Optoelectronic frequency-modulated continuous-wave terahertz spectroscopy with 4 THz bandwidth

Abstract

Broadband terahertz spectroscopy enables many promising applications in science and industry alike. However, the complexity of existing terahertz systems has as yet prevented the breakthrough of this technology. In particular, established terahertz time-domain spectroscopy (TDS) schemes rely on complex femtosecond lasers and optical delay lines. Here, we present a method for optoelectronic, frequency-modulated continuous-wave (FMCW) terahertz sensing, which is a powerful tool for broadband spectroscopy and industrial non-destructive testing. In our method, a frequency-swept optical beat signal generates the terahertz field, which is then coherently detected by photomixing, employing a time-delayed copy of the same beat signal. Consequently, the receiver current is inherently phase-modulated without additional modulator. Owing to this technique, our broadband terahertz spectrometer performs (200 Hz measurement rate, or 4 THz bandwidth and 117 dB peak dynamic range with averaging) comparably to state-of-the-art terahertz-TDS systems, yet with significantly reduced complexity. Thickness measurements of multilayer dielectric samples with layer-thicknesses down to 23 µm show its potential for real-world applications. Within only 0.2 s measurement time, an uncertainty of less than 2 % is achieved, the highest accuracy reported with continuous-wave terahertz spectroscopy. Hence, the optoelectronic FMCW approach paves the way towards broadband and compact terahertz spectrometers that combine fiber optics and photonic integration technologies.

Visualizing nanometric structures with sub-millimeter waves

“…A measured standard deviation of 40 nm was recently reported by Molter et al 26 employing such a system. Although Liebermeister et al have recently reported a CW system with a bandwidth 4 THz 24 , using such huge bandwidth will also immensely increase the overall scan duration for the proposed single-pixel based imaging system. Last but not least, no multi-layer structures were characterized.…”

Terahertz Multilayer Thickness Measurements: Comparison of Optoelectronic Time and Frequency Domain Systems

“…With this FDS system, a PET film with a thickness of 23 µm could be measured with an accuracy better than 2%, which corresponds to an uncertainty below 0.5 µm. This demonstrates that FDS can compete with TDS for accurate thickness measurements on thin dielectric layers [10]. Thereby, the main advantage of FDS compared to TDS is the simplicity of its system architecture.…”

A Review of THz Technologies for Rapid Sensing and Detection of Viruses including SARS-CoV-2

“…Meta-and nanomaterials with various unit cell geometries working in the THz region are an emerging option assuring much promise for on-site, high-speed virus detection and have the potential to go beyond the detection limit. Moreover, with recent advancements in miniaturized THz technologies [86][87][88][89][90], THz-based virus sensing platforms can be widely deployed, even to remote locations. Even though terahertz based biosensing methods have been used for several sectors [91,92], there is still a need for extensive research on SARS CoV-2 and similar virus detection.…”