Photoinduced Multistable Resonance Frequency Switching of Phase Change Microstring at Room Temperature

Vanadium dioxide (VO₂), a promising phase change material, exhibits insulator to metal transition at 68 °C, manifests a drastic change in multiple physical properties, such as electrical resistance, mechanical modulus, lattice parameters, etc. From technological perspective, the transition temperature can be reduced by precise strain engineering. Here a noncontact, all-optical, and highly energy efficient platform is demonstrated to study macroscopic dynamics related to the localized structural rearrangements at room temperature. A thin layer (≈25 nm) of polycrystalline VO₂ deposited on a platinum coated silicon nitride microstring resonator shows a fast controlled mechanical resonance frequency response upon variations in optical power and wavelength. It is shown multiple stable frequencies of the resonator, designated as different equilibrium states, can be activated at different optical powers (≈200 µW) and wavelength, i.e., 450, 520, and 635 nm. The observed multiple resonance states of the microstring are explained because of the generation of stress due to the interplay between thermal expansion and the temperature-induced phase change of VO₂. It is believed this change in frequency states under the controlled external optical excitation can have potential applications in ultrafast optical switching, intelligent temperature sensors, and neuromorphic devices operated at room temperature.