Polariton probing of attometer displacement and nanoscale strain in ultrashort acoustic pulses

Abstract Atomic displacement and lattice strain are inextricably linked in most ultrafast processes in solids, such as in optically induced phase transition [1,2] or demagnetization [3]. The visualization of lattice dynamics with femtosecond temporal and sub-angstrom spatial resolution, typically performed using X-ray [1,3-5] and electron diffraction techniques [2,5,6], yields information about the involved physics. However, the detection of tiny atomic motion localized in a nanometer volume has remained challenging. For this purpose, we suggest focussing on the acoustic pulse injected into the surrounding volume [3,7-10]. We demonstrate that its optical detection in a material possessing a narrow polariton resonance provides superior sensitivity and enables the detection of atomic motion of attometer amplitudes. In the performed experiment, we detect an acoustic pulse injected into the polaritonic layer from an optically heated 100-nm metal film. By measuring the transient reflectivity modulated by the dynamical strain, we reliably detect the thermal expansion of the film by less than 100 attometers due to its temperature increase by ~0.1 K only.