In recent times, terahertz (THz) technologies have been actively applied in many biomedical research work, including gastric cancer diagnosis. In order to provide an effective removal of tumor during surgery, it is necessary to clearly distinguish it from different membranes of the stomach. In this work, we reported an investigation of various normal and cancer fresh gastric tissues using terahertz time-domain spectroscopy in the reflection mode. Refractive index and absorption coefficient of moderately differentiated and poorly differentiated gastric adenocarcinomas, as well as both serosa and mucosa were obtained in the frequency range from 0.2 to 1 THz. All cancer tissues were distinguishable from normal ones. The influence of the morphology of the investigated tissues on the obtained optical properties is discussed. The obtained results demonstrated a potential of THz time-domain spectroscopy to discriminate a tumor from normal serous and mucous gastric membranes. Thus, this method might be applied to gastric cancer diagnosis.

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. To the best of our knowledge, this is the first work that describes axicon generated Bessel beam measurements of layered spheres with varying radii of curvature, in the submillimeter range.

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.

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.

Miniature on-chip sources based on the collapsing field domain phenomenon in GaAs have advanced compact sub-picosecond time-domain imaging schemes operating in the 100-250 GHz frequency range. Efforts have been done to broaden the emission band of on-chip sources towards 400 GHz, but increasing the peak power of the emitted picosecond-range pulses remains physically challenging but practically important. In this work, a fivefold increase in peak power has been achieved by replacing the previously used hetero-bipolar-based collapsing field domain source operating in self-oscillating mode with a bipolar-based source externally pulsed by a hetero-bipolar driver. Importantly, the suggested solution unaffected the compactness of the source and facilitates its application in time domain imaging.