Bias-selective dual-band charge-sensitive infrared phototransistor with partially mode-decoupled cross-hole plasmonic coupling

High-sensitivity, bias-selective dual-band infrared detection is essential for two-color absolute thermometry of mid- to long-wavelength infrared sources, including the picowatt-class signal regime relevant to cryogenic near-field thermal measurements. Here, we report a triple-quantum-well charge-sensitive infrared phototransistor in which two spatially isolated floating gates provide electrically separated photoresponse pathways, allowing the 10.1 and 16.4 μm channels to be addressed individually or jointly by gate bias. The cross-hole plasmonic grating supports two geometrically distinguishable resonances with different dominant tuning sensitivities: a period-dominated Rayleigh anomaly–surface plasmon polaritons (SPP) mode and an arm-length-dominated localized shape plasmon–SPP hybrid mode. This partially mode-decoupled response reduces spectral entanglement and enables practical co-design of the two infrared bands. At an optimized well doping of 8.0 × 1017 cm−3, the device delivers an integration responsivity of 6.17 × 105 A W−1 at VSD = 10 mV under a 4.24 pW, 300 K background; the band-resolved photocurrent responsivities at VSD = 120 mV are 1.35 × 103 A W−1 at 10.1 μm and 6.07 × 103 A W−1 at 16.4 μm. Quantum efficiencies of 19.4% (10.1 μm) and 16.0% (16.4 μm) yield specific detectivities of 8.5 × 1011 and 1.1 × 1012 cm Hz1/2 W−1, respectively, with a balanced (∼1:1) dual-band spectral response suitable for two-color pyrometry. The architecture establishes a route to high-gain dual-band mid- to long-wavelength infrared sensing for cryogenic two-color thermometry.