Microwave Monitoring by Compact Carbon Nanotube Photo-Thermoelectric Sensors Beyond the Diffraction Limit Toward Ultrabroadband Non-Destructive Inspections

Abstract

Electromagnetic-wave (EMW) sensing in microwave (MW) frequencies exhibits permeability even to deeper positions of various non-metallic materials (indispensable for social products) and potentially facilitates non-destructive inspections. However, conventional MW-sensor designs generally have faced difficulties in miniaturizations for longer wavelengths and the subsequent diffraction limit. While EMW sensors essentially require pixel miniaturizations for imaging, implementations of typical external antennas concentrating MW-irradiation into smaller areas than the diffraction limit fatally complicate overall fabrications and operations. Herein, this work demonstrates that carbon nanotube (CNT) film photo-thermoelectric (PTE) sensors sufficiently handle even MW-irradiation in compact configurations beyond the diffraction limit by themselves while maintaining inherent operations in shorter-wavelength millimeter-wave–infrared bands. The CNT film PTE sensors enhance MW-detection responses with particular channel dimensions (shorter length and narrower width), demonstrating a signal-to-noise ratio of 1497 with a 1-mm-square planar structure under 5 GHz irradiation (one-sixtieth size of the wavelength). In such advantageous behaviors, this work experimentally clarifies that electrically conductive wiring of the CNT film PTE sensor (inherently included within pristine device structures as response signal readout electrodes) plays a key antenna-like role. Then, the presenting devices demonstrate composition-identifying non-destructive testing of complex targets with multiple-wavelength imaging in ultrabroad MW–near-infrared bands, while compensating characteristics in respective irradiation regions.