Time‐Modulated Optomechanics Reveal Energy Flow and Inelastic Interactions Between Acoustic Phonons in Nanomembranes

An abrupt change in refractive index, known as a temporal interface, can cause temporal reflection, refraction, and diffraction of light, enabling the development of time‐varying photonic devices. Since time‐varying optics are based on ultrafast electronic excitations, the scattered light should be affected by electrons and phonons in nonequilibrium states. Here, we analyze the inelastically scattered light from ultrathin, time‐varying optical media and search for spectroscopic signatures of nonthermal phonon populations. Our approach targets hypersonic gigahertz (GHz) acoustic phonons, termed Lamb modes, confined in prototypical, submicron‐thick membranes: 260 nm‐thick bare silicon and bilayer structures with 50 nm‐thick gold layer on 200 nm‐thick silicon. Femtosecond laser irradiation of the membranes modulates their optical properties and excites nonthermal, coherent Lamb modes, which are probed via inelastic light scattering of a continuous‐wave laser beam. In metal‐semiconductor heterostructures, the anti‐Stokes signal of photoexcited Lamb modes undergoes blueshifting and an antiresonance‐to‐resonance temporal transition, which remains unobserved for Stokes scattering. This effect is attributed to inelastic interactions between thermoelastically excited acoustic phonons, resulting in an approximately 3.6% shift in the frequency of longitudinal acoustic phonons. Our work enables studies of heat transport, phonon dynamics, and optomechanics in time‐modulated systems, using principles from time‐varying optics.