Subwavelength 3D terahertz imaging with a single-pixel laser transceiver

Terahertz imaging holds great potential for non-destructive material inspection, but practical implementation has been limited by resolution constraints. In this study, we present a single-pixel THz imaging system based on a confocal microscope architecture, utilizing a quantum cascade laser as both transmitter and phase-sensitive receiver. We demonstrate, for the first time to the best of our knowledge, that laser feedback interferometry-based imaging systems achieve enhanced lateral and axial resolution compared to conventional confocal imaging. Specifically, our approach yields a twofold improvement in lateral resolution, reaching λ/2, and a two-order-of-magnitude enhancement in axial resolution, from 25λ to beyond λ/5, through interferometric phase detection. The system can produce a 0.5 megapixel image in under three minutes, surpassing both raster-scanning single-pixel and multipixel focal-plane array-based imagers. Coherent operation enables simultaneous amplitude and phase image acquisition, and a custom visualization method links amplitude to image saturation and phase to hue, enhancing material characterization. A 3D tomographic analysis of a silicon chip reveals subwavelength features, demonstrating the system's potential for high-resolution THz imaging and material analysis. This work overcomes the resolution limits of conventional lens-based imaging systems, by enabling rapid, high-fidelity imaging of subwavelength features beyond the diffraction limit.