Terahertz imaging using optically controlled Fourier-basis structured illumination

Abstract

We demonstrate that a new type of structured-illumination imaging may be migrated from the optical to the terahertz domain. This Fourier-basis technique involves illuminating a target with rapidly moving sinusoidal fringes of controllable spatial frequency and orientation, while measuring the scattered radiation on a single fast detector. This initial proof-of-concept demonstration is purely one-dimensional since the fringe orientation is fixed, but the technique is readily extensible to two dimensions. The fringes are first generated in the near-infrared (808 nm) by passing a high-power laser beam through an acousto-optic Bragg cell driven by a superposition of two RF signals slightly offset in frequency, blocking the undeflected beam, and refocusing the two diffracted beams onto a metal-backed semiconductor wafer. The laser can be amplitude modulated to slow down the moving fringes to accommodate the semiconductor’s temporal response. The semiconductor acts as an optically addressed spatiotemporal modulator for a THz beam illuminating the same area. The periodic optical fringes effectively transform the semiconductor into a reflective THz diffraction grating with a programmable period. The diffracted THz radiation is then imaged onto the remote target plane, where the diffraction orders interfere pairwise to create traveling THz fringes. Scattered radiation from the target is collected by a simple receiver operating in “light bucket” mode, which produces an output signal consisting of a superposition of sinusoidal tones, one for each spatial Fourier component of the target. We present measurements of the THz fringe projector’s performance and compare with a model of the semiconductor modulator’s operation. Finally, we present Fourier-reconstructed images of pairs of point targets as an initial demonstration of THz Fourier-basis agile structured illumination sensing imaging.