Abstract
Abstract 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 NO2 (6.25 5m) and the other to a non-absorbing reference wavelength (7.15 5m), NDIR NO2 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.
Highlights
Non-dispersive infrared (NDIR) sensing, widely applied in industrial diagnostics and process control, measures gas concentrations based on mid-infrared absorption lines unique to molecules [1, 2]
Two Metal– dielectric–metal (MDM) cavities sandwiching a quantum well infrared photodetector (QWIP) with distinct resonance wavelengths are alternately arranged in a subwavelength period
The QWIP layer used in this study consists of a single 4 nm GaAs quantum well layer sandwiched by two 50 nm Al0.3Ga0.7As barrier layers and two 48 nm GaAs contact layers [21, 25, 32], a total thickness T = 200 nm, which exhibits a photoconductive responsivity peak at 6.7 μm [21, 25]
Summary
Non-dispersive infrared (NDIR) sensing, widely applied in industrial diagnostics and process control, measures gas concentrations based on mid-infrared absorption lines unique to molecules [1, 2]. In conventional NDIR, broadband emission traverses through the gas specimen and two wavelengths, a target gas absorption line and a nonabsorbing reference, are picked out by bulky, spatially separated narrowband filters and passed to two wideband detectors for transmission measurement [1, 2]. Metal–dielectric–metal (MDM) metasurface plasmon cavities, precisely controllable narrowband absorbers [5, 6] (Δλ/λpeak ≤ 0.1, where Δλ is the full-width half maximum and λpeak is the peak wavelength of the cavity), offer a simpler NDIR approach [7] without external filters; vertically stacked MDM cavities can exhibit near-perfect absorption of normally incident light via magnetic coupling [5].
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