Pouyan Rezapoor

In this study, we propose a novel perturbed resonance frequency analysis method where the modified, THz frequency, Fabry–P´ erot resonances of a tissue loaded quartz window are used to quantify superficial skin layer thickness and water content. The superficial layer is treated as an unknown matching layer between the quartz window and deep epidermis and resolution of the longitudinal modes provides a unique mapping from resonance frequency and width to layer water content and thickness. The approach is experimentally validated with a telecentric beam scanning system coupled to VNA extender operating in the 330– 500 GHz band. The system is used to observe the dynamic effects of stinging nettle exposure on in vivo human skin over a 52 x 52 mm2 field-of-view (FOV). Spectroscopic imaging revealed an immediate decrease in resonance frequency and width of up to 15 GHz and 20 GHz, respectively, compared to unperturbed skin, which maps to a 30μm increase in thickness and 25% decrease in water content. This immediate response was followed by a gradual recovery to pre-exposed values. These findings demonstrate additional dielectric window utility in terahertz biomedical spectroscopy in imaging beyond the standard field flattening and system calibration roles.

A quasi-optical (QO) test bench was designed, simulated, and calibrated for characterizing all four S-parameters of devices in the 220–330 GHz (WR3.4) frequency range, from room temperature down to 4.8 K. Quasioptical calibration methods were applied to de-embed the impact of cryostat and optical elements on device under test measurements. The devices were measured through vacuum windows via focused beam radiation. A de-embedding method employing line-reflect-match (LRM) calibration was established to account for the effects of optical components and vacuum windows. Such a method does not require multiple line standards inside the cryostat and mechanical translation of quasioptics. System validation was performed with measurements of cryogenically cooled devices, such as bare silicon wafers and stainless-steel frequency-selective surface (FSS) bandpass filters, and superconducting bandpass FSS fabricated in niobium. A permittivity reduction of Si based on a 4 GHz resonance shift was observed concomitant with a drop in temperature from 296 to 4.8 K. The stainless steel FSS measurements revealed a relatively temperature invariant center frequency and return loss level of 263 GHz and 35 dB on average, respectively. Finally, a center frequency of 257 GHz was measured with the superconducting filters, with return loss improved by 11 dB on average at 4.8 K. To the best of our knowledge, this is the first reported attempt to scale LRM calibration to 330 GHz and use it to de-embed the impact of optics and cryostat from cryogenically cooled device S-parameters.

Terahertz (THz) imaging has emerged as a promising technology in medical diagnostics, thanks to non-ionizing radiation and the high sensitivity of THz waves to water content. However, traditional THz imaging systems face challenges such as slow mechanical scanning, limited field-of-view, and poor telecentricity. To address these limitations, we present the telecentric offset reflective imaging system, a novel dual-mirror scanning design optimized for high-speed, distortion-free imaging. Utilizing a telecentric $f-\theta$ lens and ray-tracing and physical optics simulations, the system achieves uniform resolution across a 50×50 mm² field of view without mechanical translation stages. The system demonstrates its capability through broadband spectral imaging of a USAF resolution test target across WR-2.2 (325 - 500 GHz) and WR-1.5 (500 - 700 GHz) rectangular waveguide frequency bands, achieving consistent beam focus and minimal distortion, with maximum deviation of 2.7$^{\circ }$ from normal incidence and beam waist of 2.1$\lambda$ at the edge of the field of view in WR- 1.5 frequency band. Additionally, the system's ability to monitor hydration dynamics is validated by imaging wet tissue paper, illustrating its sensitivity to temporal changes in water content. Further, in vivo imaging of human skin after capsaicin patch application reveals localized hydration variations influenced by biochemical responses and adhesive patch removal. These results confirm the scanner's potential for real-time hydration sensing and dermatological assessments. By providing high-resolution, real-time THz imaging, TORIS establishes a new standard in THz-based spectroscopy and imaging, with applications spanning clinical diagnostics, wound assessment, and material characterization.