Confinement and optimization of electromagnetic wave in photonic crystals based on SU-8 photoresist strip

Confinement and optimization of electromagnetic wave in photonic crystals based on SU-8 photoresist strip | 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 Research Article Confinement and optimization of electromagnetic wave in photonic crystals based on SU-8 photoresist strip Hanbo Shao, XiaoChen Hang, Dong Jiang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4905487/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Confinement of electromagnetic wave is key to the realization of applicable filters in optics technologies. We present a flexible cavity fabrication process that writes SU-8 photoresist microstrip onto a photonic crystal waveguide deterministically, in which electromagnetic wave can be confined excellently. Corresponding simulation shows bandgap of the waveguide shifts from 0.269c/a to 0.266c/a by adding a 0.1µm-thick layer of SU-8 with a refractive index of 1.57 on top of PhC. This is equivalent to an increment in wavelength from 1.265 µm to 1.277 µm. We also study the relationship of the Q factor with the strip width of SU-8 strip cavity. Tesults show that as the strip width is increased beyond 2 µm, Q total increases by up to approximately an order of magnitude, while V 0 increases by only a factor of 2. It provides a way to optimize Q factor and hence enables potential improvement on optical cavity mode. SU-8 photoresist photonic crystal electromagnetic wave filters Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Full Text With the development of integrated optics and quantum technology [1-3]. The concept of photon mechanics has also developed rapidly, and various electronic products at this stage are gradually entering the field of vision to many researchers in terms of smaller volume, faster computing speed and lower energy consumption. Unlike electrons, photons have specific physical properties such as polarization and frequency, and their speed is similar to the speed of light. For photons at rest, they are not affected by gravity, they will not have any effect on each other and the intersection of light rays will not interfere with the photon. Because of these properties, the photonic crystal [4-6] structure has advantages over electronic-based semiconductor materials. In 1987, Yablonovitch [7] and John [8] respectively gave their own understandings of photonic crystals. The advantage of photonic crystals is that they are small and easy to integrate, their wavelength can be selected, and the bandgap frequency can be changed according to changing material parameters. The photonic crystals with various defect states are ideal for designing optical components such as filters and waveguides. In 2003, Suh et al. [9] designed a mechanically modulated optical filter between two photonic crystal plates based on waveguide resonance. Drysdale et al. [10] developed a tunable photonic crystal filter, which can achieve frequency tunability in the range of 3.5GHz. In 2004, Nemec et al. [11] designed a one-dimensional photonic crystal filter with heat regulation in the terahertz range, which greatly increased the tunable range to 20% tunable type. The photonic crystal filter proposed by S.Upta et al. [12] can filter the infrared band to the low frequency range. Takes filtering capability to a new level. Compared with inorganic material filters, Hu et al. [13] produced organic material photonic crystal filters in 2005. The matrix was selected as polystyrene film, and then a particle beam was used to etch regular cylindrical pores on its surface, and a two-dimensional photonic crystal filter was formed by introducing line defects in the middle. On the experimental surface, the quality factor of the filter reaches 230, the peak transmittance reaches 90%, and the optical passband width is 2.4nm. In addition, these physical parameters can also be adjusted according to the defect width of the mediation line. These performance compared to inorganic photonic crystal filters achieve a very high strength improvement. To confine electromagnetic wave, some researchers [14, 15] remove a row of 3 holes from a hexagonal hole-in-slab photonic crystal (known as an L3 defect), a perturbation is introduced to the lattice which allows light to be confined to a cavity, The L3 cavity design, which can be optimized by shifting the radii and positions of neighbouring holes, is extremely popular in electromagnetic wave propagation experiments and has been featured in many high impact publications. electromagnetic wave is confined in the plane of the PhC by the photonic band gap and in the z -direction by index guiding. In a similar manner, it is also possible to create a waveguide in the PhC by introducing a linear defect. A common method of achieving this in a hole-in-slab PhC is to remove a row of holes [16]. The created linear defect can then support waveguide modes propagating along the x -direction -confined to the slab by index guiding and directed in-plane by the photonic band gap. Waveguides formed from linear defects in PhC slabs are frequently used for on-chip control of electromagnetic wave [17-20]. Most applications involve either guiding the light between components, or delaying the light by trapping it or slowing its group velocity [21] - crucial operations for the realisation of on-chip all-optical circuits. PhC waveguides can be optically coupled to defect cavity modes in the same PhC [22], allowing light to be coupled in or out of the cavity mode via the waveguide one potential use of this is for trapping and delaying of photons for optical processing operations. It is possible to fabricate PhC waveguides with very low propagation losses: for example, losses as low as ∼ 2 dBcm-1 have been achieved in Si-based slabs [23]. Therefore, the waveguides can be used to efficiently couple light into or out of PhC cavities (or components of interest) from a considerable distance, either through the use of grating couplers placed at the end of the waveguide [24, 25] (which are illuminated with a laser spot or collected from using a microscope objective above the sample) or via coupling the end of the waveguide to an optical fibre [26]. In this paper, we write a SU-8 photoresist strip on photonic crystals to confine electromagnetic wave, the relationship of the Q factor with the strip width of the SU-8 strip cavity are also investigated. It provides a way to optimize the Q factor and hence enables potential improvement on the optical cavity mode. The design is more practical because it can control wave propagation and frequency bandgap without altering the photonic crystal structure. Through our research, The Q factor can increase as the width of strip increases. The PhC slab is formed by air holes etched in the z -direction with a periodic hexagonal lattice pattern in the xy -plane (directions as defined in figure 1a). Finite-difference time-domain (FDTD) simulations were performed to model the PhC slab, using a lattice constant a = 340 nm, hole radius of 0.27a and a slab thickness of 200 nm. The refractive index of the slab is chosen to be n = 3.33 to match that of GaAs at cryogenic temperatures. The PhC exhibits an in-plane photonic band gap for TE-like modes. By introducing a row of missing holes along the x - direction through the PhC (The SEM image is shown in (Figure 1.b) is formed which can support in-plane guided modes [27-28]. A mode gap cavity is created by modifying the local structure of the PhC waveguide to have a lower energy than the waveguide band edge as obtained with a deposited SU-8 film (figure 2). A local potential minimum is then introduced which leads to confinement of electromagnetic wave in a similar fashion to that seen for electronic quantum confinement in a quantum well. Adding a 0.1μm-thick layer of SU-8 with a refractive index of 1.57 on top of the PhC shifts the waveguide band lower in energy, from 0.269c/a to 0.266c/a as shown by the dotted black curve in figure 2c, the AFM image of a successful SU-8 strip cavity with a strip height H = 0.1μm is shown in Figure 2b. This is equivalent to an increment in wavelength from 1.265 μm to 1.277 μm. This 12 nm band shift is sufficient to form an optical cavity to trap electromagnetic wave from the band edge when patterning a narrow strip of SU-8 across the PhC waveguide. This is represented schematically in figures 3a-c, for a 1μm-wide SU-8 strip positioned on a PhC waveguide. A strip width of 1μm is chosen to match the spot size of the laser used to write the SU-8 strip in the experiment. The simulated electric field intensity (| E | 2 ) profile of a cavity defined by a 1μm wide SU-8 strip is plotted in figure 3d through a central slice of the slab along the z-direction. The position of the SU-8 strip is represented by the region bounded by the dashed lines. A clear demonstration of electromagnetic wave restricted to the region of the strip is observed. Having successfully created an SU-8 strip cavity to realize confinement of electromagnetic wave, we now investigate the relationship of the Q factor with the strip width of the SU-8 strip cavity. It provides a way of optimize the Q factor and hence enables potential improvement on the optical cavity mode. We consider the effects of varying the strip width, w strip , while keeping the strip height h strip =100 nm constant. FDTD simulations were performed with w strip ranging from 0.5 µm to 6 µm; the parameters extracted from the fundamental cavity mode are presented in Fig. 4. Where Q total means the total Q factor of the cavity mode, Q in and Q out represent how well the cavity confines light in and out of the plane of the PhC. As the strip width is increased beyond ~ 2 µm, Q total increases by up to approximately an order of magnitude, while V 0 increases by only a factor of ~ 2 over the range of w strip simulated. For confinement applications, the Q total / V 0 ratio should be maximized. The results from Fig.4 confirm that, as long as the strip height is kept constant, it is possible to increase Q total / V 0 simply by increasing the width of the SU-8 strip. This suggests that cavities defined by a wider SU-8 strip could be preferable for confinement of electromagnetic wave. Before depositing the strip, the PhC is symmetric along the z -direction. The structure supports both TE-like and TM-like modes. The two types of modes can be studied independently due to their orthogonal polarizations. However, structural modification from the SU-8 breaks the z -symmetry and hence the two types of mode start to couple. This is a lossy process as the TM-like mode is allowed for propagation. The Q total of the SU-8 strip cavity mode, which is limited by in-plane TE-TM coupling losses , shows an increase with w strip that is reflected by an increase in Q in . This suggests a reduction of the in-plane losses, which is understood by considering how the cavity mode field profile changes with w strip . The losses are more explicitly illustrated via Fourier analysis. In Fig.5, We consider the spatial Fourier transform of the main E field components of the TE-like cavity mode, |FT( E x )+ FT( E y )|, in the top plane of the slab. This is compared for the cavity with w strip =1µm, h strip =100 nm [Fig. 5(a)], the cavity with w strip =4 µm, h strip =100 nm [Fig. 5(b)]. The dominant source of losses from the SU-8 strip cavity is due to coupling of the TE-like cavity mode to TM-like slab modes, which occurs along the TM contours marked on the diagrams by the dashed lines. Such coupling is facilitated by the broken z -symmetry of the structure and is clearly evident in 100 nm thick strip for the 1 µm-wide strip by the significant field components along the TM contour. However, for a 4 µm-wide strip of the same thickness, the field components along the TM contours are much weaker. This appears to be due to the increased localization of the Fourier components along k x , which is a result of the decreased spatial localization of the mode along the waveguide. We propose that this inhibits TE-TM coupling, since the field components of the TE- like cavity mode overlap less with the TM contours. We therefore attribute the increase in Q total with w strip to a reduction in TE-TM coupling losses, resulting from the more localized Fourier components of the mode, which is consistent with an increase in Q in . Conclusion In this paper, the SU-8 photoresist strip on photonic crystals is used to confine electromagnetic wave. Corresponding simulation shows that, bandgap of the waveguide shifts from 0.269c/a to 0.266c/a by Adding a 0.1μm-thick layer of SU-8 with a refractive index of 1.57 on top of the PhC. This is equivalent to an increment in wavelength from 1.265 μm to 1.277 μm. This 12 nm band shift is sufficient to form an optical cavity to trap electromagnetic wave from the band edge when patterning a narrow strip of SU-8 across the PhC waveguide. The relationship of the Q factor with the strip width of the SU-8 strip cavity is also investigated. It provides a way to optimize the Q factor and hence enables potential improvement on the optical cavity mode. Our study shows that as the strip width is increased beyond ~ 2 µm, Q total increases by up to approximately an order of magnitude, while V 0 increases by only a factor of ~2, so Q total / V 0 ratio can be maximized. This suggests that cavities defined by a wider SU-8 strip could be preferable for confinement of electromagnetic wave. Using SU-8 photoresist strip on photonic crystals to confine electromagnetic wave is more practical and flexible, the method in our paper provide new significance for research into optical filter. Declarations Notes The authors declare no competing financial interest. Data availability The datasets used and/or analysed during the current study available from the corresponding author on reasonable request. Funding Not applicable Authors Contributions Hanbo Shao developed the algorithm, constructed the structure, carried out the simulation and drafted the manuscript. Xiaochen Hang revised the manuscript and provided relevant literature experience. Dong Jiang participated in the design of the study and provided financial support. All authors have read and approved the final manuscript. References Maruf, A.: Rubayet. Integrated Waveguide Interfaces for Quantum. Optics Applications [J]. (2020) Capmany, J., Mora, J., Gasulla, I., et al.: Microw. Photonic Signal. Process. [J] J. Lightwave Technol. 31 (4), 571–586 (2013) Jacques, C.: Christopher, et al. Quantum Optics. Universal linear optics [J].Science (2015) Zhang, H.F., Liu, S.B., Li, B.X.: Investigation on the properties of omnidirectional photonic band gaps in two-dimensional plasma photonic crystals [J]. Phys. Plasmas. 23 (1), 2486 (2016) Dolan, J.A., Wilts, B.D., Vignolini, S., et al.: Optical Properties of Gyroid Structured Materials: From Photonic Crystals to Metamaterials [J]. Adv. Opt. Mater. 3 (1), 12–32 (2015) Gevorgyan, A.H., Oganesyan, K.B., Ayryan, E.A., et al.: Radiation Peculiarities in Chiral Photonic Crystals [J]. (2017) Yablonovitch, E.: Inhibited spontaneous emission in solid-state physics and electronics [J]. Phys. Rev. Lett. 58 (20), 2059–2062 (1987) John, S.: Strong localization of photons in certain disordered dielectric superlattices [J]. Phys. Rev. Lett. 58 (23), 2486–2489 (1987) Takano, H., Akahane, Y., Asano, T., et al.: In-plane-type channel drop filter in a two-dimensional photonic crystal slab [J]. Appl. Phys. Lett. 84 , 2226–2228 (2004) Drysdale, T.D., Blaikie, R.J.: Calculated and measured transmittance of a tunable metallic photonic crystal filter for terahertz frequencies [J]. Appl. Phys. Lett. 83 (26), 5362–5364 (2003) Nemec, H.: Thermally-tunable filter for terahertz range based on a one-dimensional photonic crystal with a defect [J]. J. Appl. Phys. 96 , 4072–4075 (2004) Gupta, S., Tuttle, G., Sigalas, M., et al.: Infrared filters using metallic photonic band gap structures on flexible substrates [J]. Appl. Phys. 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Express. 15 , 17458–17481 (2007) Faraon, A., Fushman, I., Englund, D., et al.: Dipole induced transparency in waveguide coupled photonic crystal cavities [J]. Opt. Express. 16 , 12154–12162 (2008) Liu, F., Brash, A.J., O’Hara, J., et al.: High Purcell factor generation of indistinguishable on-chip single photons [J]. Nat. Nanotechnol. 13 , 835–840 (2018) Lee, E.H., Song, J.D., Stobbe, S., et al.: Near-Unity Coupling Efficiency of a Quantum Emitter to a Photonic Crystal Waveguide [J]. Phys. Rev. Lett. 113 , 93603 (2014) Faraon, A., Fushman, I., Englund, D., et al.: Dipole induced transparency in waveguide coupled photonic crystal cavities [J]. Opt. Express. 16 , 12154–12162 (2008) Bose, R., Sridharan, D., Solomon, G.S., et al.: Observation of strong coupling through transmission modification of a cavity-coupled photonic crystal waveguide [J]. Opt. Express. 19 , 5398–5409 (2011) Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4905487","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":341961911,"identity":"2883c6bd-23bb-4bd5-bac0-c191f255464f","order_by":0,"name":"Hanbo Shao","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7ElEQVRIiWNgGAWjYBACAwYGNiB1gIGBvYHBoIIBwiZSC88BBoMzpGmRSGBgIEqLOfvZYw8+/Lkjzy/59kDBwTYGOb4bCYyfC/BosezJSzec2fbMcObsvAQDoBZjyRsJzNIz8DnsQI6ZNG/DYcYNt3MMjD+2MSRuuJHAxsyDT8v5N2bSPH8O22+4ecYAZEs9YS03gLbwsB0GGs4D1pJgQFjLG3OgXw4nz+zJMTA4cE7CcOaZh83S+B2WYwYMscO2/exnzAwOlNnI8x1PPvgZnxZkwAaMJAkgzdhApAYGBuYHRCsdBaNgFIyCEQUAsvpTxdRC1bIAAAAASUVORK5CYII=","orcid":"","institution":"Nanjing Forestry University","correspondingAuthor":true,"prefix":"","firstName":"Hanbo","middleName":"","lastName":"Shao","suffix":""},{"id":341961912,"identity":"efb145ed-28a8-45e9-99a2-e8e738d1ce67","order_by":1,"name":"XiaoChen Hang","email":"","orcid":"","institution":"Nanjing Forestry University","correspondingAuthor":false,"prefix":"","firstName":"XiaoChen","middleName":"","lastName":"Hang","suffix":""},{"id":341961913,"identity":"0b29cf74-6e92-4988-a1b0-2aebbc75b77a","order_by":2,"name":"Dong Jiang","email":"","orcid":"","institution":"Nanjing Forestry University","correspondingAuthor":false,"prefix":"","firstName":"Dong","middleName":"","lastName":"Jiang","suffix":""}],"badges":[],"createdAt":"2024-08-13 08:42:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4905487/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4905487/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":64369799,"identity":"c0f6bd49-c237-4069-bbff-862abb3a6e3e","added_by":"auto","created_at":"2024-09-12 09:00:54","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":63154,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic representation of the PhC slab: a hexagonal array of air holes (white), with lattice constant a, is etched into a GaAs slab (grey). A choice of unit cell is shown in blue (3 are shown), in addition to two choicesof primitive lattice vector, a1and a2. (b) SEM images of the PhC devices used in this work\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4905487/v1/61be4b66e68d9615344da009.jpg"},{"id":64369798,"identity":"49a599bc-e74f-4936-950a-46c18304869e","added_by":"auto","created_at":"2024-09-12 09:00:54","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":40601,"visible":true,"origin":"","legend":"\u003cp\u003ePhC waveguide with a layer of 0.1 µm thick cross-linked SU-8 on the top surface. (a)SEM image of a successful SU-8 strip cavity with a strip height H = 0.1μm. (b) AFM image of the deposited SU-8 film. (c) The waveguide band is shown for PhC 1(no film) and PhC 2 (0.1μm SU-8 film), in addition to the light cone and approximate position of the extended modes (light blue region) and photonic band gap edges (dark blue lines).\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4905487/v1/67c23ca2c8731a13be649d35.jpg"},{"id":64369800,"identity":"f488bb62-829a-4503-bee9-0f7cb8a1422f","added_by":"auto","created_at":"2024-09-12 09:00:54","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":58634,"visible":true,"origin":"","legend":"\u003cp\u003eSchematics diagrams showing the principle of the SU-8 strip-defined mode gap cavity. (a) and (b): side profile and top-down view of the device, which consists of a PhC waveguide with a strip of SU-8 placed on top. The strip is a cuboid with a width \u003cem\u003ew\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e and height \u003cem\u003eh\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e, which extends across the \u003cem\u003ey\u003c/em\u003e-extent of the PhC and does not infiltrate the holes of the PhC. (c) representation of light confine to the SU-8 strip region by mode gap confinement. (d) |E|\u003csup\u003e2\u003c/sup\u003e field envelope of the cavity mode\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4905487/v1/7d0d458518d6497afdbb5c32.jpg"},{"id":64369810,"identity":"b6ee7073-d6c2-49b0-b11b-8ca1c5189213","added_by":"auto","created_at":"2024-09-12 09:00:54","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":50551,"visible":true,"origin":"","legend":"\u003cp\u003eFDTD simulation results for SU-8 strip cavities with hstrip=100 nm and variable wstrip, Q factor components and the mode volume of the fundamental cavity mode are shown.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4905487/v1/45cc11410347fcd95ee2cb26.jpg"},{"id":64369801,"identity":"acc09807-294c-4e4f-8187-29da395ea93d","added_by":"auto","created_at":"2024-09-12 09:00:54","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":40714,"visible":true,"origin":"","legend":"\u003cp\u003eSpatial Fourier transform, |FT(Ex)+ FT(Ey)|, extracted from FDTD simulations for a device with (a) wstrip=1 µm, hstrip=100 nm, (b) wstrip=4 µm, hstrip=100 nm. The light cone is marked as a solid red circle; contours of the TM-like slab mode are marked by dashed red lines. Each result is normalized.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4905487/v1/f509aad6c07c1a26c80539cd.jpg"},{"id":69154259,"identity":"3f107f5c-ed3b-480d-8da0-8c75532fe4dd","added_by":"auto","created_at":"2024-11-16 15:31:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":520980,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4905487/v1/658977eb-5938-4356-8405-cc1f9d0758ec.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Confinement and optimization of electromagnetic wave in photonic crystals based on SU-8 photoresist strip","fulltext":[{"header":"Full Text","content":"\u003cp\u003eWith the development of integrated optics and quantum technology [1-3]. The concept of photon mechanics has also developed rapidly, and various electronic products at this stage are gradually entering the field of vision to many researchers in terms of smaller volume, faster computing speed and lower energy consumption. Unlike electrons, photons have specific physical properties such as polarization and frequency, and their speed is similar to the speed of light. For photons at rest, they are not affected by gravity, they will not have any effect on each other and the intersection of light rays will not interfere with the photon. Because of these properties, the photonic crystal [4-6] structure has advantages over electronic-based semiconductor materials. In 1987, Yablonovitch [7] and John [8] respectively gave their own understandings of photonic crystals. The advantage of photonic crystals is that they are small and easy to integrate, their wavelength can be selected, and the bandgap frequency can be changed according to changing material parameters. The photonic crystals with various defect states are ideal for designing optical components such as filters and waveguides.\u003c/p\u003e\n\u003cp\u003eIn 2003, Suh et al. [9] designed a mechanically modulated optical filter between two photonic crystal plates based on waveguide resonance. Drysdale et al. [10] developed a tunable photonic crystal filter, which can achieve frequency tunability in the range of 3.5GHz. In 2004, Nemec et al. [11] designed a one-dimensional photonic crystal filter with heat regulation in the terahertz range, which greatly increased the tunable range to 20% tunable type. The photonic crystal filter proposed by S.Upta et al. [12] can filter the infrared band to the low frequency range. Takes filtering capability to a new level.\u003c/p\u003e\n\u003cp\u003eCompared with inorganic material filters, Hu et al. [13] produced organic material photonic crystal filters in 2005. The matrix was selected as polystyrene film, and then a particle beam was used to etch regular cylindrical pores on its surface, and a two-dimensional photonic crystal filter was formed by introducing line defects in the middle. On the experimental surface, the quality factor of the filter reaches 230, the peak transmittance reaches 90%, and the optical passband width is 2.4nm. In addition, these physical parameters can also be adjusted according to the defect width of the mediation line. These performance compared to inorganic photonic crystal filters achieve a very high strength improvement.\u003c/p\u003e\n\u003cp\u003eTo confine electromagnetic wave, some researchers [14, 15] remove a row of 3 holes from a hexagonal hole-in-slab photonic crystal (known as an L3 defect), a perturbation is introduced to the lattice which allows light to be confined to a cavity, The L3 cavity design, which can be optimized by shifting the radii and positions of neighbouring holes, is extremely popular in electromagnetic wave propagation experiments and has been featured in many high impact publications. electromagnetic wave is confined in the plane of the PhC by the photonic band gap and in the \u003cem\u003ez\u003c/em\u003e-direction by index guiding. In a similar manner, it is also possible to create a waveguide in the PhC by introducing a linear defect. A common method of achieving this in a hole-in-slab PhC is to remove a row of holes [16]. The created linear defect can then support waveguide modes propagating along the \u003cem\u003ex\u003c/em\u003e-direction -confined to the slab by index guiding and directed in-plane by the photonic band gap.\u003c/p\u003e\n\u003cp\u003eWaveguides formed from linear defects in PhC slabs are frequently used for on-chip control of electromagnetic wave [17-20]. Most applications involve either guiding the light between components, or delaying the light by trapping it or slowing its group velocity [21] - crucial operations for the realisation of on-chip all-optical circuits. PhC waveguides can be optically coupled to defect cavity modes in the same PhC [22], allowing light to be coupled in or out of the cavity mode via the waveguide one potential use of this is for trapping and delaying of photons for optical processing operations. It is possible to fabricate PhC waveguides with very low propagation losses: for example, losses as low as \u0026sim; 2 dBcm-1 have been achieved in Si-based slabs [23]. Therefore, the waveguides can be used to efficiently couple light into or out of PhC cavities (or components of interest) from a considerable distance, either through the use of grating couplers placed at the end of the waveguide [24, 25] (which are illuminated with a laser spot or collected from using a microscope objective above the sample) or via coupling the end of the waveguide to an optical fibre [26].\u003c/p\u003e\n\u003cp\u003eIn this paper, we write a SU-8 photoresist strip on photonic crystals to confine electromagnetic wave, the relationship of the \u003cem\u003eQ\u003c/em\u003e factor with the strip width of the SU-8 strip cavity are also investigated. It provides a way to optimize the \u003cem\u003eQ\u003c/em\u003e factor and hence enables potential improvement on the optical cavity mode. The design is more practical because it can control wave propagation and frequency bandgap without altering the photonic crystal structure. Through our research, The \u003cem\u003eQ\u003c/em\u003e factor can increase as the width of strip increases.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe PhC slab is formed by air holes etched in the \u003cem\u003ez\u003c/em\u003e-direction with a periodic hexagonal lattice pattern in the \u003cem\u003exy\u003c/em\u003e-plane (directions as defined in figure 1a). Finite-difference time-domain (FDTD) simulations were performed to model the PhC slab, using a lattice constant a = 340 nm, hole radius of 0.27a and a slab thickness of 200 nm. The refractive index of the slab is chosen to be \u003cem\u003en\u003c/em\u003e = 3.33 to match that of GaAs at cryogenic temperatures. The PhC exhibits an in-plane photonic band gap for TE-like modes. By introducing a row of missing holes along the \u003cem\u003ex\u003c/em\u003e- direction through the PhC (The SEM image is shown in (Figure 1.b) is formed which can support in-plane guided modes [27-28].\u003c/p\u003e\n\u003cp\u003eA mode gap cavity is created by modifying the local structure of the PhC waveguide to have a lower energy than the waveguide band edge as obtained with a deposited SU-8 film (figure 2). A local potential minimum is then introduced which leads to confinement of electromagnetic wave in a similar fashion to that seen for electronic quantum confinement in a quantum well. Adding a 0.1\u0026mu;m-thick layer of SU-8 with a refractive index of 1.57 on top of the PhC shifts the waveguide band lower in energy, from 0.269c/a to 0.266c/a as shown by the dotted black curve in figure 2c, the AFM image of a successful SU-8 strip cavity with a strip height \u003cem\u003eH\u003c/em\u003e = 0.1\u0026mu;m is shown in Figure 2b. This is equivalent to an increment in wavelength from 1.265 \u0026mu;m to 1.277 \u0026mu;m. This 12 nm band shift is sufficient to form an optical cavity to trap electromagnetic wave from the band edge when patterning a narrow strip of SU-8 across the PhC waveguide. This is represented schematically in figures 3a-c, for a 1\u0026mu;m-wide SU-8 strip positioned on a PhC waveguide. A strip width of 1\u0026mu;m is chosen to match the spot size of the laser used to write the SU-8 strip in the experiment. The simulated electric field intensity (|\u003cem\u003eE\u003c/em\u003e|\u003csup\u003e2\u003c/sup\u003e) profile of a cavity defined by a 1\u0026mu;m wide SU-8 strip is plotted in figure 3d through a central slice of the slab along the z-direction. The position of the SU-8 strip is represented by the region bounded by the dashed lines. A clear demonstration of electromagnetic wave restricted to the region of the strip is observed.\u003c/p\u003e\n\u003cp\u003eHaving successfully created an SU-8 strip cavity to realize confinement of electromagnetic wave, we now investigate the relationship of the \u003cem\u003eQ\u003c/em\u003e factor with the strip width of the SU-8 strip cavity. It provides a way of optimize the \u003cem\u003eQ\u003c/em\u003e factor and hence enables potential improvement on the optical cavity mode.\u003c/p\u003e\n\u003cp\u003eWe consider the effects of varying the strip width, \u003cem\u003ew\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e, while keeping the strip height \u003cem\u003eh\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e=100 nm constant. FDTD simulations were performed with \u003cem\u003ew\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e ranging from 0.5 \u0026micro;m to 6 \u0026micro;m; the parameters extracted from the fundamental cavity mode are presented in Fig. 4. Where \u003cem\u003eQ\u003c/em\u003e\u003csub\u003etotal\u003c/sub\u003e means the total \u003cem\u003eQ\u003c/em\u003e factor of the cavity mode, \u003cem\u003eQ\u003c/em\u003e\u003csub\u003ein\u003c/sub\u003e and \u003cem\u003eQ\u003c/em\u003e\u003csub\u003eout\u003c/sub\u003e represent how well the cavity confines light in and out of the plane of the PhC. As the strip width is increased beyond \u003cem\u003e~\u0026nbsp;\u003c/em\u003e2 \u0026micro;m, \u003cem\u003eQ\u003c/em\u003e\u003csub\u003etotal\u003c/sub\u003e increases by up to approximately an order of magnitude, while \u003cem\u003eV\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e increases by only a factor of \u003cem\u003e~\u003c/em\u003e2 over the range of \u003cem\u003ew\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e simulated. For confinement applications, the \u003cem\u003eQ\u003c/em\u003e\u003csub\u003etotal\u003c/sub\u003e/\u003cem\u003eV\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e ratio should be maximized. The results from Fig.4 confirm that, as long as the strip height is kept constant, it is possible to increase \u003cem\u003eQ\u003c/em\u003e\u003csub\u003etotal\u003c/sub\u003e/\u003cem\u003eV\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003esimply by increasing the width of the SU-8 strip. This suggests that cavities defined by a wider SU-8 strip could be preferable for confinement of electromagnetic wave.\u003c/p\u003e\n\u003cp\u003eBefore depositing the strip, the PhC is symmetric along the \u003cem\u003ez\u003c/em\u003e-direction. The structure supports both TE-like and TM-like modes. The two types of modes can be studied independently due to their orthogonal polarizations. However, structural modification from the SU-8 breaks the \u003cem\u003ez\u003c/em\u003e-symmetry and hence the two types of mode start to couple. This is a lossy process as the TM-like mode is allowed for propagation. The \u003cem\u003eQ\u003c/em\u003e\u003csub\u003etotal\u003c/sub\u003e of the SU-8 strip cavity mode, which is limited by in-plane TE-TM coupling losses\u003csup\u003e,\u003c/sup\u003e shows an increase with \u003cem\u003ew\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e that is reflected by an increase in \u003cem\u003eQ\u003c/em\u003e\u003csub\u003ein\u003c/sub\u003e. This suggests a reduction of the in-plane losses, which is understood by considering how the cavity mode field profile changes with \u003cem\u003ew\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eThe losses are more explicitly illustrated via Fourier analysis. In Fig.5, We consider the spatial Fourier transform of the main E field components of the TE-like cavity mode, |FT(\u003cem\u003eE\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e)+ FT(\u003cem\u003eE\u003csub\u003ey\u003c/sub\u003e\u003c/em\u003e)|, in the top plane of the slab. This is compared for the cavity with \u003cem\u003ew\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e=1\u0026micro;m, \u003cem\u003eh\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e=100 nm [Fig. 5(a)], the cavity with \u003cem\u003ew\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e=4 \u0026micro;m, \u003cem\u003eh\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e=100 nm [Fig. 5(b)]. The dominant source of losses from the SU-8 strip cavity is due to coupling of the TE-like cavity mode to TM-like slab modes, which occurs along the TM contours marked on the diagrams by the dashed lines. Such coupling is facilitated by the broken \u003cem\u003ez\u003c/em\u003e-symmetry of the structure and is clearly evident in 100 nm thick strip for the 1 \u0026micro;m-wide strip by the significant field components along the TM contour. However, for a 4 \u0026micro;m-wide strip of the same thickness, the field components along the TM contours are much weaker. This appears to be due to the increased localization of the Fourier components along \u003cem\u003ek\u003csub\u003ex\u003c/sub\u003e\u003c/em\u003e, which is a result of the decreased spatial localization of the mode along the waveguide. We propose that this inhibits TE-TM coupling, since the field components of the TE- like cavity mode overlap less with the TM contours. We therefore attribute the increase in \u003cem\u003eQ\u003c/em\u003e\u003csub\u003etotal\u003c/sub\u003e with \u003cem\u003ew\u003c/em\u003e\u003csub\u003estrip\u003c/sub\u003e to a reduction in TE-TM coupling losses, resulting from the more localized Fourier components of the mode, which is consistent with an increase in \u003cem\u003eQ\u003c/em\u003e\u003csub\u003ein\u003c/sub\u003e.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this paper, the SU-8 photoresist strip on photonic crystals is used to confine electromagnetic wave. Corresponding simulation shows that, bandgap of the waveguide shifts from 0.269c/a to 0.266c/a by Adding a 0.1\u0026mu;m-thick layer of SU-8 with a refractive index of 1.57 on top of the PhC. This is equivalent to an increment in wavelength from 1.265 \u0026mu;m to 1.277 \u0026mu;m. This 12 nm band shift is sufficient to form an optical cavity to trap\u0026nbsp;electromagnetic\u0026nbsp;wave from the band edge when patterning a narrow strip of SU-8 across the PhC waveguide.\u003c/p\u003e\n\u003cp\u003eThe relationship of the \u003cem\u003eQ\u003c/em\u003e factor with the strip width of the SU-8 strip cavity is also investigated. It provides a way to optimize the \u003cem\u003eQ\u003c/em\u003e factor and hence enables potential improvement on\u0026nbsp;the optical cavity mode. Our study shows that as the strip width is increased beyond ~ 2 \u0026micro;m, \u003cem\u003eQ\u003csub\u003etotal\u003c/sub\u003e\u003c/em\u003e increases by up to approximately an order of magnitude, while V\u003csub\u003e0\u003c/sub\u003e increases by only a factor of ~2, so \u003cem\u003eQ\u003c/em\u003e\u003csub\u003etotal\u003c/sub\u003e/\u003cem\u003eV\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e ratio can be maximized. This suggests that cavities defined by a wider SU-8 strip could be preferable for confinement of electromagnetic wave. Using SU-8 photoresist strip on photonic crystals to confine electromagnetic wave is more practical and flexible, the method in our paper provide new significance for research into optical filter.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eNotes\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing financial interest.\u003c/p\u003e\n\u003cp\u003eData availability\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe datasets used and/or analysed during the current study available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003eAuthors Contributions\u003c/p\u003e\n\u003cp\u003eHanbo Shao developed the algorithm, constructed the structure, carried out the simulation and drafted the manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eXiaochen Hang revised the manuscript and provided relevant literature experience.\u003c/p\u003e\n\u003cp\u003eDong Jiang participated in the design of the study and provided financial support.\u003c/p\u003e\n\u003cp\u003eAll authors have read and approved the final manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMaruf, A.: Rubayet. 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Express. \u003cb\u003e19\u003c/b\u003e, 5398\u0026ndash;5409 (2011)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"SU-8 photoresist, photonic crystal, electromagnetic wave, filters","lastPublishedDoi":"10.21203/rs.3.rs-4905487/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4905487/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eConfinement of electromagnetic wave is key to the realization of applicable filters in optics technologies. We present a flexible cavity fabrication process that writes SU-8 photoresist microstrip onto a photonic crystal waveguide deterministically, in which electromagnetic wave can be confined excellently. Corresponding simulation shows bandgap of the waveguide shifts from 0.269c/a to 0.266c/a by adding a 0.1µm-thick layer of SU-8 with a refractive index of 1.57 on top of PhC. This is equivalent to an increment in wavelength from 1.265 µm to 1.277 µm. We also study the relationship of the \u003cem\u003eQ\u003c/em\u003e factor with the strip width of SU-8 strip cavity. Tesults show that as the strip width is increased beyond 2 µm, \u003cem\u003eQ\u003c/em\u003e\u003csub\u003e\u003cem\u003etotal\u003c/em\u003e\u003c/sub\u003e increases by up to approximately an order of magnitude, while V\u003csub\u003e0\u003c/sub\u003e increases by only a factor of 2. It provides a way to optimize \u003cem\u003eQ\u003c/em\u003e factor and hence enables potential improvement on optical cavity mode.\u003c/p\u003e","manuscriptTitle":"Confinement and optimization of electromagnetic wave in photonic crystals based on SU-8 photoresist strip","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-12 09:00:49","doi":"10.21203/rs.3.rs-4905487/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"14d982f8-f6a7-4aad-b1e2-c23531262f94","owner":[],"postedDate":"September 12th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-11-16T15:23:41+00:00","versionOfRecord":[],"versionCreatedAt":"2024-09-12 09:00:49","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4905487","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4905487","identity":"rs-4905487","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}