Photoelasticity of VO2 nanolayers in insulating and metallic phases studied by picosecond ultrasonics

We use a picosecond ultrasonic technique to evaluate the photoelastic parameters at the wavelength of 1028 nm in epitaxial vanadium dioxide (VO2) nanolayers grown on c-cut sapphire substrates. In the experiments, we monitor the picosecond evolution of the reflectivity of VO2 in insulating and metallic phases under the impact of a picosecond longitudinal strain pulse injected into the nanolayer from the side of the substrate. We show that in a 145-nm-thick granular nanolayer, the temporal features of the reflectivity are clearly dependent on the phase state of VO2, showing the change of the photoelastic parameters upon the insulator-metal transition. Analytical consideration and numerical simulations of the optical response to the picosecond strain pulse show that the temporal evolution of the reflectivity strongly depends on the complex photoelastic parameter. The analysis enables us to obtain the values of the photoelastic parameters in the studied nanolayer in both insulating and metallic phases. We find that for a 145-nm film of VO2 in an insulating state the imaginary part of the photoelastic constant is negligible. This means that in the insulating phase the strain does not affect the optical absorption of VO2. In the metallic phase, the photoelastic parameter of VO2 is found to be similar to that typical for metals with positive real and negative imaginary parts. We further show that the optical response to the strain pulse in the layer consisting of disconnected VO2 nanohillocks with an average height of 70 nm is governed by their morphology and is different from what is predicted in the plane VO2 films.