Tuning of Optical Phonons in α-MoO₃–VO₂ Multilayers

Merging the properties of VO₂ and van der Waals (vdW) materials has given rise to novel tunable photonic devices. Despite recent studies on the effect of the phase change of VO₂ on tuning near-field optical response of phonon polaritons in the infrared range, active tuning of optical phonons (OPhs) using far-field techniques has been scarce. Here, we investigate the tunability of OPhs of α-MoO₃ in a multilayer structure with VO₂. Our experiments show the frequency and intensity tuning of 2 cm^-1 and 11% for OPhs in the [100] direction and 2 cm^-1 and 28% for OPhs in the [010] crystal direction of α-MoO₃. Using the effective medium theory and dielectric models of each layer, we verify these findings with simulations. We then use loss tangent analysis and remove the effect of the substrate to understand the origin of these spectral characteristics. We expect that these findings will assist in intelligently designing tunable photonic devices for infrared applications, such as tunable camouflage and radiative cooling devices.