Terahertz Reflection Spectroscopy Research Articles

An Improved Theoretical Model to Extract the Optical Conductivity of Two-Dimensional Material from Terahertz Transmission or Reflection Spectroscopy

The technique of terahertz time-domain spectroscopy (THz-TDS) enables us to simultaneously determine the real and imaginary parts of optical parameters. However, it is still a challenge to extract the optical parameters of a two-dimensional (2D) material (or an ultra-thin film) on a substrate accurately and flexibly for an arbitrary incident angle and different polarization. By treating a 2D material as a conductive boundary without thickness, we propose an improved theoretical model to extract the optical conductivity of the 2D material on a substrate from THz transmission or reflection spectroscopy. Importantly, the effects of wave polarization, incident angle, and multiple reflections in the substrate are considered in our model and the analytical formulae associated with the optical conductivity of the 2D material are provided. Furthermore, we verify the validation of our model based on the THz transmission and reflection experiments for mono- and few-layer MoS2 on sapphire substrates. These results not only are of practical significance for investigating the THz properties of 2D materials but can also be extended to the situations of ultra-thin films and/or incoherent detection such as Fourier transform infrared spectroscopy.

Investigation of Formalin-Fixed Tissue Optical Characteristics in the Range of 200–500 GHz Using Pulsed Terahertz Reflection Spectroscopy to Differentiate Oral Malignant, Benign, and Cyst

The application of Terahertz electromagnetic waves to diagnose oral cancer was investigated. A single case of formalin-fixed oral squamous cell carcinoma (malignant), ameloblastoma (benign), and odontogenic keratocyst was examined using terahertz pulsed spectroscopy in the frequency span of 0.1–2 THz. The measured absorption coefficient, refractive index, and the extinction coefficient were reported to be high for malignant samples than benign and cyst. The THz results are validated with hematoxylin and eosin-stained microscopic images. The results demonstrate that the THz signal was shown to be consistently higher for the malignant sample compared to benign and the cyst. These results indicate that THz signals responded to the cell density by eliminating the effect of water.

Hyperspectral terahertz imaging and optical clearance for cancer classification in breast tumor surgical specimen.

Purpose: We investigate the enhancement in terahertz (THz) images of freshly excised breast tumors upon treatment with an optical clearance agent. The hyperspectral imaging and spectral classifications are used to quantitatively demonstrate the image enhancement. Glycerol solution with 60% concentration is applied to excised breast tumor specimens for various time durations to investigate the effectiveness on image enhancement. Approach: THz reflection spectroscopy is utilized to obtain the absorption coefficient and the index of refraction of untreated and glycerol-treated tissues at each frequency up to 3THz. Two classifiers, spectral angular mapping (SAM) based on several kernels and Euclidean minimum distance (EMD) are implemented to evaluate the effectiveness of the treatment. The testing raw data is obtained from five breast cancer specimens: two untreated specimens and three specimens treated with glycerol solution for 20, 40, or 60min. All tumors used in the testing data have healthy tissues adjacent to cancerous ones consistent with the challenge faced in lumpectomy surgeries. Results: The glycerol-treated tissues showed a decrease in the absorption coefficients compared with untreated tissues, especially as the period of treatment increased. Although the sensitivity metric of the classifier presented higher values in the untreated tissues compared with the treated ones, the specificity and accuracy metrics demonstrated higher values for the treated tissues compared with the untreated ones. Conclusions: The biocompatible glycerol solution is a potential optical clearance agent in THz imaging while keeping the histopathology imaging intact. The SAM technique provided a good classification of cancerous tissues despite the small amount of cancer in the training data (only 7%). The SAM exponential kernel and EMD presented classification accuracy of to 85% compared with linear and polynomial kernels that provided accuracy ranging from 70% to 80%. Overall, glycerol treatment provides a potential improvement in cancer classification in freshly excised breast tumors.

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

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.

In vivo estimation of water diffusivity in occluded human skin using terahertz reflection spectroscopy.

Water diffusion and the concentration profile within the skin significantly affect the surrounding chemical absorption and molecular synthesis. Occluding the skin causes water to accumulate in the top layer of the skin (the stratum corneum [SC]) and also affects the water diffusivity. Scar treatments such as silicone gel and silicone sheets make use of occlusion to increase skin hydration. However with existing techniques, it is not possible to quantitatively measure the diffusivity of the water during occlusion: current methods determine water diffusivity by measuring the water evaporated through the skin and thus require the skin to breathe. In this work, we use the high sensitivity of terahertz light to water to study how the water content in the SC changes upon occlusion. From our measurements, we can solve the diffusion equations in the SC to deduce the water concentration profile in occluded skin and subsequently to determine the diffusivity. To our knowledge, this is the first work showing how the diffusivity of human skin can be measured during occlusion and we envisage this paper as being used as a guide for non-invasively determining the diffusivity of occluded human skin in vivo.

Observation of terahertz plasmon and plasmon-polariton splitting in a grating-coupled AlGaN/GaN heterostructure.

Plasmon in two-dimensional electron gas (2DEG) has long been considered as a promising active medium for terahertz emitters and detectors. However, the efficiency of terahertz plasmonic devices is severely limited by the high damping rate of plasma wave in solid state. In addition to the enhancement of plasmon lifetime by using 2DEGs with higher carrier mobility, engineering on the boundary condition and electromagnetic environment of plasmon cavity helps to preserve the plasmon states. Here we report on terahertz reflection spectroscopy of plasmon states in a grating-coupled AlGaN/GaN-2DEG plasmonic device at 7 K in equilibrium with ambient blackbody irradiation. Localized plasmon states and plasmon-polariton states were observed when the core plasmonic device is integrated with a silicon lens and when it is embedded in a terahertz Fabry-Pérot cavity, respectively. Simulation results including the reflection spectra and total reflection power agree well with the measured results. The Rabi splitting is found to be inversely proportional to the resonance frequency, and follows a linear relation with the square root of the sheet electron density. A normalized coupling ratio, ΩRω0≈0.13, is achieved between the Rabi splitting ΩR and the resonance frequency ω0. The coupling ratio could be further increased to allow for ultrastrong coupling between terahertz photons and plasmons.

Antireflection self-reference method based on ultrathin metallic nanofilms for improving terahertz reflection spectroscopy.

We present the potential of an antireflection self-reference method based on ultra-thin tantalum nitride (TaN) nanofilms for improving terahertz (THz) reflection spectroscopy. The antireflection self-reference method is proposed to eliminate mutual interference caused by unwanted reflections, which significantly interferes with the important reflection from the actual sample in THz reflection measurement. The antireflection self-reference model was investigated using a wave-impedance matching approach, and the theoretical model was verified in experimental studies. We experimentally demonstrated this antireflection self-reference method can completely eliminate the effect of mutual interference, accurately recover the actual sample's reflection and improve THz reflection spectroscopy. Our method paves the way to implement a straightforward, accurate and efficient approach to investigate THz properties of the liquids and biological samples.

THz Study on the Role of ZnO Crystallinity in ZnO/AgNW/ZnO Composite Electrodes

We present the conducting mechanism of silver nanowire (AgNW) networks embedded between zinc oxide (ZnO) bilayers studied by terahertz (THz) reflection spectroscopy. Percolatively connected AgNWs have been considered as one of the most promising alternatives for indium-tin-oxide transparent conductive electrodes (TCEs). Although AgNWs can be spin-coated on nearly any kind of substrate, they leave a large open area between randomly scattered NWs. In ZnO/AgNW/ZnO composite electrodes, the conductive ZnO underlayer has the advantage in charge transport through electrodes and another ZnO layer covering the AgNW/ZnO layer can suppress oxidation of NW surface and improve the thermal stability. The performance of AgNW-based TCEs is highly correlated to the connectivity of NWs at junctions, which can be realized by percolational transition near a critical NW density. Terahertz reflection spectroscopic measurement shows that the photoconductivity of ZnO/AgNW/ZnO composite electrodes is strongly influenced by the ZnO crystallinity. The percolational behavior of the confinement factor in THz spectroscopic analysis reveals that single-crystalline ZnO layers in ZnO/AgNW/ZnO increase the photoconductivity by forming wider effective charge collection area around AgNWs and filling the gap between AgNWs to improve the ohmic contact.

Sensitive monitoring of photocarrier densities in the active layer of a photovoltaic device with time-resolved terahertz reflection spectroscopy

We demonstrate the sensitive measurement of photocarriers in an active layer of a GaAs-based photovoltaic device using time-resolved terahertz reflection spectroscopy. We found that the reflection dip caused by Fabry-Pérot interference is strongly affected by the carrier profile in the active layer of the p-i-n structure. The experimental results show that this method is suitable for quantitative evaluation of carrier dynamics in active layers of solar cells under operating conditions.

Calculation and Study of Graphene Conductivity Based on Terahertz Spectroscopy

Based on terahertz time-domain spectroscopy system and two-dimensional scanning control system, terahertz transmission and reflection intensity mapping images on a graphene film are obtained, respectively. Then, graphene conductivity mapping images in the frequency range 0.5 to 2.5 THz are acquired according to the calculation formula. The conductivity of graphene at some typical regions is fitted by Drude-Smith formula to quantitatively compare the transmission and reflection measurements. The results show that terahertz reflection spectroscopy has a higher signal-to-noise ratio with less interference of impurities on the back of substrates. The effect of a red laser excitation on the graphene conductivity by terahertz time-domain transmission spectroscopy is also studied. The results show that the graphene conductivity in the excitation region is enhanced while that in the adjacent area is weakened which indicates carriers transport in graphene under laser excitation. This paper can make great contribution to the study on graphene electrical and optical properties in the terahertz regime and help design graphene terahertz devices.

Extracting Complex Refractive Index from Polycrystalline Glucose with Self-Referenced Method for Terahertz Time-Domain Reflection Spectroscopy

A self-referenced method for extracting the complex refractive index of material was proposed. The method utilized signals reflected from the front and rear surfaces of a slice sample as reference and sample signals, respectively. Before using the self-referenced method, a hybrid filtering technique for eliminating systematic and random noises of time-domain terahertz reflection spectroscopy was used. A terahertz reflection spectrum of crystalline glucose was measured and three feature absorption peaks were obtained from 0.2 to 2.0 THz. We suggest that intermolecular vibrational modes may contribute to the observed absorption spectra in the THz frequency range.

Terahertz spectroscopic imaging and properties of gastrointestinal tract in a rat model.

We have investigated basic properties of normal gastrointestinal (GI) tract tissues, including glandular stomach (GS), fore stomach (FS), large intestine (LI), small intestine (SI), and esophagus (ESO), from a rat model using terahertz (THz) reflection imaging and spectroscopy. The THz images collected from stratified squamous epithelia (SSE) of FS and ESO show a lower peak-to-peak value compared to those from columnar epithelia (CE) of GS, LI, or SI because the SSE contains less water than CE. The refractive index and absorption coefficient of FS were less than those of GS or LI, both having values similar to those of water. Additionally, we report internal reflection THz signals from ESO, although we were unable to determine the exact interface for this internal reflection.

Realization of Metal–Insulator Transition and Oxidation in Silver Nanowire Percolating Networks by Terahertz Reflection Spectroscopy

Metal nanowires (NWs) enable versatile applications in printed electronics and optoelectronics by serving as thin and flexible transparent electrodes. The performance of metal NWs as thin electrodes is highly correlated to the connectivity of NW meshes. The percolation threshold of metal NW films corresponds to the minimum density of NWs to form the transparent, yet conductive metal NW networks. Here, we determine the percolation threshold of silver NW (AgNW) networks by using morphological analysis and terahertz (THz) reflection spectroscopy. From the divergent behavior of carrier scattering time and the increase of carrier backscattering factor, the critical NW density at which crossover from Drude to non-Drude behavior of THz conductivity occurs can be unambiguously determined for AgNW thin films. Furthermore, the natural oxidation of AgNWs which causes the gradual reduction of the connectivity of the AgNW network is also realized by the THz spectroscopy. The selective oxidation of NW-to-NW junctions weakens the ohmic contact, and for AgNWs near a critical density, it can even lead to metal-insulator transition. The presented results offer invaluable information to accelerate the deployment of metal nanowires for next-generation electronics and optoelectronics on flexible substrates.

Identification of concealed materials, including explosives, by terahertz reflection spectroscopy

We report on a method for extracting the characteristic features of covered materials, including hexogen, in the range from 0.5 to 1.8 THz. This time-domain spectroscopy-based technique takes into account only part of the signal reflected from a covered sample and analyzes it by using the Fourier transform. The obtained power spectrum has distinctive peaks that correspond to peaks measured in the transmission configuration and can be used for further identification of the materials. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduc- tion of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10.1117/1.OE.53.3.031202) spectral dynamics analysis method for identification of explosives in the reflection regime. In this article, we propose a new signal processing method to reveal characteristic THz fingerprints of materials covered with semitransparent layers such as a sheet of polyethylene or paper in the range of about 0.5 to 1.8 THz. The method takes into account the only part of the signal reflected from the covered sample that is then analyzed in the frequency domain with the Fourier transform. The obtained spectrum has very distinctive peaks that correspond to the main peaks measured in the transmission configuration, and this obtained spectrum is what in turn makes reliable identifica- tion possible.

Terahertz time-domain spectroscopy of absorbing materials

In this paper, we study the noise-limited precision on the complex refractive index value of materials in the terahertz frequency range as determined by terahertz time-domain spectroscopy. As absorbing samples are almost opaque, transmission measurements are not suitable to determine the refractive index and reflection indices have to be employed instead, but at the expense of precision. We give rules to select the most adapted technique, i.e. in transmission or in reflection, for any given sample of arbitrary transparence Full Text: PDF References M. van Exter and D. Grischkowsky, "Optical and electronic properties of doped silicon from 0.1 to 2 THz", Appl. Phys. Lett. 56, 1694 (1990); CrossRef Sensing with Terahertz Radiation, ed. D. Mittleman, Springer Series in Optical Sciences Vol. 85, Berlin (2003). DirectLink For example: http://www.menlosystems.com; , http://www.rainbowphotonics.com; , http://www.teraview.com; , http://www.ekspla.com; , http://www.picometrix.com; , http://www.zomega-terahertz.com; . T. Dorney, R. Baraniuk, and D. Mittleman, "Material parameter estimation with terahertz time-domain spectroscopy", J. Opt. Soc. Am. A 18, 1562 (2001); CrossRef I. Pupeza, R. Wilk, and M. Koch, "Highly accurate optical material parameter determination with THz time-domain spectroscopy", Opt. Exp. 15, Issue 7, 4335 (2007). CrossRef L. Thrane, R. H. Jacobsen, P. U. Jepsen, and S. R. Keiding, "THz reflection spectroscopy of liquid water", Chem. Phys. Lett. 240, 330 (1995); CrossRef S. C. Howells and L. A. Schlie, "Transient terahertz reflection spectroscopy of undoped InSb from 0.1 to 1.1 THz", Appl. Phys. Lett. 69, 550 (1996). CrossRef P. Uhd Jepsen and B. M. Fischer, "Dynamic range in terahertz time-domain transmission and reflection spectroscopy", Optics Letters 30, 29 (2005). CrossRef L. Duvillaret, F. Garet, J.-L. Coutaz, "Influence of noise on the characterization of materials by terahertz time-domain spectroscopy", J. Opt. Soc. Amer. B17, 452 (2000). CrossRef

Model-Based Material Parameter Estimation for Terahertz Reflection Spectroscopy

One important yet commonly overlooked phenomenon in terahertz (THz) reflection spectroscopy is the etalon effect-interference caused by multiple reflections from dielectric layers. Material layers are present in many THz applications from the monitoring of pharmaceutical tablet coating thickness to the detection of drugs, explosives, or other contraband concealed beneath layers of packaging and/or clothing. This paper focuses on the development and implementation of a model-based material parameter estimation technique, primarily for use in reflection mode THz spectroscopy that takes the etalon effect into account. The technique is adapted from techniques developed for transmission spectroscopy of thin samples and includes a priori information; namely, the assumption that a material's complex refractive index behaves consistently with the Lorentz dispersion model. The inclusion of the multiple returns provides additional information about the material and its structure, alleviating the need for short time windows in the fast Fourier transform (FFT) processing, which can limit spectral resolution. In addition, the parametrization of the material's dielectric function offers the potential for material classification based on these estimated dispersion model parameters. The parametric model-based method is validated by comparison with results from a conventional, non-parametric method applied to transmission mode data before being applied to reflection mode data for further validation with transmission mode results. Tests are also conducted to evaluate the parametric technique's robustness against measurement noise and ambiguity in sample thickness.

Improved sample characterization in terahertz reflection imaging and spectroscopy: erratum

Improved sample characterization in terahertz reflection imaging and spectroscopy: erratum

Terahertz spectroscopy of liver cirrhosis: investigating the origin of contrast

We have previously demonstrated that terahertz pulsed imaging is able to distinguish between rat tissues from different healthy organs. In this paper we report our measurements of healthy and cirrhotic liver tissues using terahertz reflection spectroscopy. The water content of the fresh tissue samples was also measured in order to investigate the correlations between the terahertz properties, water content, structural changes and cirrhosis. Finally, the samples were fixed in formalin to determine whether water was the sole source of image contrast in this study. We found that the cirrhotic tissue had a higher water content and absorption coefficient than the normal tissue and that even after formalin fixing there were significant differences between the normal and cirrhotic tissues’ terahertz properties. Our results show that terahertz pulsed imaging can distinguish between healthy and diseased tissue due to differences in absorption originating from both water content and tissue structure.

Terahertz reflection spectroscopy of Debye relaxation in polar liquids [Invited

Terahertz (THz) radiation interacts strongly with the intermolecular hydrogen-bond network in aqueous liquids. The dielectric properties of liquid water and aqueous solutions in the THz spectral region are closely linked to the microscopic dynamics of the liquid solution, and hence THz spectroscopy offers an important insight into fundamental intermolecular interactions in polar liquids. At the same time, the strong and characteristic interaction between THz radiation and liquids offers a methodology for the classification of liquids inside containers, and hence the THz region is suitable for remote detection of some of the properties of bottled liquids. Here we present a review of THz spectroscopy and modeling of water-ethanol mixtures, and establish a link between the dielectric function of water-ethanol mixtures and some of their thermodynamic properties. We then review how the knowledge of the dielectric function of aqueous mixtures can be used for inspection of liquids inside bottles. Finally we draw up some of the limits to the applicability of THz reflection spectroscopy in the identification of dangerous liquids.

Improved sample characterization in terahertz reflection imaging and spectroscopy

For imaging applications involving biological subjects, the strong attenuation of terahertz radiation by water means that terahertz pulsed imaging is most likely to be successfully implemented in a reflection geometry. Many terahertz reflection geometry systems have a window onto which the sample is placed - this window may introduce unwanted reflections which interfere with the reflection of interest from the sample. In this paper we derive a new approach to account for the effects of these reflections and illustrate its success with improved calculations of sample optical properties.