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We present the development and implementation of a novel wavelet shrinkage technique for the retrieval of obscured characteristic resonant signatures in the scattered terahertz (THz) reflectivity of molecular crystals. In this implementation, the wavelet basis functions associated with the absorption features were identified using the second-order total variation of the wavelet coefficients. Additionally, wavelet coefficients at certain scales were modified using the phase function corrections and wavelet hard thresholding. Reconstruction of the original spectra using these modified wavelet coefficients yielded the exact resonant frequencies of the chemicals, which were otherwise unrecognizable in the spectral artifacts of the rough surface scattering. We examined the robustness of this method over controlled levels of rough surface scattering, validated using the Kirchhoff approximation, in spectroscopic targets made from $\alpha $ -lactose monohydrate and 4-aminobenzoic acid (PABA), which have close spectral lines. We successfully retrieved the spectral absorption fingerprints in both specular and off-specular reflection geometries. This technique can be utilized for stand-off material characterization using the THz reflection spectroscopy in uncontrolled environments and potentially can be adopted for other broadband spectroscopic modalities.

We demonstrate spectroscopic THz reflection imaging through turbid media. We show that the detection of resonant features in the extinction spectrum can be made possible using multiresolution analysis based on wavelet transforms. We determine that the spatial resolution of this imaging technique is only diffraction limited.

Current terahertz (THz) spectroscopy techniques only use the coherent light beam for spectral imaging. In the presence of electromagnetic scattering, however, the scattering-mitigated incoherent beams allow for flexible emitter-detector geometries, which enable applications such as seeing through turbid media. Despite this potential, THz spectroscopy using diffuse waves has not been demonstrated. The main obstacles are the very poor signal to noise ratios of the diffused fields and the resonance-like spectral artifacts due to multiple Mie scattering events that obscure the material absorption signatures. In this work, we demonstrate diffuse THz spectroscopy of a heterogeneous sample through turbid media using a novel technique based on the wavelet multiresolution analysis and the bimodality coefficient spectrum, which we define here for the first time using the skewness and kurtosis of the spectral images. The proposed method yields broadband and simultaneous material characterization at detection angles as high as 90o with respect to the incident beam. We determined the accuracy of the wavelet-based diffuse spectroscopy at oblique detection angles, by evaluating the area under the Receiver Operating Characteristic (ROC) curves, to be higher than 95%. This technique is agnostic to any a priori information on the spectral signatures of the sample materials or the characteristics of the scattering medium, and can be expanded for other broadband spectroscopic modalities.

Topographical abnormality in corneal tissue is a common diagnostic marker for many eye diseases and injuries. Using an asynchronous optical sampling terahertz time-domain spectroscopy setup, we developed a non-contact and normal-incidence imaging system to measure topographic changes along the surface of spherical samples. We obtained orthogonal 1D scans of calibration spheres to evaluate the minimum axial resolution of our system. We determined the axial and spatial resolution of the scanner using 3D-printed spherical cross and Boehler star targets. Furthermore, we characterized the asymmetrical performance of the scanner due to the use of an off-axis parabolic mirror. Finally, we developed an edge-detection filter to aid with improving the topographic scans. We showed that when imaging samples were comparable in size to the human cornea, the axial and spherical spatial resolutions were limited to about 15 µm ( ∼ λ / 67 ) and 1 mm, respectively.

We present an upgraded handheld THz-TDS polarimetric scanner for use in clinical settings and nondestructive testing applications. We have converted an ASOPS system to perform ECOPS time-domain data acquisition to improve measurement rate to about 800 Hz, resulting in TDS measurement time of 1.25 ms. Moreover, the addition of a second detection channel allows co- and cross-polarization elements of the reflected beam to be measured. We present polarization-sensitive imaging results from partial thickness in vivo porcine burns.

Dielectric heterogeneity in addition to wavelength-scale air voids and granular particles lead to significant electromagnetic scattering in the transmission-mode THz-TDS. These scattering processes result in the distorted or obscured resonant signatures, a frequency-dependent extinction loss, and the appearance of anomalous spectral artifacts, imposing severe challenges on the real-world automatic material characterization schemes. In this work, we use a combination of the wavelet multiresolution analysis, the principal component analysis, and the bimodality coefficient to simultaneously identify all the resonant frequencies of a heterogeneous sample embedded beneath a highly scattering turbid medium.

To solve the phase ambiguity problem in the THz reflection spectroscopy, current material parameter extraction methods require the sample to make an ideal contact with an optically transparent window. We treat these samples as a multilayer system and present a new algorithm that estimates the thickness of the potential air gap and calculates the complex refractive index of the samples.

The rough surface scattering in the reflection-mode THz-TDS can result in inaccurate phase spectra, reduced signal to noise ratios, distorted or obscured resonant fingerprints, and the appearance of anomalous spectral artifacts. In this work, we demonstrate the implementation of a wavelet shrinkage technique for the retrieval of obscured resonant frequencies from the derivative of THz reflectivity of solid dielectric materials. We examine the robustness of this technique over controlled levels of rough surface scattering created using sandpapers of different grits on sample disks made from 4-aminobenzoic acid (PABA).

We have developed a terahertz time-domain polarimetry (THz-TDP) system by applying frequency modulation to electro-optic sampling detection in a nonlinear crystal. We characterized the precision of this system in determining the polarization angles to be 1.3° for fixed time delay, and 0.5° for complete time-domain waveform. Furthermore, we calculated the Jones matrix of the optical components used for beam propagation to calibrate the induced systematic error. The advantages of employing this calibration approach are demonstrated on a sapphire crystal investigated at different sample test positions in transmission configuration, and using high resistivity Si, AlN and quartz in reflection geometry. The new THz-TDP technique has the advantage of not using any external polarizers, and therefore is not constrained by their optical performance limitations, such as restricted bandwidths and frequency-dependent extinction ratio. Finally, the THz-TDP technique can be easily implemented on existing time-domain spectroscopy (TDS) systems.

Terahertz imaging often involves seeing through inhomogeneous and highly scattering media. We propose a wavelet multiresolution analysis approach to mitigate the scattering effects and produce highly-resolved reflection spectral images for chemical mapping through turbid media.

We present a handheld THz-TDS scanner for use in clinical settings and non-destructive testing applications. The ASOPS scanner is based on a gimbaled galvanoscanner and custom-designed f-$\theta$ lenses to achieve telecentric imaging in a 3D-printed housing. Imaging results from Bohler star calibration targets for determination of spatial resolution will be shown. We will also present non-destructive testing of airplane wing samples. Finally, in the full paper, application of this scanner in clinical diagnosis of skin burns will be presented.

Topographic imaging of corneal tissue would be a prominent indicator of many ocular diseases. Using the time of arrival of a reflected THz beam from the surface, we develop a setup where depth profiles can be extracted from a spherical target. Topographic information associated with the contour of corneal tissue can be extracted, such as radius, surface height and thickness variations. In this study, we image several 3D printed spherical targets to determine the lateral and axial resolutions, and asymmetrical characteristics of our imaging system.

We report on the development and performance characterization of a telecentric terahertz spectroscopic scanner using an f-θ objective lens and a single gimballed scanning mirror for image formation. We derived a beam steering transform to compensate for the intercoupling of the gimballed mirror axes and the distortions caused by an imperfect scanning lens. We characterize the optical performance of the system in both the time and spatial domains, demonstrating a constant diffraction-limited imaging resolution over the entire field of view. Finally, given the large depth of focus of the objective lens, we demonstrate the broadband imaging capability at different depths using a Boehler star target. This imaging setup has the potential to be miniaturized into portable form factors for field-deployable scenarios.

Terahertz (THz) imaging is a widely used technique in the study and detection of many chemicals and biomolecules in polycrystalline form because the spectral absorption signatures of these target materials often lie in the THz frequencies. When the size of dielectric grain boundaries are comparable to the THz wavelengths, spectral features can be obscured due to electromagnetic scattering. In this study, we first investigate this granular scattering effect in identification of chemicals with THz spectral absorption features. We then will propose a signal processing technique in the so-called "quefrency" domain to improve the ability to resolve these spectral features in the diffuse scattered THz images. We created a pellet with α-lactose monohydrate and riboflavin, two biologically significant materials with well-known vibrational spectral resonances, and buried the pellet in a highly scattering medium. THz transmission measurements were taken at all angles covering the half focal plane. We show that, while spectral features of lactose and riboflavin cannot be distinguished in the scattered image, application of cepstrum filtering can mitigate these scattering effects. By employing our quefrency-domain signal processing technique, we were able to unambiguously detect the dielectric resonance of lactose in the diffused scattering geometries. Finally we will discuss the limitation of the new proposed technique in spectral identification of chemicals.

Perturbation of normal corneal water content is a common manifestation of many eye diseases. Terahertz (THz) imaging has the potential to serve as a clinical tool for screening and diagnosing such corneal diseases. In this study, we first investigate the diffusive properties of a corneal phantom using simultaneous THz time-domain spectroscopy (THz-TDS) and gravimetric measurements. We will then utilize a variable-thickness diffusion model combined with a stratified composite-media model to simulate changes in thickness, hydration profile, and the THz-TDS signal as a function of time. The simulated THz-TDS signals show very good agreement with the reflection measurements. Results show that the THz-TDS technique can be used to understand water diffusion dynamics in corneal phantoms as a step towards future in vivo quantitative hydration sensing.

We present experimental results from the application of our new terahertz full-polarimetry instrument, to measuring the thickness of ultra-thin films in a multi-layer coating system. While the standard time-of-flight measurement is limited by the bandwidth of the THz pulse in the minimum thickness resolution of the film that can be resolved using a time-domain system, the new terahertz time-domain polarimetry (THz-TDP) instrument can extend this limit to a few micrometers. This unique ability to measure the thickness of ultra-thin films (approximately λ/1000) is due to the accuracy of THz-TDP to resolve the polarization direction of the THz field within 1 degrees.

We have designed and fabricated a terahertz full-polarimetry instrument, based on control of terahertz polarization in a gas-plasma source, and fast measurement of polarization direction and chirality of the terahertz field in the time-domain without using any external polarizers. We will present details on this new experimental technique including synchronization and calibration of the measurement apparatus against wire-grid polarizers, and derivation of the Jones matrix representation of the propagation optics. The new terahertz time-domain polarimetry (THz-TDP) system extends the polarimetry frequency range to the full bandwidth of gas-plasma sources, and offers accuracy of approximately 1 degrees.

The accuracy rates of the clinical assessment techniques used in grading burn injuries remain significantly low for partial thickness burns. In this paper, we present experimental results from terahertz characterization of 2nd and 3rd degree burn wounds induced on a rat model. Reflection measurements were obtained from the surface of both burned and normal skin using pulsed terahertz spectroscopy. Signal processing techniques are described for interpretation of the acquired terahertz waveform and differentiation of burn wounds. Furthermore, the progression of burn injuries is shown by comparison between acute characterization and 72-hours survival studies. While the water content of healthy and desiccated skin has been considered as a source of terahertz signal contrast, it is demonstrated that other biological effects such as formation of post-burn interstitial edema as well as the density of the discrete scattering structures in the skin (such as hair follicles, sweat glands, etc.) play a significant role in the terahertz response of the burn wounds.

Previously, it has been shown that scattering of terahertz waves by surface roughness of a target can alter the terahertz absorption spectrum and thus obscure the detection of some chemicals in both transmission and reflection geometries. In this paper it is demonstrated that by employing Maximal Overlap Discrete Wavelet Transform (MODWT) coefficients, wavelet-based methods can be used to retrieve spectroscopic information from a broadband terahertz signal reflected from a rough surface target. It is concluded that while the commonly used direct frequency domain deconvolution method fails to accurately characterize and detect the resonance in the dielectric constant of rough surface lactose pellets, wavelet techniques were able to successfully identify such features.

The existence of unique absorption spectrum patterns for many chemicals at terahertz frequencies has opened an exciting avenue in non-contact safe detection of such materials by terahertz spectroscopy. However, scattering of THz waves, which have wavelengths on the order of material grain sizes, by surface roughness challenges the sensitivity of this detection scheme in practice. In this work, we present terahertz time domain spectroscopy results for materials with rough surfaces. Both reflection from and transmission through lactose, which has sharp absorption peaks in the terahertz regime, are studied and the effect of increasing scattering through controlled surface roughness is investigated. Such electromagnetic scattering can alter the terahertz absorption spectrum and thus obscure the detection of chemicals. Furthermore we examine electro-optic detection of terahertz signals reflected from randomly rough targets with a theoretical electromagnetic system perspective and provide a method to retrieve coherent reflection responses from rough surface targets.