Sensing infrared radiation is done inexpensively with pyroelectric detectors that generate a temporary voltage when they are heated by the incident infrared radiation. Unfortunately the performance of these detectors deteriorates for longer wavelengths, leaving the detection of, for instance, millimetre-wave radiation to expensive approaches. We propose here a simple and effective method to enhance pyroelectric detection of the millimetre-wave radiation by combining a compact commercial infrared pyro-sensor with a metasurface-enabled ultra-thin absorber, which provides spectrally- and polarization-discriminated response and is 136 times thinner than the operating wavelength. It is demonstrated that, due to the small thickness and therefore the thermal capacity of the absorber, the detector keeps the high response speed and sensitivity to millimetre waves as the original infrared pyro-sensor does against the regime of infrared detection. An in-depth electromagnetic analysis of the ultra-thin resonant absorbers along with their complex characterization by a BWO-spectroscopy technique is presented. Built upon this initial study, integrated metasurface absorber pyroelectric sensors are implemented and tested experimentally, showing high sensitivity and very fast response to millimetre-wave radiation. The proposed approach paves the way for creating highly-efficient inexpensive compact sensors for spectro-polarimetric applications in the millimetre-wave and terahertz bands.
Abstract-A multispectral 24 × 24 bolometric matrix structure of terahertz (THz) absorbers operating at 0.3-0.4 THz was proposed and experimentally investigated. Each pixel of the structure was implemented as a fragment of an ultra-thin metamaterial absorber. The matrix structure consisted of four types of pixels with nearly perfect absorptivity. Three pixels were at 0.30, 0.33, 0.36 THz respectively with identically oriented polarization sensitivity, and the fourth pixel was at 0.33 THz oriented with polarization sensitivity orthogonal to foregoing ones. The backside of the structure included a high-performance infrared emissive layer. Resonant absorption of THz radiation induced the structure heating and increasing IR emission from the emissive layer, which was henceforth detected by the IR camera. The terahertz imaging system, capable to operate in real time, with spectral and polarization discrimination was demonstrated. The experimental results showed good spectral and polarization resolution together with acceptable spatial resolution.
The quasi-optical bolometric converter of terahertz (THz) waves into infrared (IR) radiation is proposed and experimentally investigated. The converter includes an ultra-thin THz absorber (with a thickness 1/50 of the operating free-space wavelength) based on an artificial impedance surface with close to perfect resonant absorptivity at 0.3 THz and a high-performance IR emissive layer. Absorption of THz waves induces converter heating that yields enhancement of IR emission from the emissive layer. The experimental testing of the THz-to-IR converter demonstrates the applicability of a converter to THz imaging with spectral and polarization discrimination in real-time operation.
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