Quantum-inspired terahertz spectroscopy with visible photons

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 timedomain-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 -lactose monohydrate and paraaminobenzoic acid by detecting only visible photons.

Mid-Infrared Microscopy via Position Correlations of Undetected Photons

“…The clear success of this approach for imaging and microscopy in the mid-IR motivates the question of its limitations -can we envisage this approach to wide field imaging stretching beyond its near and mid-IR, towards the far IR and even terahertz wavelengths [19], where detection technologies are even more limited? Fig.…”

“…When the two processes are aligned such that any information distinguishing whether the biphoton was born on the first or second crystal is erased, the two processes interfere. The strength (visibility) and phase of this interference signature serve as the contrast mechanism for imaging -analogously so for similar applications in varied tasks including optical co-herence tomography [12][13][14], spectroscopy [15][16][17][18][19] and polarimetry [20]. In contrast to ghost imaging schemes [21][22][23], in QIUP the detection of only one of the photons of the pair suffices to yield the information imprinted on the other.…”

Sample thickness measurements by phase-sensitive terahertz upconversion detection

“…The transfer of these concepts to the terahertz frequency range is highly attractive, as the detection of terahertz radiation is challenging due to its inherently low photon energy. Even though the quantum-optical sensing and spectroscopy approach has been demonstrated in this spectral range [6,7], its measurement time is not yet competitive. But the use of spontaneous parametric down-conversion and the correlation of the involved photons is not the only possibility to transfer photon properties to the visible range.…”

Mid-IR spectroscopy with NIR grating spectrometers

“…Instead, by exploiting widely non-degenerate correlated photonpairs generated via spontaneous parametric down-conversion, one can realise sensing and detection at vastly separated wavelengths [11]. This technique of sensing with undetected photons has also been applied to other modalities of sensing, including imaging [12,13,14,15], optical coherence tomography [16,17,18], refractometry [19], and even THz-sensing [20,21].…”