Study of the photothermal response of a multilayer structure doped with VO2@Au nanoshells

Abstract

In this paper, we demonstrate a theoretical study of a multiphysics problem to solve for the photothermal response of a one-dimensional multilayer structure containing a layer doped with VO2@Au nanoshells. The VO2@Au nanoshell consists of a gold (Au) shell and a core of the phase change material vanadium dioxide (VO2) where the VO2 core transitions from a semiconductor state to a conductor state at the critical temperature of 68 °C. This behaviour results in thermal induced optical tunability through this reversible phase change of the VO2, due to the temperature dependent optical and thermal properties. The presence of the VO2 core, functioning as an ultra-fast and reversible optical phase-change material, leads to the emergence of photothermal induced bistability. The layer doped with the VO2@Au nanoshell is approximated as an effective medium using the Maxwell-Garnett Theory to enable an analytical solution. In this study, the optical response of the multilayer structure is obtained using the Transfer Matrix Method, while the thermal response for both stationary and transient states is solved using the Green’s Function Method and Kirchhoff’s Transformation. These equations are interconnected through the heat source term in the heat diffusion equations, representing the local heat generation induced by the continuous-wave laser applied to the structure. Our findings indicate that at the wavelengths of 658 nm and 747 nm, there are two distinct photothermal responses arising from the phase change of the VO2 core. At these wavelengths, the absorption of light increases and decreases, respectively, because of the VO2 phase change. This analytical method not only offers a thorough exploration of the fundamentals of induced photothermal responses in multilayer structures but also holds considerable potential for various applications, including solar cells, photothermal therapy, and nanothermal sensors.