The surface conductivity of (111)-and (100)-oriented hydrogen-terminated diamonds was investigated at low temperatures for different carrier densities. The carrier density was controlled in a wide range in an electric doublelayer transistor configuration using ionic liquids. As the carrier density was increased, the temperature dependences of sheet resistance and mobility changed from semiconducting to metallic ones: the sheet resistance and mobility for the (111) surface were nearly independent of temperature for a sheet carrier density of %4 Â 10 13 cm À2 , indicating metallic carrier transport. It was also found that the interface capacitance, determined from the gate voltage dependence of the Hall carrier density, depended significantly on the crystal orientation.

Optical patch antennas sandwiching dielectrics between metal layers have been used as deep subwavelength building blocks of metasurfaces for perfect absorbers and thermal emitters. However, for applications of these metasurfaces for optoelectronic devices, wiring to each electrically isolated antenna is indispensable for biasing and current flow. Here we show that geometrically engineered metallic wires interconnecting the antennas can function to synchronize the optical phases for promoting coherent resonance, not only as electrical conductors. Antennas connected with optimally folded wires are applied to intersubband infrared photodetectors with a single 4-nm-thick quantum well, and a polarization-independent external quantum efficiency as high as 61% (responsivity 3.3 A W −1 , peak wavelength 6.7 μm) at 78 K, even extending to room temperature, is demonstrated. Applications of synchronously wired antennas are not limited to photodetectors, but are expected to serve as a fundamental architecture of arrayed subwavelength resonators for optoelectronic devices such as emitters and modulators.

AbstractThe integration of quantum well infrared photodetectors with plasmonic cavities has allowed for demonstration of sensitive photodetectors in the mid-infrared up to room-temperature operating conditions. However, clear guidelines for optimizing device structure for these detectors have not been developed. Using simple stripe cavity detectors as a model system, we clarify the fundamental factors that improve photodetector performance. By etching semiconductor material between the stripes, the cavity resonance wavelength was expected to blue-shift, and the electric field was predicted to strongly increase, resulting in higher responsivity than unetched stripe detectors. Contrary to our predictions, etched stripe detectors showed lower responsivities, indicating surface effects at the sidewalls and reduced absorption. Nevertheless, etching led to higher detectivity due to significantly reduced detector dark current. These results suggest that etched structures are the superior photodetector design, and that appropriate sidewall surface treatments could further improve device performance. Finally, through polarization and incidence angle dependence measurements of the stripe detectors, we clarify how the design of previously demonstrated wired patch antennas led to improved device performance. These results are widely applicable for cavity designs over a broad range of wavelengths within the infrared, and can serve as a roadmap for improving next-generation infrared photodetectors.

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Two photodetectors for measuring transmission and two bulky, separated narrowband filters for picking a target gas absorption line and a non-absorbing reference from broadband emission are typically required for dual-band non-dispersive infrared (NDIR) gas sensing. Metal-dielectric-metal (MDM) metasurface plasmon cavities, precisely controllable narrowband absorbers, suggest a next-generation, nanophotonic approach. Here, we demonstrate a dual-band MDM cavity detector that consolidates the function of two detectors and two filters into a single device by employing resonant photon sorting-a function unique to metasurfaces. Two MDM cavities sandwiching a quantum well infrared photodetector (QWIP) with distinct resonance wavelengths are alternately arranged in a subwavelength period. The large absorption cross section of the cavities ensures ~95% efficient lateral sorting of photons by wavelength into the corresponding detector within a near-field region. The flow of incident photons is thus converted into two independent photocurrents for dual-band detection. Our dual-band photodetectors show competitive external quantum efficiencies up to 38% (responsivity 2.1 A/W, peak wavelength 6.9 5m) at 78 K. By tailoring one resonance to an absorption peak of NO 2 (6.25 5m) and the other to a non-absorbing reference wavelength (7.15 5m), NDIR NO 2 gas sensing with 10 ppm accuracy and 1 ms response times is demonstrated. Through experiment and numerical simulation, we confirm near-perfect absorption at the resonant cavity and suppressed absorption at its non-resonant counterpart, characteristic of resonant photon sorting. Dual-band sensing across the mid-infrared should be possible by tailoring the cavities and quantum well to desired wavelengths.

Complex lightwave manipulation such as broadband absorption has been realized with metasurfaces based on laterally arranged metal-dielectric-metal cavities with different geometries. However, application of these metasurfaces for optoelectronic devices by incorporating functional dielectrics remains challenging. Here, we integrate a quantum well infrared photodetector (QWIP) with a metasurface made of a patchwork of square cavities with different dimensions arranged in a subwavelength unit cell. Our detector realizes wideband photoresponse approaching the entire responsivity spectrum of the QWIP—single-sized square cavities can utilize only 60% of the possible bandwidth—and external quantum efficiencies of up to 78% at 6.8 µm. Our highly flexible design scheme enables integration of photodetectors and metasurfaces with arbitrary arrangements of cavities selectively responding to incidence with a specific wavefront.

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