Quantum-inspired terahertz spectroscopy with visible photons

TL;DR

This paper introduces a quantum-inspired method for terahertz spectroscopy that uses visible photons for detection, eliminating the need for cooled detectors or ultrafast lasers, and enabling simpler, more accessible terahertz analysis.

Contribution

It presents the first demonstration of terahertz spectroscopy using correlated visible photons, inspired by quantum optics, to detect terahertz spectral features without specialized terahertz detectors.

Findings

Successful detection of terahertz absorption features of lactose and para-aminobenzoic acid.

Elimination of cooled detectors and ultrafast lasers in terahertz spectroscopy.

Use of visible photon detection to infer terahertz spectral information.

Abstract

Terahertz spectroscopy allows for identifying different isomers of materials, for drug discrimination as well as for detecting hazardous substances. As many dielectric materials used for packaging are transparent in the terahertz spectral range, substances might even be identified if packaged. Despite these useful applications, terahertz spectroscopy suffers from the still technically demanding detection of terahertz radiation. Thus, either coherent time-domain-spectroscopy schemes employing ultrafast pulsed lasers or continuous-wave detection with photomixers requiring two laser systems are used to circumvent the challenge to detect such low-energetic radiation without using cooled detectors. Here, we report on the first demonstration of terahertz spectroscopy, in which the sample interacts with terahertz idler photons, while only correlated visible signal photons are detected - a concept inspired by quantum optics. To generate these correlated signal-idler photon pairs, a periodically poled lithium niobate crystal and a 660 nm continuous-wave pump source are used. After propagating through a single-crystal nonlinear interferometer, the pump photons are separated from the signal radiation by highly efficient and narrowband volume Bragg gratings. An uncooled scientific CMOS camera detects the frequency-angular spectra of the remaining visible signal and reveals terahertz-spectral information in the Stokes as well as the anti-Stokes part of collinear forward generation. Neither cooled detectors nor expensive pulsed lasers for coherent detection are required. We demonstrate spectroscopy on the well-known absorption features in the terahertz spectral range of $\alpha$-lactose monohydrate and para-aminobenzoic acid by detecting only visible photons.