Controlling the Sign and Magnitude of the Thermoreflectance Coefficient of Gold-MoS$_2$ Metamaterials

Abstract

Heating a material changes how much light it reflects. This effect, quantified by the thermoreflectance coefficient $C_\mathrm{TR}$, enables temperature sensing, while it is a nuisance for situations where constant reflection is crucial. Whereas temperature detection requires a large $C_\mathrm{TR}$, the latter situation demands it to be as close to zero as possible. Controlling the $C_\mathrm{TR}$ has so far relied on the selection of materials and wavelengths, rather than engineering the coefficient itself. This work demonstrates that the $C_\mathrm{TR}$ of a 50 nm thin gold film on SiO$_2$/Si can be controlled in sign and magnitude by an ultrathin ($<$8 nm) MoS$_2$ film underneath it. Time-resolved thermoreflectance measurements reveal suppression, sign inversion, and revival of the thermoreflectance signal, controlled by three knobs: ($i$) the buried MoS$_2$ thickness, ($ii$) probe wavelength, and ($iii$) ambient temperature. Transfer-matrix simulations using temperature-dependent dielectric functions of gold and MoS$_2$ reproduce all experimental observations, demonstrating an optothermal metamaterial platform with controllable properties. In particular, these (anti-)thermoreflective metamaterial mirrors bring $C_\mathrm{TR}$ under the same interference design rules previously applied to the optical response itself, opening ways to maximize or minimize the temperature dependence of the optical reflectance, tailored to the specific application.