Bernd M. Fischer

Terahertz scattering by granular composite materials: An effective medium theory

Terahertz (THz) spectroscopy and imaging have emerged as important tools for identification and classification of various substances, which exhibit absorption characteristics at distinct frequencies in the THz range. The spectral fingerprints can potentially be distorted or obscured by electromagnetic scattering caused by the granular nature of some substances. In this paper, we present THz time domain transmission measurements of granular polyethylene powders in order to investigate an effective medium theory that yields a parameterized model, which can be used to estimate the empirical measurements to good accuracy.

Low-cost ultra-thin broadband terahertz beam-splitter

A low-cost terahertz beam-splitter is fabricated using ultra-thin LDPE plastic sheeting coated with a conducting silver layer. The beam splitting ratio is determined as a function of the thickness of the silver layer--thus any required splitting ratio can be printed on demand with a suitable rapid prototyping technology. The low-cost aspect is a consequence of the fact that ultra-thin LDPE sheeting is readily obtainable, known more commonly as domestic plastic wrap or cling wrap. The proposed beam-splitter has numerous advantages over float zone silicon wafers commonly used within the terahertz frequency range. These advantages include low-cost, ease of handling, ultra-thin thickness, and any required beam splitting ratio can be readily fabricated. Furthermore, as the beam-splitter is ultra-thin, it presents low loss and does not suffer from Fabry-Pérot effects. Measurements performed on manufactured prototypes with different splitting ratios demonstrate a good agreement with our theoretical model in both P and S polarizations, exhibiting nearly frequency-independent splitting ratios in the terahertz frequency range.

Low-Frequency Spectroscopic Analysis of Monomeric and Fibrillar Lysozyme

Terahertz time-domain spectroscopy (THz-TDS) and Fourier transform infrared (FT-IR) spectroscopy were used to generate far-infrared and low-frequency spectral measurements of monomeric lysozyme and lysozyme fibrils. The formation of lysozyme fibrils was verified by the Thioflavin T assay and transmission electron microscopy (TEM). It was evident in the FT-IR spectra that between 150 and 350 cm−1 the two spectra diverge, with the lysozyme fibrils showing higher absorbance intensity than the monomeric form. The broad absorption phenomenon is likely due to light scattered from the fibrillar architecture of lysozyme fibrils as supported by simulation of Rayleigh light scattering. The lack of discrete phonon-like peaks suggest that far-infrared spectroscopy cannot detect vibrational modes between the highly ordered hydrogen-bonded beta-pleated sheets of the lysozyme subunit.

Dual-Mode Terahertz Time-Domain Spectroscopy System

Terahertz time-domain spectroscopy (THz-TDS) systems traditionally operate in a single mode, either in reflection or transmission. In cases where the sample has nonunity permeability, measurements in both reflection and transmission geometries are required. The process of shifting and swapping the samples during an experiment increases the measurement uncertainty. This paper therefore presents a system where both reflection and transmission measurements can be performed simultaneously to reduce both experimental error and acquisition time. The measurement results are validated against findings in literature.

Reduction of Scattering Effects in THz-TDS Signals

The scattering of terahertz radiation from the granular nature of a sample can potentially distort or obscure its characteristic spectral features. Several techniques have been proposed to reduce the effects of scattering in terahertz time-domain spectroscopy (THz-TDS) measurements, that usually require a complex measurement apparatus or rely on specific information about the sample under study. However, in real-world applications a priori information of the sample is not always known and therefore the applicability of these techniques may be limited. In this letter, we present a method for estimating and mitigating scattering effects in THz-TDS measurements for samples made of material exhibiting sharp and sparse absorption features, without requiring information of its granularity, refractive index, and density.

Terahertz scattering by two phased media with optically soft scatterers

In the last few decades, terahertz time domain spectroscopy has seen an increasing popularity in a variety of applications such as pharmaceutical material characterization, food quality control, investigation of explosive materials, and biomedical sensing. Many researchers have contributed to the development of several databases containing THz spectral signatures of various pure reagent-grade materials. 1,2 The availability of such databases has spurred the deployment of THz-TDS systems for real world applications. A common sample preparation technique is to mill or grind the sample material into fine powder and mix it with spectroscopic grade polyethylene powder to form rigid sample pellets. Such a preparation technique ensures a uniform sample structure with small particle sizes. However, in absence of laboratory conditions, invasive access to the sample material may not always be possible. Under such conditions, due to variations in refractive index within the sample, caused by granularity, impurities and the non-uniform internal structure, the THz radiation undergoes multiple scattering which can significantly alter/obscure the spectral fingerprints of the material. In this letter, we present a numerical approach based on the modified Rayleigh-Gans-Debye approximation to mitigate the scattering contribution in transmission mode THzTDS measurements of two phased media with absorbing constituents. The resulting expression describes the scattering attenuation in terms of the refractive indices of the sample constituents. The proposed technique not only eliminates the increased baseline, but also corrects the extinction spectrum for asymmetrically distorted absorption bands, often observed as consequence of multiple scattering in the sam

Terahertz fingerprinting in presence of quasi-ballistic scattering

characteristic molecular vibrational and rotational modes in the terahertz spectral range (0.3‐3THz), enabling them to absorb terahertz (THz) radiation at specific frequencies. These absorption features are unique to every material that absorbs in the THz spectral range and therefore may be used as spectral fingerprints for their classification and identification. 5 However, variations in refractive index within the sample, caused by sample granularity and impurities, cause the THz radiation to scatter, which can significantly alter or obscure the spectral fingerprints of the material under study. Usually in the case of the solids, the material of interest is quite dense and causes multiple scattering of THz radiation within the sample. The response of a dense medium, as a consequence of multiple scattering, can be classified into three regimes: ballistic, quasi-ballistic, and diffusive transport. 6 While the THz time domain spectroscopy (TDS) technique is sensitive to both quasi-ballistic and diffusive scattering, the criteria to determine which scattering regime is dominant, depends on the scattering (ksc) and transport mean free path lengths (ktr) in the medium, given by, ksc ¼ c=2nix; ktr ¼ ksc=ð1 �h cosðhÞi;

Terahertz scattering by dense media

Frequency dependent absorption of a given material at distinct frequencies in the terahertz (THz) range is commonly used as a spectral fingerprint for material identification and classification. However, in the presence of strong scattering, these features can often become distorted or altered. Thus, there is an important need to understand how scattering from a sample alters the THz signal. In this letter, we propose an iterative algorithm that builds on the effective field theory proposed by P. C. Waterman and R. Truell [J. Math. Phys. 2, 512–537 (1961)] and offers a rather simple and computationally efficient method for accurately explaining the multiple scattering response of a medium.

Double-layered nitrocellulose membrane sample holding technique for THz and FIR spectroscopic measurements

In terahertz (THz) and far-infrared (FIR) spectroscopic measurements, weak absorption spectral features due to small quantities of test sample can be masked by undesirable etalon fringe artifacts caused by multiple reflections within a pellet or a rigid sample holder. A double-layered nitrocellulose (NC) membrane structure is proposed in this paper as an alternative holder for small quantities of either dry or wet pure (no added polyethylene powder) samples with significantly reduced etalon artifacts. Utilizing a THz time-domain spectroscopy system and a synchrotron source, we demonstrate the performance of the NC structure across the THz/FIR spectrum, benchmarking against pellets holding similarly small quantities of α-lactose powder either with or without different grades of polyethylene powder. With only pure samples to consider, scattering can be mitigated effectively in NC-derived spectra to reduce their baselines.

Mitigating scattering effects in THz-TDS measurements

Scattering is a major problem in precise measurement of quasi-optical parameters of material samples. In this paper, we review some popular scattering mitigating techniques and propose a novel method that allows calculating true absorption spectra for samples with unknown thickness and granularity.