Chemical Identification in the Specular and Off-Specular Rough-Surface Scattered Terahertz Spectra Using Wavelet Shrinkage.

Mahmoud E Khani,M Hassan Arbab

doi:10.1109/access.2021.3059424

Abstract

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 α-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.

Highlights

A broadband terahertz (THz) pulse can resolve the low-frequency vibrational and rotational modes of molecular crystals [1]
We show that the wavelet shrinkage technique enables identification of resonant frequencies obscured by the rough surface scattering in both specular and off-specular detection geometries
We illustrate the results of the proposed wavelet shrinkage technique in specular reflectivities of PABA and α-lactose monohydrate

Summary

Introduction

A broadband terahertz (THz) pulse can resolve the low-frequency vibrational and rotational modes of molecular crystals [1] These molecular motions, which are associated with intra- or inter-molecular interactions, such as the weak hydrogen bonds or the crystalline lattice modes, appear as resonant signatures in the dielectric functions measured using THz time-domain spectroscopy (THz-TDS) [2]. In the reflection-mode THz spectroscopy, surface height variations on the order of the illumination wavelengths result in significant rough surface scattering, which can distort or obscure the resonant signatures [8]–[10] It may reduce the signal-to-noise ratio (SNR) and cause spectral signal distortions, rough surface scattering would allow for flexible emitter-detector geometries. Random variations in a sample’s surface height result in a random change in the phase of the THz fields reflected from that sample as compared to a perfect reflector, which is often used as a reference for the Fourier-domain deconvolution, causing additional phase ambiguity in the extracted dielectric

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