectively, as shown in Fig. 4 c. The travel distance of the movable waveguide determined by the mechanical stopper is measured to be 880 nm as shown in Fig. 4 d, indicating a 25-nm-wide gap when the switch is ON. The gentle discrepancy between the designed and fabricated dimensions is attributed to the imperfect fabrication. The measured transmission spectra T 12 with different driving voltage are given in Fig. 4 e, showing that there is very low crosstalk in the OFF state (0 V). As the driving voltage increases, T 12 increases as well because the gap between the two waveguides is narrowed by the MEMS actuator. The threshold voltage setting the switch to be ON was measured to be 22 V, which agrees well with the simulated result of 24 V. The gentle discrepancy is attributed to the lowered spring constant caused by fabrication errors for the mechanical structures. The switch remains in the ON state without any damage even operating with a driving voltage up to 65 V, indicating a maximum driving voltage rating beyond 65 V. Such a large tolerance warrants robust drive schemes for large-scale switch arrays. For precise measurement of the excess loss of the switch, ten elementary switches in cascade were measured (see Supplementary Section 4). Figure 4 f gives the averaged transmission spectra of the switch in OFF state, showing an excess loss of 0.11–0.43 dB and crosstalk of − 61.8 – − 42 dB over the large bandwidth of 1420–1600 nm. Note that the measured bandwidth is limited by the grating couplers. The excess loss in the OFF state is not as low as the simulated one due to the relatively high propagation loss caused by rough-sidewall scattering, excess mode conversion losses (see Supplementary Section 3), and extra scattering losses of the slab-anchors due to slight EBL alignment offset. In the ON state, the measured transmission spectra are given in Fig. 4 g, which shows that the excess loss is 0.05–0.99 dB and the crosstalk is − 24.9 – − 10.3 dB over the same large bandwidth. It can be seen that the experimental results are not as excellent as the simulated ones due to the waveguide buckling. By using a laser confocal microscope (LCM), the maximum buckling in the z direction of the stationary waveguide in the OFF state from 10 devices is measured to be ~ 100 nm consistently (see the red curve and shaded area in Fig. 4 h), which is much larger than the simulated results due to the waveguide narrowing and the slab-anchor over-etching. Note that the simulated results for the buckling of the stationary waveguide with the parameters of w 1 = 440 nm, w 2 = 340 nm and t 1 = 40 nm agree well with the experimental results, as shown by the blue curve in Fig. 4 h, while the designed parameters are w 1 = 450 nm, w 2 = 350 nm and t 1 = 70 nm. Also note that the consistent buckling profiles from all the measured devices indicate excellent uniformity of the fabricated devices. The temporal experimental results show that the present switch works well with fast ON/OFF switching time of 2.2/1.9 µs, as shown in Fig. 4 i. To demonstrate the durability, the switch was operated over 7.2 billion on-off cycles with a square-wave voltage at the frequency of 50 kHz for more than 40 hours. Negligible performance degradations were observed from the measured transmission at 1550 nm shown in Fig. 4 j and the measured transmission spectra T 12 given in Supplementary Section 5. The present 2×2 HADC switch is then scaled to a 64×64 array designed with the Benes topology, which consists of 352 switch cells and has a footprint of 8.4×5.2 mm 2 , as shown in Fig. 5 a. Here varied-width multimode interference (VWMMI) ridge waveguide crossings are employed to achieve low loss and low crosstalk 30 , while grating couplers are used for fiber-chip coupling with two 66-channel fiber arrays at the input/output ports. The switch array in the initial all-OFF state (i.e., all 352 switch cells are OFF, labelled as ‘A-0’) was characterized by coupling light into the i -th ( i = 1–64) input port in sequence, and the measured transmission T A-0 i,j from the i-th input port to the j -th output port are shown in Fig. 5 b, where i, j = 1–64. The maximum transmission T A-0 i,j with i ≠ j is 36.7 dB lower than the transmission T A-0 i,j with i = j over the bandwidth of 1530–1590 nm, exhibiting high extinction ratios. The excess losses of all the transmission T A-0 i,i at the center wavelength of 1550 nm are given by the red circles in Fig. 5 c, showing a variation ranging from 4.06 dB to 9.73 dB. Notably, among the 64 signal paths, path T A-0 i,j ( i = 40) exhibits a significantly higher loss, as indicated by the grey cross in Fig. 5 c. This is attributed to fabrication defects occasionally introduced during the EBL process. To prevent confusion between the defective signal path and non-signal paths, the transmission spectrum of this particular path has been excluded from Fig. 5 b. Here, since the chip is not packaged yet, each 2×2 switch cell can electrically be addressed by a pair of probes manually in the experiment, in which case we go further characterize the 32 specific states with only one cell switched on (labelled as ‘S-1’, i.e., all switch cells are still OFF except the selected one,). In the experiment, the 32 switch cells at the center stage (stage 5) of the Benes array were switched on one by one. As a result, the signal incident from the i -th input port is alternatively routed to the j -th output port, where j = i + 32 when i = 1–32 or j = i – 32 when i = 33–64), and the corresponding measured transmission spectra T S-1 i,j are shown in Fig. 5 d. As it can be seen, the extinction ratios (given as T S-1 i,j – T A-0 i,j ) are higher than 38.5 dB over the bandwidth of 1530–1590 nm, while the excess losses of these 64 transmissions T S-1 i,j are 5.31–8.75 dB at 1550 nm (see the red rings in Fig. 5 e). The defective path i = 40 is marked in Fig. 5 e and excluded in Fig. 5 d for the same reason mentioned above. On the other hand, according to the measured excess/propagation losses of ridge waveguide crossings, 90° arc-bends, Euler S-bends and 600-nm-wide ridge waveguides (see Supplementary Section 3), the total excess losses of the signal paths in the all-OFF and single-ON states are respectively estimated as 3.92–8.93 dB and 5.65–8.15 dB (see Supplementary Section 6), which agrees well with the directly measured results, as shown by the blue rings in Fig. 5 c and 5 e, indicating that all the elements in the array work well with high uniformity, while the slight discrepancy is attributed to some slight fabrication defects, minor variations of grating couplers, inter-channel variation of the fiber arrays as well as some random particulate contamination on top of the chip. Discussions Residual strain of the SOI wafer For silicon photonic MEMS device design, residual strain management is crucial for doubly-clamped structures, and thus it is demanded to co-design optical and mechanical structures optimally. The residual strain of the SOI wafer is estimated to be 5×10 –4 by measuring the buckling of suspended doubly-clamped beams and fitting the measured data with the FEM simulation result. Particularly, we introduce extra waveguide S-bends and slab anchors to reduce the buckling of the HADC waveguides to be less than 40 nm in theory, thus alleviating the negative impact on the performance of the switch. Such a mechano-optical co-design approach is beneficial to the development of most photonic MEMS devices. In experiment, the buckling of the fabricated HADC was alleviated to be ~ 100 nm, while the corresponding excess loss and crosstalk of the switch in the ON state are 0.05–0.99 dB and − 24.9 – − 10.3 dB, respectively, which is not as good as the design. The reason is due to the fabrication deviations, including the waveguide narrowing and the slab-anchor over-etching, which makes the HADC more sensitive to the release of residual strain. The performance of the switch can be improved by enhancing the fabrication precision. Scalability In this paper, the switches were fabricated with manual EBL processes in order to make rapid experimental verification. Unfortunately, the propagation loss of the fabricated silicon photonic waveguides is 8.3 dB/cm due to the rough sidewalls’ scattering. When using standard silicon photonic foundry processes, the propagation loss of 600-nm-wide ridge waveguides can be as low as 1.0 dB/cm or less, which is nine times lower than the waveguide fabricated with EBL processes in the lab. Meanwhile, as demonstrated in our previous work, the excess loss of the waveguide crossings can potentially be reduced to < 10 mdB when fabricated with silicon photonic foundry processes 31 . Accordingly, the total excess loss could be reduced greatly to 4.5 dB on average for the 64×64 array. When scaled to be 128×128, the array has an average excess loss of 6.9 dB, which is still acceptably low, showing the great potential for high scalability. As a summary, the present HADC switch provides a highly viable solution for realizing large-scale silicon photonic switches available for a broad spectrum of practical applications such as photonic interconnect, programmable and reconfigurable PICs, photonic FPGAs, ONNs/PNNs, quantum information processors, and FPSA LiDAR. Declarations Availability of data and materials The data and materials that support the plots within this paper are available from the corresponding authors upon request. Competing interests H.L., Y.H., and D.D. have filed a patent application (application number: CN115826143A) on the silicon photonic MEMS switches based on adiabatic directional couplers presented in this paper, which is also in progress of PCT patent application (application number: PCT/CN2023/118315). All other authors declare they have no competing interests. Funding This work is funded by National Key Research and Development Program of China (2024YFB2908302), National Science Fund for Distinguished Young Scholars (61725503), National Natural Science Foundation of China (U23B2047, 62321166651, 92150302), Leading Innovative and Entrepreneur Team Introduction Program of Zhejiang (2021R01001), Zhejiang Provincial Major Research and Development Program (2021C01199), Natural Science Foundation of Zhejiang Province (LZ22F050006), Fundamental Research Funds for the Central Universities, and Startup Foundation for Hundred-Talent Program of Zhejiang University. Author’s contributions Y. H. and J. Z. contributed equally to this manuscript. Y. H., Q. M., L. Lyu, H. L., and D. D. designed the structures. Y. H., J. Z., Y. Lu, and Y. Liu fabricated the devices. Y. H. and J. Z. characterized the devices. Y. H., J. Z., H. L. and D. D. contributed to the data analyses. Y. H., J. Z., H. L. and D. D. wrote the manuscript. All authors discussed the results and contributed to the manuscript. H. L. and D. D. conceived the project. Acknowledgements The authors thank the ZJU Micro-Nano Fabrication Center and the Westlake Center for Micro/Nano Fabrication and Instrumentation for the facility support. References Bernal S, et al. 12.1 terabit/second data center interconnects using O-band coherent transmission with QD-MLL frequency combs. Nature Communications 15, 7741 (2024). Bogaerts W, et al. Programmable photonic circuits. Nature 586, 207–216 (2020). Zhang W, Yao J. Photonic integrated field-programmable disk array signal processor. Nature Communications 11, 406 (2020). 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1","display":"","copyAsset":false,"role":"figure","size":953817,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePrinciple of the switch. a,\u003c/strong\u003e Schematic diagram of the MEMS structures. \u003cstrong\u003eb, c\u003c/strong\u003e, Schematic diagram of the gap adjustable horizontal adiabatic coupler (HADC) in OFF and ON state, respectively.\u003c/p\u003e","description":"","filename":"image1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7519103/v1/80a5454fcbd20290f9d0d9f1.jpeg"},{"id":95428255,"identity":"83cf4927-21f2-4db1-bc48-712002b4a8f6","added_by":"auto","created_at":"2025-11-08 07:10:46","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":325884,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eResidual strain analysis of the used SOI wafer.\u003c/strong\u003e \u003cstrong\u003ea,\u003c/strong\u003eOptical microscope image of the suspended beams. \u003cstrong\u003eb,\u003c/strong\u003e Laser confocal microscope (LCM) image of the suspended beams. \u003cstrong\u003ec,\u003c/strong\u003e Measured and simulated buckling of a suspended beam.\u003c/p\u003e","description":"","filename":"image2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7519103/v1/7a1b139a5022063011bc46a5.jpeg"},{"id":95428253,"identity":"2ba08ecf-cac4-4b01-b904-81d4ad607a09","added_by":"auto","created_at":"2025-11-08 07:10:45","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":538594,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDesign of the device.\u003c/strong\u003e \u003cstrong\u003ea,\u003c/strong\u003e Schematic diagram of the HADC. \u003cstrong\u003eb,\u003c/strong\u003e Cross-section view through the A–A’ line in \u003cstrong\u003ea\u003c/strong\u003e. \u003cstrong\u003ec,\u003c/strong\u003e Transmission spectra of the HADC in OFF state with different \u003cem\u003eL\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e. \u003cstrong\u003ed, e,\u003c/strong\u003e Buckling of the stationary waveguide in the \u003cem\u003ey\u003c/em\u003e and \u003cem\u003ez\u003c/em\u003e directions with different \u003cem\u003eL\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e. \u003cstrong\u003ef,\u003c/strong\u003e Buckling of the stationary waveguide in the \u003cem\u003ey\u003c/em\u003e and \u003cem\u003ez\u003c/em\u003e directions with extra S-bend. \u003cstrong\u003eg, h,\u003c/strong\u003e Optimized alignment of the HADC in the ON state enabled by the minimized buckling. \u003cstrong\u003ei, j,\u003c/strong\u003e Significant misalignment of the HADC in the ON state without the residual strain management structures including the slab anchors and extra S-bends. The buckling is magnified by a factor of 2 for clarity. \u003cstrong\u003ek, l,\u003c/strong\u003e Transmission spectra of the HADC in OFF and ON state respectively. \u003cstrong\u003em, n,\u003c/strong\u003e Light propagation of the HADC in OFF and ON state respectively. \u003cstrong\u003eo,\u003c/strong\u003e Simulated displacement and maximum principal strain of the device in ON state. Residual strain of the SOI wafer has been considered. \u003cstrong\u003ep\u003c/strong\u003e, Simulated electrostatic force arisen by different voltage and corresponding displacement of the movable mechanical structures.\u003c/p\u003e","description":"","filename":"image3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7519103/v1/87b66052ad68f7dbe0b0b964.jpeg"},{"id":95428274,"identity":"7aaed34c-34cf-418b-903d-8e2c97eb8c3d","added_by":"auto","created_at":"2025-11-08 07:10:51","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":330340,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eExperimental results of the elementary switch. a,\u003c/strong\u003e Perspective SEM image of the elementary switch. \u003cstrong\u003eb,\u003c/strong\u003e Stitched close-up view of the HADC. \u003cstrong\u003ec, d,\u003c/strong\u003e Close-up view of the taper waveguides and the mechanical stopper. \u003cstrong\u003ee,\u003c/strong\u003e Transmission \u003cem\u003eT\u003c/em\u003e\u003csub\u003e12\u003c/sub\u003e with different actuation voltages. The threshold voltage is measured to be 22 V and the maximum driving voltage rating is beyond 65 V. \u003cstrong\u003ef, g,\u003c/strong\u003e Measured transmission spectra of the switch in OFF and ON state respectively. \u003cstrong\u003eh,\u003c/strong\u003e The red shaded area shows the measured buckling in the \u003cem\u003ez\u003c/em\u003e direction of the stationary waveguide in the OFF state from 10 devices. The red curve shows the averaged buckling. The blue curve shows the simulated results with the fabrication errors including the narrowing of waveguides and over-etch of the slab anchors. \u003cstrong\u003ei,\u003c/strong\u003e Temporal response of the switch, which demonstrates ON/OFF switching time of 2.2 μs and 1.9 μs, respectively. \u003cstrong\u003ej,\u003c/strong\u003e Durability measurement of the switch. The switch is driven by a square voltage with 50 kHz frequency for over 40 hours and the total operating cycles exceeds 7.2 billion, after which the switch shows no performance degradation. Scale bars: a, 20 μm; b, 10 μm; c, 500 nm; d, 500 nm.\u003c/p\u003e","description":"","filename":"image4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7519103/v1/89522ccb2009c31107487720.jpeg"},{"id":95428273,"identity":"4584d8ac-5a6a-4212-84a4-db5b83c0111e","added_by":"auto","created_at":"2025-11-08 07:10:50","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":898264,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eExperimental results of the switch array. a,\u003c/strong\u003eOptical microscope image of the switch array. Scale bars: 500 μm. \u003cstrong\u003eb,\u003c/strong\u003e Transmission spectra \u003cem\u003eT\u003c/em\u003eA-0 i,j \u0026nbsp;(i, j = 1–64) of the switch array in all-OFF state. \u003cstrong\u003ec,\u003c/strong\u003e Measured and estimated excess loss of the 64 signal paths in all-OFF state at 1550 nm. \u003cstrong\u003ed,\u003c/strong\u003e Transmission spectra \u003cem\u003eT\u003c/em\u003eS-1 i,j \u0026nbsp;of the switch array in single-ON state. \u003cstrong\u003ee,\u003c/strong\u003eMeasured and estimated excess loss of the 64 signal paths in single-ON state at 1550 nm.\u003c/p\u003e","description":"","filename":"image5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7519103/v1/6327093000d83b130423d4be.jpeg"},{"id":95531571,"identity":"4b7c397a-0d5f-4737-8707-dc2a38da775b","added_by":"auto","created_at":"2025-11-10 10:23:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3808465,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7519103/v1/190001ff-cd9f-4198-8781-6628b45db807.pdf"},{"id":95428256,"identity":"97a4e038-0c1f-4f3d-83a0-42fb5eb37d69","added_by":"auto","created_at":"2025-11-08 07:10:46","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2251682,"visible":true,"origin":"","legend":"Supplement information of the article","description":"","filename":"Supplementaryinformation20250827.docx","url":"https://assets-eu.researchsquare.com/files/rs-7519103/v1/35c98c30735712faf41de4d4.docx"}],"financialInterests":"There is a conflict of interest\nH.L., Y.H., and D.D. have filed a patent application (application number: CN115826143A) on the silicon photonic MEMS switches based on adiabatic directional couplers presented in this paper, which is also in progress of PCT patent application (application number: PCT/CN2023/118315). All other authors declare they have no competing interests.","formattedTitle":"Mechano-optically Co-designed Highly-scalable Silicon Photonic MEMS Switches with Buckling-free 2×2 Horizontal Adiabatic Directional Couplers","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe rapid development of artificial intelligence, 5G/6G networks, and Internet of Things (IoT) has generated substantial demands for data transmission and processing, making photonic information transmission and processing an attractive solution due to its ultrahigh parallelism, large bandwidth, and low latency. Consequently, various large-scale photonic integrated circuits (PICs) have been demonstrated, including on-chip photonic networks\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, programmable photonic chips\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, photonic field-programmable gate arrays (FPGAs)\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e, optical neural networks (ONNs)\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e, as well as quantum information processors\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, where highly-scalable photonic switches are essential components to enable flexible circuit reconfigurations.\u003c/p\u003e\u003cp\u003eThe scaling of conventional photonic switches\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e utilizing Mach-Zehnder interferometers or micro-ring resonators is challenging due to the issues including high power consumption for thermo-optic devices, high carrier absorption loss for electro-optic devices, and sophisticated calibration processes due to the analog nature of the phase shifters. In contrast, micro-electromechanical system (MEMS) photonic switches\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e, which operate by modulating the mode coupling between waveguides in directional couplers (DCs) or adiabatic directional couplers (ADCs), offer a promising alternative. These switches potentially feature (1) low switching energy consumption of pJ-level and near-zero static power consumption\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e for electrostatic actuation and (2) no carrier absorption, making them ideal for large-scale switch arrays.\u003c/p\u003e\u003cp\u003eThe pioneering works on MEMS switches utilizing DCs have been demonstrated with a large scale up to 50\u0026times;50\u003csup\u003e15\u003c/sup\u003e. However, their practical applications are compromised due to the narrow bandwidth of DCs. Also, due to the analog nature of their operation principle, the voltage for electrostatic actuation must be accurately controlled for each switch cell, which requires cumbersome electronic circuits for control and drive. More recently, ADC-based MEMS switches\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e emerged as a promising solution due to their advantages of low excess losses, large bandwidths, digital operation, and exceptional fabrication tolerance. Particularly, vertical ADCs (VADCs)\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e and horizontal ADCs (HADCs)\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e are employed successfully for MEMS switches. Unfortunately, both these previously reported VADC- and HADC-based elementary switches are limited to a 1\u0026times;2 configuration, which are incompatible with widely-used topologies such as Benes, PI-Loss, Switch-and-Select, etc.\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, while each of these array topologies has its own uniqueness in scalability and functionality. In contrast, this paper focuses on the 2\u0026times;2 elementary switch as the preference for flexible compatibility with various topologies available for diverse switching scales and applications. Besides, it is crucial to achieve high robustness and high performances with simple fabrication processes for elementary switches, especially when scaled to an N\u0026times;N optical switch array with a large port count.\u003c/p\u003e\u003cp\u003eFor the VADC-based MEMS switches demonstrated previously\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e, impressive performances were achieved with low excess losses of \u0026lt;\u0026thinsp;0.7 dB, high extinction ratios of \u0026gt;\u0026thinsp;70 dB, large bandwidths of 300 nm and fast ON/OFF switching speed of 400/300 ns. Leveraging the elementary switch with such high performance, a 240\u0026times;240 switch array with Cross-Bar topology\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e and a 128\u0026times;128 LiDAR system incorporating a focal plane switch array (FPSA)\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e have been demonstrated. However, two silicon layers are required for the VADC design, which are not available in most silicon photonic foundry processes. Consequently, complicated back-end processes are required, which may limit the scalability due to high fabrication cost and compromised yield. Furthermore, the driving voltage of the elementary switch is as large as 60 V or beyond, which complicates the driving circuits due to the difficulties in implementing a multi-channel (100s or even 10,000s channels) voltage source with such a high voltage.\u003c/p\u003e\u003cp\u003eIn contrast, developing HADC-based MEMS switches is recognized as another attractive scheme because it only needs a single layer of silicon and is fully compatible with standard silicon photonic foundry processes, simplifying fabrication processes compared to VADC switches\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. To date, 1\u0026times;2 elementary switch\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e and single-pole double-throw (SPDT) elementary switch\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e have been proposed based on HADCs. For the pioneering HADC-based 1\u0026times;2 elementary switch\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e, a three-comb-drive device design and 8\u0026times;8 Cross-Bar switch array have been implemented. Unfortunately, the fabricated switch exhibits high excess losses of ~\u0026thinsp;4 dB, high crosstalk of ~\u0026thinsp;\u0026minus;\u0026thinsp;10 dB over the bandwidth of 108 nm, and ON/OFF switching speed of 36.7/21.4 \u0026micro;s in experiment. Furthermore, the measured excess loss in the ON state varies up to ~\u0026thinsp;1.7 dB, depending on the ADC gap, which indicates slightly nonadiabatic coupling in the fabricated device. For the HADC-based SPDT elementary switch\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e, a unique switch design and fast ON/OFF switching speed of 0.82/0.73 \u0026micro;s have been achieved. However, the excess-loss difference between the two output ports is as high as ~\u0026thinsp;5 dB and the bandwidth is only\u0026thinsp;~\u0026thinsp;25 nm in experiment. In addition, each SPDT switch requires two, instead of one, electrical pads, which doubles the number of driving circuits and compromises the scalability to the array. These limitations highlight the need for significant performance improvements in HADC-based MEMS switches.\u003c/p\u003e\u003cp\u003eIt is important to note that suspended waveguide structures used in MEMS switches are susceptible to significant inadvertent structural deformation, such as beam buckling\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, due to the release of the residual strain in the silicon-on-insulator (SOI) wafer used dominantly for PICs. The resultant waveguide deformation and/or misalignment often leads to significant and random switch performance deterioration, which greatly compromises the scalability. This issue is especially pronounced for digital photonic switches, for which fine-tuning to compensate the coupling change due to the waveguide deformation and/or misalignment is often undesired or infeasible. Therefore, systematic residue strain management is extremely critical for MEMS photonic switches by co-designing the photonic/mechanical structures, which has not been thoroughly investigated previously and will be the key focus here.\u003c/p\u003e\u003cp\u003eIn this paper, we propose and experimentally demonstrate a highly-scalable 2\u0026times;2 silicon photonic MEMS switch based on a mechano-optically co-designed buckling-free HADC by introducing extra S-bends and partially-etched taper waveguides with slab anchors. All these structures of the HADC are designed optimally to achieve high photonic switching performances with low excess losses and high extinction ratios over a broad bandwidth. Meanwhile, these structures are also designed mechanically to effectively suppress the waveguide buckling caused by the release of the residual strain in the SOI wafer, which often causes significant misalignment between the two waveguides in conventional HADCs, leading to photonic performance deterioration or even failure. With such a novel structural design, the present HADC switch developed with a single layer of silicon with standard silicon photonic foundry processes is buckling-free and achieves unprecedented photonic performance for HADC switches, including ultra-low excess losses, high extinction ratios and broad bandwidth, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Fast ON/OFF switching speed of 2.2/1.9 \u0026micro;s and reliable durability with \u0026gt;\u0026thinsp;7.2\u0026nbsp;billion switching-cycles are also demonstrated. Besides, the present MEMS switch features 2\u0026times;2 design instead of the previous 1\u0026times;2 design (see Supplementary Section 1), making it compatible with all widely used array topologies including Benes, Cross-Bar, PI-Loss and Switch-and-Select etc. Furthermore, a 64\u0026times;64 Benes switch array is realized as a proof of concept with high performances shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The switch array can be scaled to as large as 128\u0026times;128 when the waveguide loss is lowered further by optimizing the fabrication processes, showing great potential for photonic interconnects, programmable PICs, photonic FPGAs, ONNs, quantum information processors and FPSA LiDAR.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eTheoretical and experimental photonic performance of the devices\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDevices\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eData type\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eState\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eExcess loss (dB)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eCrosstalk (dB)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eBandwidth (nm)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e\u003cp\u003eElementary\u003c/p\u003e\u003cp\u003eswitch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003esimulated\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eOFF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0\u0026ndash;0.07\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026ndash;52.5 \u0026ndash; \u0026minus;\u0026thinsp;28.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e1400\u0026ndash;1700\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eON\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0\u0026ndash;0.06\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026ndash;47.7 \u0026ndash; \u0026minus;\u0026thinsp;21.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003emeasured\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eOFF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.11\u0026ndash;0.43\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026ndash;61.8 \u0026ndash; \u0026minus;\u0026thinsp;42.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e1420\u0026ndash;1600\u003c/p\u003e\u003cp\u003e(Limited by the used grating coupler)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eON\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.05\u0026ndash;0.99\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026ndash;24.9 \u0026ndash; \u0026minus;\u0026thinsp;10.3\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e64\u0026times;64\u003c/p\u003e\u003cp\u003eBenes\u003c/p\u003e\u003cp\u003eswitch\u003c/p\u003e\u003cp\u003earray\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003emeasured\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAll-OFF\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e4.1\u0026ndash;9.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026le; \u0026minus;\u0026thinsp;28.3 \u0026ndash; \u0026minus;\u0026thinsp;36.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003e1530\u0026ndash;1590\u003c/p\u003e\u003cp\u003e(Limited by the used grating coupler)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSingle-ON\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.3\u0026ndash;8.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026ge;\u0026thinsp;38.5\u003c/p\u003e\u003cp\u003e(ON/OFF extinction ratio)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cb\u003eMechano-optical co-design of the buckling-free HADC MEMS switch.\u003c/b\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea shows the schematic configuration of the present buckling-free MEMS switch based on a gap-adjustable HADC, which includes a pair of centrosymmetric taper waveguides with a constant gap. Especially, the length of the linear taper waveguides used for ADCs is usually tens or even hundreds of micrometers to ensure adiabatic mode evolution over a broad bandwidth. However, such long suspended waveguides often buckle strongly due to the release of the residual strain in the used SOI wafer, which may cause significant misalignment between the two waveguides in the coupling region, leading to photonic performance degradation or even failure. To address this issue, the taper parts used here for the HADC is designed to be partially etched with the slabs, which then function as the mechanical anchors. In particular, the slab anchors are tapered as well to avoid scattering losses due to the abrupt change of the waveguide cross-section, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. Additionally, extra S-bends are introduced to further reduce the waveguide buckling (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). For the suspended waveguides of the HADC, one is connected to the stationary silicon core-layer by a slab anchor, while the other one is connected through another slab anchor to the MEMS actuator consisting of a mechanical stopper, a perforated shuttle beam, a structural aligner, folded springs, and electrostatic combs, which enables the movement in the \u003cem\u003ey\u003c/em\u003e direction by the electrostatic actuation\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. In the OFF state, the two waveguides are separated with a sufficiently large gap to be decoupled and thus the incident light eventually outputs at the bar port on the same side, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. In the ON state, the gap between the two waveguides is narrowed electrostatically and the input light is then coupled adiabatically to the adjacent waveguide and output to the cross port on the other side, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo achieve a buckling-free HADC with precise residue strain management, it is essential to first accurately estimate the residual strain in the SOI wafer used. To this end, we have designed, fabricated, and characterized a group of doubly-clamped suspended beams along \u0026lt;\u0026thinsp;110\u0026thinsp;\u0026gt;\u0026thinsp;and \u0026lt;\u0026thinsp;100\u0026thinsp;\u0026gt;\u0026thinsp;crystallographic directions of silicon. The length and width of the beams are 50 \u0026micro;m and 4 \u0026micro;m, respectively. The optical microscope image of the doubly-clamped suspended beams is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. Laser confocal microscope (LCM) with 20 nm \u003cem\u003ez\u003c/em\u003e-axis step size is used to measure the buckling of the suspended beams, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb. All beams along different directions have equivalent buckling in the \u003cem\u003ez\u003c/em\u003e direction, either up or down. Therefore, we show the measured results along A\u0026ndash;A\u0026rsquo; in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea to represent the buckling behavior, as the black circles show in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. Finally, the buckling with different residual strain is simulated with the finite element method (FEM) and the results with isotropic residual strain of 5 \u0026times; 10\u003csup\u003e\u0026ndash;4\u003c/sup\u003e agree well with the measured results, as shown by the red curve in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. This confirms the accuracy of the residual strain estimation and provides a foundation for mechano-optical co-design of the buckling-free HADC structures with effective strain management.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe three-dimensional (3D) finite-difference time-domain (FDTD) and finite element method (FEM) are used to mechano-optically co-design the photonic and mechanical structures of the HADC systematically. The schematic diagram of the HADC is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, b and the parameters are chosen preliminarily as listed in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e to achieve high-performance adiabatic coupling. Here the total length of the coupling region is chosen as 60 \u0026micro;m, i.e., \u003cem\u003eL\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e \u003cem\u003e= L\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e + 2\u003cem\u003eL\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e = 60 \u0026micro;m, where \u003cem\u003eL\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e and \u003cem\u003eL\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e are respectively the lengths of the taper and the slab anchors. Initially, the lengths \u003cem\u003eL\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e and \u003cem\u003eL\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e are optimized without including the extra S-bends for achieving sufficiently low excess losses and avoid notable waveguide-buckling. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec gives the simulated excess losses of the HADC designed with \u003cem\u003eL\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e = 25, 20, 15, 10, and 5 \u0026micro;m, respectively. Definitely, in order to minimize the scattering loss due to the introduction of the slab anchors, the length \u003cem\u003eL\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e should be sufficiently long. For example, here we choose \u003cem\u003eL\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e \u0026ge; 15 \u0026micro;m and obtain low scattering losses of 0\u0026ndash;0.07 dB over the broad bandwidth of 1.4\u0026ndash;1.7 \u0026micro;m. On the other hand, larger \u003cem\u003eL\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e results in a shorter anchor length \u003cem\u003eL\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e, which reduces the stability of the waveguides and might lead to stronger buckling both in the \u003cem\u003ey\u003c/em\u003e and \u003cem\u003ez\u003c/em\u003e directions. Note that the buckling of the stationary waveguide is stronger than the movable waveguide because the former is directly doubly-clamped by a pair of mode converters while the latter has many other waveguide structures including S-bends and meandering waveguides to help release the residual strain. Consequently, only the buckling of the stationary waveguide is analyzed, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed, e. The residual strain of the used SOI wafer is estimated to be 5\u0026times;10\u003csup\u003e\u0026ndash;4\u003c/sup\u003e according to our measurement described above. Evidently, the waveguide buckling increases with decreased \u003cem\u003eL\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e. Therefore, the length \u003cem\u003eL\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e is chosen to be the maximum of 30 \u0026micro;m for weakening the buckling (\u003cem\u003ed\u003c/em\u003e\u003csub\u003ey\u003c/sub\u003e \u0026le; 97 nm in the \u003cem\u003ey\u003c/em\u003e direction, and \u003cem\u003ed\u003c/em\u003e\u003csub\u003ez\u003c/sub\u003e \u0026le; 560 nm in the \u003cem\u003ez\u003c/em\u003e direction) regarding that the condition of \u003cem\u003eL\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e \u0026ge;15 \u0026micro;m for achieving low scattering losses. Nevertheless, the waveguide buckling offsets \u003cem\u003ed\u003c/em\u003e\u003csub\u003ey\u003c/sub\u003e and \u003cem\u003ed\u003c/em\u003e\u003csub\u003ez\u003c/sub\u003e are still too large for the HADC to work.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eParameters of the designed HADC\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"2\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eParameters\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eValues\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u003cp\u003ePreliminary parameters\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003ew\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e450 nm\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003ew\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e350 nm\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eL\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e17.5 \u0026micro;m\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eW\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1 \u0026micro;m\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eL\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e60 \u0026micro;m\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003ew\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1 \u0026micro;m\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003et\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e70 nm\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003et\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e150 nm\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eg\u003c/em\u003e\u003csub\u003eON\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25 nm\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u003cp\u003eOptimized parameters\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eL\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e30 \u0026micro;m\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eL\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e15 \u0026micro;m, (\u003cem\u003eL\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e \u0026ndash; \u003cem\u003eL\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e)/2\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eg\u003c/em\u003e\u003csub\u003eOFF\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e900 nm\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eTherefore, extra S-bends are also introduced at the both ends of the stationary waveguide to further decrease the buckling. Theoretically, a larger S-bends enables lower bending loss and smaller waveguide buckling, but introduce larger footprints and higher propagation losses. Therefore, the extra S-bends are designed to achieve low excess loss and compact footprints (see Supplementary Section 2). As a result, the waveguide buckling is minimized to be \u0026lt;\u0026thinsp;42 nm as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef. Thus, residual strain management structures have been implemented with the mechano-optically co-design, so that the misalignment between the stationary and movable waveguides is effectively minimized in the ON state, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg, h. Here, a direct comparison is given by simulating a conventional MEMS switch without any mechano-optically co-designed structure consisting of the slab anchors and the extra S-bends, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ei, j, indicating that there occurs a significant buckling offset of 1.3 \u0026micro;m, which seriously prohibits the desired coupling between the waveguides of the HADC in the ON state.\u003c/p\u003e\u003cp\u003eTo ensure low crosstalk in the OFF state and acceptably low driving voltages for switching on, the initial width \u003cem\u003eg\u003c/em\u003e\u003csub\u003eOFF\u003c/sub\u003e of the gap between the two waveguides in the OFF state is designed to be 900 nm. The simulated transmission spectra and light propagation at 1550 nm of the HADC in the OFF state are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ek, m, which shows low excess losses of 0\u0026ndash;0.07 dB and low crosstalk of \u0026minus;\u0026thinsp;52.5 \u0026ndash; \u0026minus;\u0026thinsp;28.5 dB over the broad bandwidth of 1400\u0026ndash;1700 nm. Higher crosstalk at longer wavelengths is attributed to the weaker optical mode confinement and thus stronger evanescent field coupling. In the ON state, the width \u003cem\u003eg\u003c/em\u003e\u003csub\u003eON\u003c/sub\u003e of the gap between the two waveguides is designed to be 25 nm to ensure adiabatic mode evolution within a short coupling region over a broad bandwidth. The simulated transmission spectra and light propagation at 1550 nm in the ON state are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003el, n, showing low excess losses of 0\u0026ndash;0.06 dB and low crosstalk of \u0026minus;\u0026thinsp;47.7 \u0026ndash; \u0026minus;\u0026thinsp;21.1 dB over a large bandwidth. Furthermore, the entire MEMS structure in the ON state is simulated numerically as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eo, which indicates\u0026thinsp;~\u0026thinsp;2.5\u0026times;10\u003csup\u003e\u0026ndash;3\u003c/sup\u003e maximum principal strain at the structural aligners, well below the damage threshold of silicon. The actuation voltage of the device is simulated as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ep. Here the red line shows the quadratic relationship between the driving force generated by the electrostatic combs and the applied voltage, while the blue line shows the linear relationship between the driving force and the displacement. The elastic deformation of the mechanical structures follows Hooke's law, with a spring constant of ~\u0026thinsp;0.36 N/m. According to the electromechanical simulation results, the threshold voltage for switching on is ~\u0026thinsp;24 V for the designed switch with an initial 900-nm gap. Furthermore, the energy consumption of the elementary switch is estimated to be ~\u0026thinsp;0.64 pJ (see Supplementary Section 7).\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eExperiment results\u003c/h2\u003e\u003cp\u003eThe designed devices were fabricated by electron beam lithography (EBL) processes. The perspective scanning electron microscope (SEM) image of the elementary switch is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, showing that the footprint of the device is approximately 100\u0026times;100 \u0026micro;m\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. The stationary waveguide, the movable waveguide and the S-bends are all suspended as shown in the stitched close-up view in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. The width of the taper waveguides and the gap between the waveguides are measured to be \u003cem\u003ew\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;440 nm, \u003cem\u003ew\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;340 nm, and \u003cem\u003eg\u003c/em\u003e\u003csub\u003eOFF\u003c/sub\u003e = 905 nm, respectively, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec. The travel distance of the movable waveguide determined by the mechanical stopper is measured to be 880 nm as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, indicating a 25-nm-wide gap when the switch is ON. The gentle discrepancy between the designed and fabricated dimensions is attributed to the imperfect fabrication. The measured transmission spectra \u003cem\u003eT\u003c/em\u003e\u003csub\u003e12\u003c/sub\u003e with different driving voltage are given in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, showing that there is very low crosstalk in the OFF state (0 V). As the driving voltage increases, \u003cem\u003eT\u003c/em\u003e\u003csub\u003e12\u003c/sub\u003e increases as well because the gap between the two waveguides is narrowed by the MEMS actuator. The threshold voltage setting the switch to be ON was measured to be 22 V, which agrees well with the simulated result of 24 V. The gentle discrepancy is attributed to the lowered spring constant caused by fabrication errors for the mechanical structures. The switch remains in the ON state without any damage even operating with a driving voltage up to 65 V, indicating a maximum driving voltage rating beyond 65 V. Such a large tolerance warrants robust drive schemes for large-scale switch arrays. For precise measurement of the excess loss of the switch, ten elementary switches in cascade were measured (see Supplementary Section 4). Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef gives the averaged transmission spectra of the switch in OFF state, showing an excess loss of 0.11\u0026ndash;0.43 dB and crosstalk of \u0026minus;\u0026thinsp;61.8 \u0026ndash; \u0026minus;\u0026thinsp;42 dB over the large bandwidth of 1420\u0026ndash;1600 nm. Note that the measured bandwidth is limited by the grating couplers. The excess loss in the OFF state is not as low as the simulated one due to the relatively high propagation loss caused by rough-sidewall scattering, excess mode conversion losses (see Supplementary Section 3), and extra scattering losses of the slab-anchors due to slight EBL alignment offset. In the ON state, the measured transmission spectra are given in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg, which shows that the excess loss is 0.05\u0026ndash;0.99 dB and the crosstalk is \u0026minus;\u0026thinsp;24.9 \u0026ndash; \u0026minus;\u0026thinsp;10.3 dB over the same large bandwidth. It can be seen that the experimental results are not as excellent as the simulated ones due to the waveguide buckling. By using a laser confocal microscope (LCM), the maximum buckling in the \u003cem\u003ez\u003c/em\u003e direction of the stationary waveguide in the OFF state from 10 devices is measured to be ~\u0026thinsp;100 nm consistently (see the red curve and shaded area in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh), which is much larger than the simulated results due to the waveguide narrowing and the slab-anchor over-etching. Note that the simulated results for the buckling of the stationary waveguide with the parameters of \u003cem\u003ew\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;440 nm, \u003cem\u003ew\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;340 nm and \u003cem\u003et\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;40 nm agree well with the experimental results, as shown by the blue curve in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh, while the designed parameters are \u003cem\u003ew\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;450 nm, \u003cem\u003ew\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;350 nm and \u003cem\u003et\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;70 nm. Also note that the consistent buckling profiles from all the measured devices indicate excellent uniformity of the fabricated devices. The temporal experimental results show that the present switch works well with fast ON/OFF switching time of 2.2/1.9 \u0026micro;s, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ei. To demonstrate the durability, the switch was operated over 7.2\u0026nbsp;billion on-off cycles with a square-wave voltage at the frequency of 50 kHz for more than 40 hours. Negligible performance degradations were observed from the measured transmission at 1550 nm shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ej and the measured transmission spectra \u003cem\u003eT\u003c/em\u003e\u003csub\u003e12\u003c/sub\u003e given in Supplementary Section 5.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe present 2\u0026times;2 HADC switch is then scaled to a 64\u0026times;64 array designed with the Benes topology, which consists of 352 switch cells and has a footprint of 8.4\u0026times;5.2 mm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea. Here varied-width multimode interference (VWMMI) ridge waveguide crossings are employed to achieve low loss and low crosstalk\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e, while grating couplers are used for fiber-chip coupling with two 66-channel fiber arrays at the input/output ports. The switch array in the initial all-OFF state (i.e., all 352 switch cells are OFF, labelled as \u0026lsquo;A-0\u0026rsquo;) was characterized by coupling light into the \u003cem\u003ei\u003c/em\u003e-th (\u003cem\u003ei\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1\u0026ndash;64) input port in sequence, and the measured transmission \u003cem\u003eT\u003c/em\u003eA-0 i,j from the i-th input port to the \u003cem\u003ej\u003c/em\u003e-th output port are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb, where \u003cem\u003ei, j\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1\u0026ndash;64. The maximum transmission \u003cem\u003eT\u003c/em\u003eA-0 i,j with \u003cem\u003ei\u003c/em\u003e\u0026thinsp;\u0026ne;\u0026thinsp;\u003cem\u003ej\u003c/em\u003e is 36.7 dB lower than the transmission \u003cem\u003eT\u003c/em\u003eA-0 i,j with \u003cem\u003ei\u003c/em\u003e\u0026thinsp;=\u0026thinsp;\u003cem\u003ej\u003c/em\u003e over the bandwidth of 1530\u0026ndash;1590 nm, exhibiting high extinction ratios. The excess losses of all the transmission \u003cem\u003eT\u003c/em\u003eA-0 i,i at the center wavelength of 1550 nm are given by the red circles in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec, showing a variation ranging from 4.06 dB to 9.73 dB. Notably, among the 64 signal paths, path \u003cem\u003eT\u003c/em\u003eA-0 i,j ( \u003cem\u003ei\u003c/em\u003e\u0026thinsp;=\u0026thinsp;40) exhibits a significantly higher loss, as indicated by the grey cross in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec. This is attributed to fabrication defects occasionally introduced during the EBL process. To prevent confusion between the defective signal path and non-signal paths, the transmission spectrum of this particular path has been excluded from Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb. Here, since the chip is not packaged yet, each 2\u0026times;2 switch cell can electrically be addressed by a pair of probes manually in the experiment, in which case we go further characterize the 32 specific states with only one cell switched on (labelled as \u0026lsquo;S-1\u0026rsquo;, i.e., all switch cells are still OFF except the selected one,). In the experiment, the 32 switch cells at the center stage (stage 5) of the Benes array were switched on one by one. As a result, the signal incident from the \u003cem\u003ei\u003c/em\u003e-th input port is alternatively routed to the \u003cem\u003ej\u003c/em\u003e-th output port, where \u003cem\u003ej\u003c/em\u003e\u0026thinsp;=\u0026thinsp;\u003cem\u003ei\u003c/em\u003e\u0026thinsp;+\u0026thinsp;32 when \u003cem\u003ei\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1\u0026ndash;32 or \u003cem\u003ej\u003c/em\u003e\u0026thinsp;=\u0026thinsp;\u003cem\u003ei\u003c/em\u003e \u0026ndash; 32 when \u003cem\u003ei\u003c/em\u003e\u0026thinsp;=\u0026thinsp;33\u0026ndash;64), and the corresponding measured transmission spectra \u003cem\u003eT\u003c/em\u003eS-1 i,j are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed. As it can be seen, the extinction ratios (given as \u003cem\u003eT\u003c/em\u003eS-1 i,j \u0026ndash;\u003cem\u003eT\u003c/em\u003eA-0 i,j ) are higher than 38.5 dB over the bandwidth of 1530\u0026ndash;1590 nm, while the excess losses of these 64 transmissions \u003cem\u003eT\u003c/em\u003eS-1 i,j are 5.31\u0026ndash;8.75 dB at 1550 nm (see the red rings in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee). The defective path \u003cem\u003ei\u003c/em\u003e\u0026thinsp;=\u0026thinsp;40 is marked in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee and excluded in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed for the same reason mentioned above. On the other hand, according to the measured excess/propagation losses of ridge waveguide crossings, 90\u0026deg; arc-bends, Euler S-bends and 600-nm-wide ridge waveguides (see Supplementary Section 3), the total excess losses of the signal paths in the all-OFF and single-ON states are respectively estimated as 3.92\u0026ndash;8.93 dB and 5.65\u0026ndash;8.15 dB (see Supplementary Section 6), which agrees well with the directly measured results, as shown by the blue rings in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee, indicating that all the elements in the array work well with high uniformity, while the slight discrepancy is attributed to some slight fabrication defects, minor variations of grating couplers, inter-channel variation of the fiber arrays as well as some random particulate contamination on top of the chip.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"Discussions","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003eResidual strain of the SOI wafer\u003c/h2\u003e\u003cp\u003eFor silicon photonic MEMS device design, residual strain management is crucial for doubly-clamped structures, and thus it is demanded to co-design optical and mechanical structures optimally. The residual strain of the SOI wafer is estimated to be 5\u0026times;10\u003csup\u003e\u0026ndash;4\u003c/sup\u003e by measuring the buckling of suspended doubly-clamped beams and fitting the measured data with the FEM simulation result. Particularly, we introduce extra waveguide S-bends and slab anchors to reduce the buckling of the HADC waveguides to be less than 40 nm in theory, thus alleviating the negative impact on the performance of the switch. Such a mechano-optical co-design approach is beneficial to the development of most photonic MEMS devices. In experiment, the buckling of the fabricated HADC was alleviated to be ~\u0026thinsp;100 nm, while the corresponding excess loss and crosstalk of the switch in the ON state are 0.05\u0026ndash;0.99 dB and \u0026minus;\u0026thinsp;24.9 \u0026ndash; \u0026minus;\u0026thinsp;10.3 dB, respectively, which is not as good as the design. The reason is due to the fabrication deviations, including the waveguide narrowing and the slab-anchor over-etching, which makes the HADC more sensitive to the release of residual strain. The performance of the switch can be improved by enhancing the fabrication precision.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eScalability\u003c/h3\u003e\n\u003cp\u003eIn this paper, the switches were fabricated with manual EBL processes in order to make rapid experimental verification. Unfortunately, the propagation loss of the fabricated silicon photonic waveguides is 8.3 dB/cm due to the rough sidewalls\u0026rsquo; scattering. When using standard silicon photonic foundry processes, the propagation loss of 600-nm-wide ridge waveguides can be as low as 1.0 dB/cm or less, which is nine times lower than the waveguide fabricated with EBL processes in the lab. Meanwhile, as demonstrated in our previous work, the excess loss of the waveguide crossings can potentially be reduced to \u0026lt;\u0026thinsp;10 mdB when fabricated with silicon photonic foundry processes\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. Accordingly, the total excess loss could be reduced greatly to 4.5 dB on average for the 64\u0026times;64 array. When scaled to be 128\u0026times;128, the array has an average excess loss of 6.9 dB, which is still acceptably low, showing the great potential for high scalability. As a summary, the present HADC switch provides a highly viable solution for realizing large-scale silicon photonic switches available for a broad spectrum of practical applications such as photonic interconnect, programmable and reconfigurable PICs, photonic FPGAs, ONNs/PNNs, quantum information processors, and FPSA LiDAR.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAvailability of data and materials\u003c/h2\u003e\n\u003cp\u003eThe data and materials that support the plots within this paper are available from the corresponding authors upon request.\u003c/p\u003e\n\u003ch2\u003eCompeting interests\u003c/h2\u003e\n\u003cp\u003eH.L., Y.H., and D.D. have filed a patent application (application number: CN115826143A) on the silicon photonic MEMS switches based on adiabatic directional couplers presented in this paper, which is also in progress of PCT patent application (application number: PCT/CN2023/118315).\u0026nbsp;All other authors declare they have no competing interests.\u003c/p\u003e\n\u003ch2\u003eFunding\u003c/h2\u003e\n\u003cp\u003eThis work is funded by National Key Research and Development Program of China (2024YFB2908302), National Science Fund for Distinguished Young Scholars (61725503), National Natural Science Foundation of China (U23B2047, 62321166651, 92150302), Leading Innovative and Entrepreneur Team Introduction Program of Zhejiang (2021R01001), Zhejiang Provincial Major Research and Development Program (2021C01199), Natural Science Foundation of Zhejiang Province (LZ22F050006), Fundamental Research Funds for the Central Universities, and Startup Foundation for Hundred-Talent Program of Zhejiang University.\u003c/p\u003e\n\u003ch2\u003eAuthor’s contributions\u003c/h2\u003e\n\u003cp\u003eY. H. and J. Z. contributed equally to this manuscript. Y. H., Q. M., L. Lyu, H. L., and D. D. designed the structures. Y. H., J. Z., Y. Lu, and Y. Liu fabricated the devices. Y. H. and J. Z. characterized the devices. Y. H., J. Z., H. L. and D. D. contributed to the data analyses. Y. H., J. Z., H. L. and D. D. wrote the manuscript. All authors discussed the results and contributed to the manuscript. H. L. and D. D. conceived the project.\u003c/p\u003e\n\u003ch2\u003eAcknowledgements\u003c/h2\u003e\n\u003cp\u003eThe authors thank the ZJU Micro-Nano Fabrication Center and the Westlake Center for Micro/Nano Fabrication and Instrumentation for the facility support.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBernal S, \u003cem\u003eet al.\u003c/em\u003e 12.1 terabit/second data center interconnects using O-band coherent transmission with QD-MLL frequency combs. \u003cem\u003eNature Communications\u003c/em\u003e 15, 7741 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBogaerts W, \u003cem\u003eet al.\u003c/em\u003e Programmable photonic circuits. \u003cem\u003eNature\u003c/em\u003e 586, 207\u0026ndash;216 (2020).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang W, Yao J. 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Optica Publishing Group (2021).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"microsystems-and-nanoengineering","isNatureJournal":false,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"micronano","sideBox":"Learn more about [Microsystems \u0026 Nanoengineering](http://www.nature.com/micronano/)","snPcode":"41378","submissionUrl":"https://mts-micronano.nature.com/","title":"Microsystems \u0026 Nanoengineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7519103/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7519103/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMicro-electromechanical system (MEMS) photonic switches based on adiabatic directional couplers (ADCs) potentially offer the advantages including large fabrication tolerance and broad bandwidth, which is essential for large-scale reconfigurable photonic integrated circuits in optical networks and artificial intelligence computing infrastructures. However, the scalability of previous ADC-based MEMS switches is challenging due to the complicated fabrication of vertical ADC switches and buckling-induced performance deterioration of horizontal ADC switches. Here we propose a mechano-optically co-designed 2\u0026times;2 MEMS switch based on a unique buckling-free horizontal ADC by introducing residual-strain management structures including slab anchors and extra S-bends. For the fabricated device, the waveguide buckling is alleviated significantly and high optical performance is achieved with low loss and low crosstalk over a broad bandwidth of 180 nm. It also features fast switching speed of ~\u0026thinsp;2 \u0026micro;s, reliable durability with \u0026gt;\u0026thinsp;7.2\u0026nbsp;billion switching cycles, and exceptional scalability with the realization of a 64\u0026times;64 Benes switch array. The present 2\u0026times;2 horizontal ADC switches are compatible with all mainstream array topologies and can be fabricated using simple standard silicon photonic foundry processes, which are not accessible for those 1\u0026times;2 ADC switches reported previously. With these advantages, the present design provides a highly-scalable solution with great potential for MEMS/NEMS photonic devices used in versatile applications.\u003c/p\u003e","manuscriptTitle":"Mechano-optically Co-designed Highly-scalable Silicon Photonic MEMS Switches with Buckling-free 2×2 Horizontal Adiabatic Directional Couplers","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-08 07:10:41","doi":"10.21203/rs.3.rs-7519103/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"revise","date":"2025-11-28T06:12:57+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"This content is not available.","date":"2025-11-25T13:17:14+00:00","index":3,"fulltext":"This content is not available."},{"type":"editorInvitedReview","content":"This content is not available.","date":"2025-11-17T08:27:43+00:00","index":1,"fulltext":"This content is not available."},{"type":"editorInvitedReview","content":"This content is not available.","date":"2025-11-16T10:03:21+00:00","index":2,"fulltext":"This content is not available."},{"type":"reviewerAgreed","content":"This content is not available.","date":"2025-11-01T09:55:52+00:00","index":3,"fulltext":"This content is not available."},{"type":"reviewerAgreed","content":"This content is not available.","date":"2025-10-29T03:22:47+00:00","index":2,"fulltext":"This content is not available."},{"type":"reviewerAgreed","content":"This content is not available.","date":"2025-10-29T03:18:50+00:00","index":1,"fulltext":"This content is not available."},{"type":"reviewersInvited","content":"","date":"2025-10-29T02:39:45+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-09-05T00:55:23+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-09-02T14:58:51+00:00","index":"","fulltext":""},{"type":"submitted","content":"Microsystems \u0026 Nanoengineering","date":"2025-09-02T14:58:50+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"microsystems-and-nanoengineering","isNatureJournal":false,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"micronano","sideBox":"Learn more about [Microsystems \u0026 Nanoengineering](http://www.nature.com/micronano/)","snPcode":"41378","submissionUrl":"https://mts-micronano.nature.com/","title":"Microsystems \u0026 Nanoengineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"0e899c1e-9660-4bc0-b56d-c00dcd91b1a0","owner":[],"postedDate":"November 8th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":57055145,"name":"Physical sciences/Optics and photonics/Other photonics/Micro-optics"},{"id":57055146,"name":"Physical sciences/Nanoscience and technology/Nanoscale devices/NEMS"}],"tags":[],"updatedAt":"2026-03-02T04:00:11+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-08 07:10:41","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7519103","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7519103","identity":"rs-7519103","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}