Absolute thickness and free water content gradients in gelatin-based corneal phantoms with physiologically accurate radii of curvature, and aqueous backing were extracted via coherent submillimeter wave reflectometry at 220 330 GHz. Fourier-domain based calibration methods, utilizing temporal and spatial gating, were developed and yielded peak-to-peak amplitude and phase clutter of 10-3 and 0.1, respectively for signal to noise ratios between 40 dB and 50 dB. Twelve phantoms were fabricated. Calibration methods enabled quantification of target sphericity that strongly correlated with optical coherence tomography-based sphericity metrics via image segmentation. Extracted free water volume fraction varied less than 5 % in the 5 phantoms whose fabrication yielded the most spherical geometry. Submillimeter wave-based thickness accuracy was better than 111 m (~/9) with average of 65 m (~/17) and standard deviation of 44 m (~/25) for phantoms with physiologically relevant geometry. Monte Carlo simulations of measurement noise and uncertainty limits agree with experimental data analysis and indicates a lower thickness accuracy limit of 33 m, and water content sensitivities of 0.5 % and 11.8 % for the anterior and posterior segments respectively. Numerical analysis suggests measurement fidelity was SNR limited and identified optical path length ambiguities within the cornea where a continuum of thickness/water gradient pairs produce statistically insignificant differences in complex reflection coefficient for finite SNR. This is the first known submillimeter-wave measurement technique able to extract both the thickness and water content gradients from a soft-tissue phantom, with a water backing, without the need for ancillary measurements.

This paper presents measurements of the permittivity of gelatin hydrogels between 220 and 330 GHz. Hydrated gelatin was treated as a binary mixture of free water and a compound consisting of water bound to collagen. Submillimeter-wave reflectometry was used to estimate the hydrated gelatin permittivity, hydrated gelatin density, and free-water volume fraction in phantoms composed of 62, 67, 72, and 77% water by weight. A hydrated dry/wet density ratio of 0.335 was validated with optical-coherence tomography. A constant non-freezing bound-water mass of 0.6 g/g was observed and confirmed with differential-scanning calorimetry. Good agreement between results from different modalities supports the dielectric spectroscopy methods and data analysis. Depending on the hydrodynamics at the sample/air interface, measurements indicate a bound-water compound permittivity of 3.77 - j2.52 to 3.95 - j2.49 contrasting the pure-water average permittivity of 5.16 j5.63. The loss related to bound water was much higher than anticipated and characterization will help reduce uncertainty in measurements of gelatin hydrogel-based tissue phantoms; particularly corneal phantoms where adjacent free water creates complex hydration gradients. This is the first known, submillimeter-wave, frequency domain measurement of complex permittivity of the bound-water component in solid, proteinaceous matter.

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.

Improving the longitudinal modes coupling in layered spherical structure contributes significantly to corneal terahertz sensing, which plays a crucial role in the early diagnosis of cornea dystrophies. Using a steel sphere to calibrate reflection from the cornea sample assists in enhancing the resolution of longitudinal modes. The requirement and challenges toward applying the calibration sphere are introduced and addressed. Six corneas with different properties are spotted to study the effect of perturbations in the calibration sphere in a frequency range from 100 GHz to 600 GHz. A particle-swarm optimization algorithm is employed to quantify corneal characteristics considering cases of accurately calibrated and perturbed calibrated scenarios. For the first case, the study is carried out with signal-to-noise values of 40 dB, 50 dB and 60 dB at waveguide bands WR-$5.1$, WR-$3.4$, and WR-$2.2$. As expected, better estimation is achieved in high-SNR cases. Furthermore, the lower waveguide band is revealed as the most proper band for the assessment of corneal features. For perturbed cases, the analysis is continued for the noise level of 60 dB in the three waveguide bands. Consequently, the error in the estimation of corneal properties rises significantly (around $30\%$).

Thin layers of silver nanowires are commonly studied for transparent electronics. However, reports of their terahertz (THz) properties are scarce. Here, we present the electrical and optical properties of thin silver nanowire layers with increasing densities at THz frequencies. We demonstrate that the absorbance, transmittance and reflectance of the metal nanowire layers in the frequency range of 0.2 THz to 1.3 THz is non-monotonic and depends on the nanowire dimensions and filling factor. We also present and validate a theoretical approach describing well the experimental results and allowing the fitting of the THz response of the nanowire layers by a Drude–Smith model of conductivity. Our results pave the way toward the application of silver nanowires as a prospective material for transparent and conductive coatings, and printable antennas operating in the terahertz range—significant for future wireless communication devices.

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.