No abstract

Thermal radiation transfer between two objects separated by a subwavelength gap (near-field thermal radiation transfer) can be orders of magnitude larger than that in free space, which is attracting increasing attention with respect to both fundamental nanoscience and its potential for high-power-density and high-efficiency conversion of heat to electricity in thermophotovoltaic (TPV) systems. However, the realization of near-field thermal radiation transfer in TPV systems involves significant challenges because it requires a subwavelength gap and large temperature difference between the emitter and the PV cell while minimizing the heat transfer that does not contribute to the photocurrent generation. To overcome these challenges, here we demonstrate a one-chip near-field TPV device consisting of a thin-film Si emitter and InGaAs PV cell with an intermediate Si substrate, which enables the suppression of the heat transfer due to sub-bandgap radiation by free carriers and surface modes. Through the one-chip integration and thermal isolation using Si process technologies, we realize a deep subwavelength gap (<150 nm) between the emitter and the intermediate substrate without using any external positioners while maintaining a large temperature difference (>700 K). Compared to the equivalent device operating in the far-field regime, we achieve 10-fold enhancement of the photocurrent in the PV cell without degrading the open-circuit voltage and fill factor, demonstrating the potential of our onechip device for the future applications of near-field thermal radiation transfer.

Silicon carbide (SiC) is a promising optical material for stable and broadband nanophotonics. To date, thin crystalline SiC layers for nanophotonic platforms have been created by ion implantation or growth on other materials, which may cause optical absorption in the SiC layer. We fabricated SiC nanobeam photonic crystal cavities directly from a crystalline (4H) SiC bulk wafer using oblique plasma etching to avoid material-based optical absorptions. The measured quality (Q) factor of the nanobeam photonic crystal cavity reaches 4 × 104, which is the highest recorded Q factor in crystalline SiC cavities. Furthermore, we investigated theoretical Q factors by taking into account structural imperfections unique to this fabrication process and compared them with the experimental results.

Narrowband thermal emitters operating in the mid-wavelength infrared (MWIR, 3−8 μm) are important for spectroscopic sensing systems in various fields of research such as chemistry, healthcare, and environmental science. To increase the signal-to-noise ratio in these spectroscopic applications, it is required that only thermal emission in a narrow target wavelength range be modulated electrically while other wavelength components are unmodulated. In addition, an increase of the emitter's temperature is highly desired for high-power operation in the MWIR. To date, a number of efforts have been put into the demonstration of electrical modulation of thermal emission by using semiconductors, phase-change materials, and graphene. However, these emitters have not achieved selected modulation of narrowband thermal emission in the MWIR at high temperatures. Here, we demonstrate the electrical modulation of a narrowband MWIR thermal emission at high temperatures of up to 500 °C using GaN/AlGaN multiple quantum well (MQW) photonic crystals. Our emitter exhibits a narrowband thermal emission (Q = 40) owing to the combination of intersubband absorption in the MQWs and optical resonances of the photonic crystals, the intensity of which can be electrically modulated at high speed (50 kHz) through the control of the electron density in the MQWs. Our demonstration of electrical modulation of MWIR narrowband thermal emitters at high temperature will accelerate the practical use of narrowband thermal emitters in various spectroscopy applications such as optical gas sensors, including CO 2 sensors.