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Detection of terahertz (THz) radiation with high sensitivity and efficiency remains a challenging problem for THz technology development. A photoconductive THz detector based on the Auston switch concept is among the most sensitive and widely-used room-temperature coherent THz detectors. Plasmonic nano-antennas and gratings, and ultrathin optical cavities recently were introduced in the structure of THz optoelectronic devices to improve their efficiencies. It allowed obtaining higher photocurrents and smaller capacitance, which lead to improvements in the response speed and efficiency of THz detectors.
Plasmonic nanostructures however introduce Ohmic losses, which can substantially reduce the responsivity of THz detectors. Alternatively, efficiency of photoconductive THz detectors can be enhanced by using all-dielectric metasurfaces. A metasurface containing only dielectric, instead of plasmonic nanostructures, can be designed to trap the incident light in a selected spectral range and thus enhance optical absorption.
We designed an optically-thin photoconductive channel as an all-dielectric metasurface, which exhibited enhanced optical absorption at laser excitation wavelength. The metasurface comprised an array of low-temperature grown GaAs nanobeams and a sub-surface distributed Bragg reflector. Integrated into a photoconductive (Auston) switch, the metasurface improved the efficiency and sensitivity of the THz detector. Specifically, the detector produced photocurrents over one order of magnitude higher compared to a similar detector with unstructured surface with only 0.5 mW of optical excitation, while exhibiting high dark resistance required for low-noise detection in THz time-domain spectroscopy and imaging. At that level of optical excitation the metasurface detector showed a high signal to noise ratio of 10^6. We will discuss mechanisms responsible for the efficiency improvement, as well as the application of THz detectors with all-dielectric metasurfaces for enabling more sensitive integrated THz near-field probes.
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