Metasurface‐Enhanced Thermal Photodetector Operating at Gigahertz Frequencies

Abstract Thermal photodetectors, including thermoelectric and pyroelectric detectors, are critical for the detection of long‐wave infrared light, but they are limited by slow response times due to thermal diffusion. If thermal photodetectors with high speeds and sensitivities can be realized, they would allow for flexible sensing at any wavelength, as all selectivity of the absorber would be reproduced in the detectivity. Here, a room‐temperature, pyroelectric photodetector is reported that couples a thermally‐sensitive aluminum nitride (AlN) layer with a metallic metasurface that acts as an efficient, spectrally‐selective photothermal converter. These pyroelectric detectors exhibit record‐breaking speeds, with 3 dB bandwidths up to 2.8 GHz, which corresponds to a rise time of 125 ps. These ultrafast speeds are attained while maintaining competitive responsivities and noise equivalent powers as small as 96 pW √Hz −1 . By varying the active area of devices, it is determined that even the smallest attainable devices are limited by their resistance‐capacitance (RC) time constants, and finite element simulations suggest that thermal response times as fast as 30 ps may be realized. These metasurface‐enabled pyroelectric photodetectors approach the speeds of commercial semiconductor photodiodes, contrary to the common conception of thermal detectors as slow, and present opportunities for advanced sensing such as spectrally broad multispectral imaging or polarimetry.