Low aspect ratio nanophotonic filled cavities with Q-matching for scalable thermophotovoltaic power conversion

Selective thermal emitters with a metallic 2D photonic crystal structure, such as shown in Fig. 1a), have been demonstrated to control the thermal emission spectrum of selective emitters for thermophotovoltaic applications. Traditionally, tungsten has been the metal of choice for thermal emitters due to their natural thermal emission selectivity to wavelengths > 2μm and high melting temperatures. However, due to the unweldable nature of tungsten, system integration of tungsten is difficult. As a result, weldable tantalum nanophotonic cavities have been demonstrated as effective thermal emitters with long wavelength emissivity below that of tungsten, allowing for improved selectivity performance compared to tungsten. However, the aspect ratios (AR) of the tantalum cavities have been demonstrated with a depth to radius aspect ratio of 11.2. The high aspect ratios in relation to tungsten (AR = 2.9) are due to the smaller material absorption at optical wavelengths around 2 μm, which causes Q_abs per unit length to be reduced thus affecting the Q matching condition. The aspect ratio is determined based on Q-matching, where the radiative Q (Q_rad) and absorption Q (Q_abs) of the cavity must be equal for maximum absorption. The high cavity aspect ratio significantly hinders the fabrication methods available to manufacture the tantalum devices. For example, because of the high aspect ratio cavities, low-cost and scalable fabrication methods such as surface deposition of tantalum via sputtering or evaporation are not possible due to poor step coverage. Thus, previous methods used expensive and difficult to prepare 2" diameter tantalum wafers with custom etching methods. Accordingly, low aspect ratio cavities are key for a low-cost and scalable deployment of thermophotovoltaic devices.