Enhanced selective thermal emission with a meta-mirror following Generalized Snell’s Law

Thermal emission plays a critical role in a wide variety of applications, including adjusting radiative losses in photovoltaics and selective solar absorbers, as well as enhancing the emission of high energy photons for thermophotovoltaics and photon-enhanced thermionic emission. In this work, we consider the benefit to thermal emission associated with replacing conventional mirrors with meta-mirrors following Generalized Snell’s Law. By reflecting light at a different angle than incident, they can couple internally guided thermal radiation modes to the escape cone, ideally starting from any internally-guided angle. We illustrate the concept with two metamirror structures: a graded index material and a xylophone structure. Even without optimization, angle-averaged selective thermal emission is significantly enhanced compared to the planar case at selected wavelengths. Furthermore, the central wavelength and bandwidth of the enhancement can be matched with the requirements of each application. INTRODUCTION The selective enhancement of radiative thermal emission is an emerging scientific theme in a wide variety of applications, including photovoltaics (PV), selective solar absorbers (SSA), thermophotovoltaics (TPV), and photon-enhanced thermionic emission (PETE), as shown in Fig. 1. For instance, photovoltaic cells could benefit from radiative cooling [1,2], particularly in indirect bandgap materials such as crystalline silicon [3]. Selective solar absorbers benefit from the simultaneous ability to emit little in the mid-infrared as they heat up, while trapping solar wavelengths in the visible and near-IR [4,5]. In terms of selective thermal emission, both TPV [6] and PETE [7] benefit from strong emission above a certain energy. While the efficiency of solid state energy conversion has reached 23% at 970 °C for TPV [8] and 2% at 120 °C for PETE [7], researchers have yet to realize the full potential of these technologies, which is more than twice as high. The efficiencies of PV, SSA, TPV, and PETE are all dependent on the precise operating temperatures of both the photon sources and receivers, as well as the wavelengths of light exchanged. Certain wavelengths, generally close to 357