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.

In this study, we propose a novel perturbed resonance frequency analysis method where the modified, THz frequency, Fabry-P & eacute;rot 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 & times; 52 mm2 field-of-view. 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 30 mu m 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.

Terahertz (THz) imaging has emerged as a promising technology in medical diagnostics due to its non-ionizing radiation and high sensitivity to water content. However, conventional THz imaging systems face limitations such as slow mechanical scanning, restricted field of view, and poor telecentricity. To overcome these challenges, we introduce the Telecentric Offset Reflective Imaging System (TORIS), a novel dual-mirror scanning design optimized for high-speed, distortion-free imaging. The system employs a telecentric f-theta lens and is validated using ray tracing and physical optics simulations. It achieves uniform resolution across a 50 mm x 50 mm field of view without the need for mechanical translation stages. Broadband spectral imaging of a USAF resolution test target across WR-2.2 (325-500 GHz) and WR-1.5 (500-700 GHz) frequency bands demonstrates consistent beam focus and minimal distortion, with a maximum deviation of 2.7 degrees from normal incidence and a beam waist of 2.1 lambda at the field edge in the WR-1.5 band. The system's sensitivity to hydration dynamics is further validated through imaging of wet tissue paper, capturing temporal changes in water content. In vivo imaging of human skin after capsaicin patch application reveals localized hydration variations due to biochemical response and adhesive removal. These findings confirm the system's potential for real-time hydration sensing and dermatological evaluation. TORIS sets a new benchmark in THz imaging, with applications in clinical diagnostics, wound assessment, and material characterization.

A quasi-optical (QO) test bench was designed, simulated, and calibrated for characterizing S-parameters of devices in the 220-330 GHz (WR-3.4) frequency range, from room temperature down to 4.8 K. 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. The setup provides all four S-parameters with the reference plane located inside the cryostat, and achieves a return loss of 30 dB with an empty holder. 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 4-GHz resonance shift was observed concomitant with a drop in temperature from 296 K 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 7 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.

In this work, superconducting niobium-film (Nb) filters have been designed and simulated based on Mattis-Bardeen theory in CST Microwave Studio. Proposed structures have been fabricated and measured in a WR-3.4 (220-330 GHz) quasioptical setup coupled to a cryostat. To de-embed measurements from the quasioptics, a Line-Reflect-Match calibration approach has been used. The measurements correspond well with numerical simulations, giving improved matching at 261 GHz versus 257 GHz in numerical simulations, however, with lower quality factors.

This study aims to explore the effect of the Gouy phase shift correction on determining refractive index and physical thickness of concentric spherical shells measured by quasioptical terahertz (THz) spectroscopy. The shells consisted of a loss-free quartz layer sitting on a water core which serves as an aqueous half space similar to the cornea's aqueous humour. The reflection of the water-backed quartz shells were measured with a focused Gaussian beam in the 220-330 GHz range. The optics generated a beam with a frequency-independent confocal distance resulting in equal radius of curvature and thus optimal wavefront matching to the sample curvature across the band. Thickness and refractive index were estimated from the measurements using Fresnel's equations and a fixed phase velocity. Parameter extraction was performed a second time where the frequency and axial location dependent phase velocity was corrected by incorporating the expected Gouy phase shift. The correction improved both the thickness and refractive index accuracy. The utility of Gouy phase correction was explored on hydrated corneal phantoms and increased the accuracy of thickness, and anterior and posterior water content estimates.

Conventional terahertz time-domain spectroscopy (THz-TDS) systems lack the frequency resolution necessary for accurate assessment of ultra-thin skin layers such as the stratum corneum (SC) below 30 μm. We introduce a Fabry–Pérot resonance-based technique, implemented with the high-resolution Telecentric Offset Reflective Imaging System (TORIS), which enables precise detection of spectral perturbations in a window-tissue multilayer geometry. This approach facilitates non-invasive, real-time mapping of epidermal thickness and hydration. In this study, we employ the validated TORIS system to investigate the dynamic epidermal response following exposure to stinging nettle, highlighting the method’s capability to detect rapid physiological changes in skin properties.

Diffractive optical elements (DOEs) in the THz range provide unparalleled, multifunctional control over radiation. Neural network-based design approaches offer significant potential in optimizing the unique phase maps of DOEs, enabling nearly arbitrary modulation of radiation. However, these neural network (NN) design methods typically require continuous values for the DOE phase profile synthesis. Often, the methods do not consider the physical quantization in DOE fabrication due to the discrete material layers produced with 3D printers. To address this, we apply automatic differentiation, a technique commonly used in neural networks, to develop a novel method for optimizing the phase profile of heavily quantized DOEs. Our simulation experiments show that this approach facilitates fast and flexible DOE design, while considering the fabrication limitations.

Galvanometers are commonly used in terahertz imaging systems due to their increased scanning speed in two dimensions. Typically, a telecentric f-theta lens is employed to ensure normal incidence on the target plane in reflective imaging systems. Galvanometric scanners, however, introduce beam aberrations in the target plane due to the use of a separate reflector for each scanning axis. In this work, we present a galvanometric scanner with a modified geometry, which reduces beam aberrations and increases telecentricity. Ray tracing and physical optics simulations show that by scanning the first reflector with respect to an offset point located at the center of the second reflector, the scanned beam behaves as if it is being scanned from a single point. By fixing this point in the focal point of the lens, we can minimize beam aberrations. Comparative analysis shows improved telecentricity (+/- 1 degrees at the scanning edge) and beam spot size, with acceptable f-theta distortion and negligible deviation from normal incidence.

The angular spectrum method is an efficient approach for synthesizing electromagnetic beams from planar electric field distributions. The electric field definition is restricted to a plane, which can introduce inaccuracy when applying the synthesized beam to curved surface features. The angular spectrum method can also be interpreted as a pure source method defining the field symmetrically with respect to the creation plane. Recently, we generalized that symmetric field method to arbitrary source distributions, which are valid at any point on compact, regular surface Ω in R3. We call this approach the Curved Boundary Integral method. The electromagnetic fields synthesized with this method satisfy the Helmholtz equation and are adjusted via amplitude and phase at the desired surface. The fields are obtained as a relatively simple integral. However, restrictions on where in space the synthesized field is valid were included in the mathematical proof length to avoid obscuring the main points. These restrictions can be significant depending on the shape and degree of curvature of surface Ω. In this article, we remove these restrictions so that the integral representation of the electromagnetic beam becomes valid at all points r∈R3∖Ω, with a minor restriction. Its modification can work even on Ω. We demonstrate the importance of this extended legality with a source field parametrized into the torus surface. The electromagnetic radiation of this structure would not be valid at any point in space without this extension. Finally, we show that by changing the order of integration, the field singularity at each source point is eliminated.

A quasioptical setup based on a Gaussian beam telescope system has been created to analyze the room-temperature and cryogenic millimeter-wave S-parameters of materials and devices for astronomical instrumentation, simulated and tested. The room temperature tests were performed in the WR-3.4 (220-330 GHz) frequency range after completing the thru-reflect-line calibration with a set of custom calibration standards. The system capabilities have been tested during the characterization of the planar devices and materials.

We recently introduced a new boundary integral method for numerical electromagnetic beam synthesis on curved surfaces. This study applies the methodology to computing gaussian beam reflection from an off-axis parabolic mirror and validates the results with physical optics simulation. Good agreement was observed.

This study investigates the impact of varying the illuminating frequency spectrum over time in a 340 GHz FMCW radar system with a holographic setup. Experiments using a corner cube reflector explored the behavior of spatially varying frequency-dependent field patterns across bandwidths ranging from 326 to 356 GHz. Findings suggest that varying the illuminating frequency spectrum over time can capture more spatial information from the target than using the radar’s entire bandwidth allowing for improved target discrimination and imaging capabilities in holographic radar systems.

The angular spectrum method is a rigorous method to synthesize near and far-field electromagnetic beams from planar field distributions. However, this limitation of planar surfaces has restricted its applicability to beams with simple focal planes. We propose a curved boundary integral method (CBIM) to synthesize electromagnetic beams from arbitrary surfaces to address this limitation and expand the method's scope to synthesize beams from and between shaped objects. This study presents a detailed theoretical framework behind the CBIM and validates its effectiveness and accuracy with a comprehensive set of simulations. Additionally, we present mathematical proof to support our proposal. The proposed method satisfies Maxwell's equations and significantly benefits optical systems and inverse beam design. It allows for analyzing electromagnetic forward/backward propagation between optical elements using a single method. It is also valuable for optical force beam design and analysis.

The feasibility of a 220 - 330 GHz zero order axicon generated Bessel beam for corneal water content was explored. Simulation and experimental data from the 25-degree cone angle hyperbolic-axicon lens illuminating metallic spherical targets demonstrate a monotonically decreasing, band integrated, backscatter intensity for increasing radius of curvature from 7 - 11 mm, when lens reflector and optical axis are aligned. Further, for radii >= 9.5 mm, maximum signal was obtained with a 1 mm transverse displacement between lens and reflector optical axes arising from spatial correlation between main lobe and out of phase side lobes. Thickness and permittivity parameter estimation experiments were performed on an 8 mm radius of curvature, 1 mm thick fused quartz dome over a 10 mm axial span. Extracted thickness and permittivity varied by less than 25 μm and 0.2 respectively after correction for superluminal velocity. Estimated water permittivity and thickness of water backed gelatin phantoms showed significantly more variation due to a time varying radius of curvature.

Ocular diseases can be detected in early-stage with non-destructive terahertz imaging by analyzing cornea thickness and water content changes. We propose wavefront-modified spherical vector beam imaging, which reduces nominal 4-5 % super-confocally focused Gaussian beam imaging errors to less than 0.1 %.

A design technique for frequency-diverse phase holograms using spatial filtering to enhance submillimeter-wave imaging performance is presented. The frequency-diverse holograms are designed to create complex, spatially varying field patterns for an imaging system operating at 325-355 GHz. The proposed technique uses spatial filtering during the hologram synthesizing process to eliminate the wide-angle plane-wave components. This minimizes the field dispersion outside the region of interest, improving the signal-to-noise ratio and imaging performance.

Mie theory is a powerful method to model electromagnetic scattering from a multilayered sphere. Usually, the incident beam is expanded to its vector spherical harmonic representation defined by beam shape coefficients, and the multilayer sphere scattering is obtained by the T-matrix method. However, obtaining the beam shape coefficients for arbitrarily shaped incident beams has limitations on source locations and requires different methods when the incident beam is defined inside or outside the computational domain or at the scatterer surface. We propose a 3D angular spectrum method for defining beam shape coefficients from arbitrary source field distributions. This method enables the placement of the sources freely within the computational domain without singularities, allowing flexibility in beam design. We demonstrate incident field synthesis and spherical scattering by comparing morphology-dependent resonances to known values, achieving excellent matching and high accuracy. The proposed method has significant benefits for optical systems and inverse beam design. It allows for the analysis of electromagnetic forward/backward propagation between optical elements and spherical targets using a single method. It is also valuable for optical force beam design and analysis.

We present recent developments of a standoff imaging system based on a frequency-diverse phase hologram and deep neural networks. The single-pixel imaging system operates in a monostatic configuration consisting of a 340-GHz FMCW radar and a frequency-diverse phase hologram to interrogate the radar down range direction with spatially varying, frequency-dependent field patterns. The measured back-reflected signal contains spatial reflectivity information from the target, and the fast chirp rate of the radar enables real-time imaging performance. Together with simultaneously acquired visible-light images, a deep neural network integrated into the submillimeter-wave data readout electronics can map the received signal onto a 2D image without mechanical or active electrical beam scanning. In experiments, we have collected submillimeter-wave and visible-light data of a moving target in the region of interest with a 60-Hz frame rate. The results suggest that the system can image the moving target with a resolution comparable to the theoretical diffraction limit. The minimal hardware complexity and good imaging performance of the demonstrated computational submillimeter-wave imaging system support its potential as a cost-effective and easily deployable solution for various imaging applications.

We present simulation results on transmission-type element with optimized local transmission function at submillimeter waves. The element can be considered as a part of illuminating quasioptics for single-pixel imaging. The planar element is based on stacked layers of 497-μm silicon wafers. The wide-band transmission function is achieved with perforations in each layer, which realize the desired effective permittivity in the volume of the stacked layers. We have optimized the element for 220-330 GHz, with the transmission function producing frequency-multiplexed Hadamard-basis patterns.