Reflection Terahertz Time-domain Spectroscopy Research Articles

Contactless graphene conductivity mapping on a wide range of substrates with terahertz time-domain reflection spectroscopy

We demonstrate how terahertz time-domain spectroscopy (THz-TDS) operating in reflection geometry can be used for quantitative conductivity mapping of large area chemical vapour deposited graphene films on sapphire, silicon dioxide/silicon and germanium. We validate the technique against measurements performed with previously established conventional transmission based THz-TDS and are able to resolve conductivity changes in response to induced back-gate voltages. Compared to the transmission geometry, measurement in reflection mode requires careful alignment and complex analysis, but circumvents the need of a terahertz transparent substrate, potentially enabling fast, contactless, in-line characterisation of graphene films on non-insulating substrates such as germanium.

Rapid analysis of tetracycline hydrochloride solution by attenuated total reflection terahertz time-domain spectroscopy.

Despite numerous methods for the detection of antibiotic residues, they are usually destructive and require tedious pre-treatment. Terahertz time-domain spectroscopy (THz-TDS) is an emerging technology that has advantages for analyzing chemical and biological compounds since THz waves are very sensitive to the molecular vibrational modes. Here we incorporated attenuated total reflection technique into the THz-TDS and demonstrated that this technology (ATR THz-TDS) allowed to determine the complex refractive indices of tetracycline hydrochloride (TCH) solutions with high accuracy and could be used to predict their concentrations. Our results from the simple linear regression models indicated that the complex refractive index exhibited a monotonic decrease with an increase in the TCH concentration. This study will provide new knowledge about the concentration determination of a liquid sample that couldn't be elucidated with the conventional THz-TDS technologies.

Brain tumor imaging of rat fresh tissue using terahertz spectroscopy.

Tumor imaging by terahertz spectroscopy of fresh tissue without dye is demonstrated using samples from a rat glioma model. The complex refractive index spectrum obtained by a reflection terahertz time-domain spectroscopy system can discriminate between normal and tumor tissues. Both the refractive index and absorption coefficient of tumor tissues are higher than those of normal tissues and can be attributed to the higher cell density and water content of the tumor region. The results of this study indicate that terahertz technology is useful for detecting brain tumor tissue.

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.

Evaluation of complex conductivity in a circularly polarized field by terahertz time-domain reflection spectroscopy with a phase shifter

We propose a smart technique for directly evaluating complex conductivity in a circularly polarized field with no polarimeter by time-domain reflection spectroscopy using a terahertz (THz) phase shifter and a polarized beam splitter. We performed THz time-domain spectroscopy on a doped InSb wafer under a magnetic field with a δ-phase shifter based on parallel metal waveguides. Despite the π/2 phase shifter with low accuracy, we obtained the conductivity spectrum shifted by cyclotron frequency while maintaining a Drude-like spectral shape. This technique paves the way for new magneto-optical spectroscopy methods.

24pDC-10 反射型テラヘルツ時間領域分光で観測したL_ SrxCuO_4の超伝導ゆらぎ

24pDC-10 反射型テラヘルツ時間領域分光で観測したL_ SrxCuO_4の超伝導ゆらぎ

Electrical characterization of silver nanowire-graphene hybrid films from terahertz transmission and reflection measurements

We determined the electrical sheet conductivity of silver nanowire-graphene hybrid films from transmission and reflection terahertz time-domain spectroscopy measurements. The sheet resistance extracted from noncontact terahertz measurement is in good agreement with one measured with a classical 4-point-probe technique. The conductivity is well described by a Drude-Smith model and is calculated to peak around 10 THz.

Reflection terahertz time-domain spectroscopy of RDX and HMX explosives

We report on our study of RDX and HMX, two of the most commonly used explosive materials, in bulk pellets with reflection terahertz time-domain spectroscopy in the frequency range of 0.3–3 THz. The maximum entropy method was utilized to correct our raw reflection data against the phase error due to the relative displacement between the sample and the reference. Both the refractive index n and the extinction coefficient k in the terahertz region were acquired for these two explosives without a Kramers-Kronig analysis. Both RDX and HMX exhibit a series of distinct peaks not quite detectable in the more conventional transmission-type measurements due to their high terahertz absorptivity. Our results are compared with the literature data on powder samples.

Nondestructive Analysis of Structure and Components of Tablet Coated with Film by the Usage of Terahertz Time-Domain Reflection Spectroscopy

Nondestructive analysis of tablet is of great importance from the aspect of productivity and safety. In terahertz (THz) region, however, the analysis of core of coated tablet has not been progressed. In this study, we have measured a flat-surface push–pull osmotic pump tablet, having no orifice, having bilayer core and a coating film layer. The bilayer core was made from the drug layer and the push layer, and acetaminophen was contained in the drug layer as a model drug. To study its structure and components, we have obtained reflection spectra from the drug layer side and the push layer side measurements using THz time-domain reflection spectroscopy (THz-TDRS). From these results, detection of the peak of acetaminophen in the spectrum from the drug layer side measurements was confirmed. We have made ridges approximated toward the peak using a general method of linear regression analysis in both spectra. Two-sample t-test was applied to their gradients, and significant difference between the drug layer and the push layer was shown. These results suggested that THz-TDRS is applicable to the analysis of structure and component of a coated tablet.

Precise Determination of the Refractive Index of Samples Showing Low Transmission Bands by THz Time-Domain Spectroscopy

In this paper, we propose a method to determine the refractive index and the absorption coefficient of samples showing spectral bands of very low transmission. The method, takes advantages of both transmission and reflection terahertz time-domain spectroscopy (THz-TDS) to obtain more precise optical constants than when only reflection measurement is used. It has been then validated using pellets made of maltose, which exhibits resonant lines within the THz spectral range.

Beating the wavelength limit: three-dimensional imaging of buried subwavelength fractures in sculpture and construction materials by terahertz time-domain reflection spectroscopy

We use reflection terahertz spectroscopy to locate and produce three-dimensional images of air gaps between stones that resemble fractures, even of subwavelength thicknesses. This technique is found to be promising tool for sculpture and building damage evaluation as well as structural quality control in other dielectric materials.

Reflective Terahertz Time-Domain Spectroscopy Measurement on the Stripe-Ordered Superconductor La1.84-yNdySr0.16CuO4

We measured reflectivity on static stripe-ordered La 1.84- y Nd y Sr 0.16 CuO 4 (LNSCO) y =0.1, 0.2, 0.3, and 0.4 by means of reflective terahertz time-domain spectroscopy (THz-TDS) with electric field polarized along c -axis, in which one can obtain lower frequency information than the conventional Fourier transform type spectrometer. Recently, two-dimensional superconducting (2DSC) property [tajima,schafgans,li], intra-layer perfect conductivity not accompanied with inter-layer superconducting behavior, was reported on LNSCO ( x =0.15, y ≥0.2). We observed the existence of Josephson plasma edge, which suggests that 2DSC is not realized in these compounds.

Hiding a Realistic Object Using a Broadband Terahertz Invisibility Cloak

The invisibility cloak has been a long-standing dream for many researchers over the decades. Using transformation optics, a three-dimensional (3D) object is perceived as having a reduced number of dimensions, making it “undetectable” judging from the scattered field12345. Despite successful experimental demonstration at microwave and optical frequencies6789101112, the spectroscopically important Terahertz (THz) domain13141516 remains unexplored due to difficulties in fabricating cloaking devices that are optically large in all three dimensions. Here, we report the first experimental demonstration of a 3D THz cloaking device fabricated using a scalable Projection Microstereolithography process. The cloak operates at a broad frequency range between 0.3 and 0.6 THz, and is placed over an α-lactose monohydrate absorber with rectangular shape. Characterized using angular-resolved reflection THz time-domain spectroscopy (THz-TDS), the results indicate that the THz invisibility cloak has successfully concealed both the geometrical and spectroscopic signatures of the absorber, making it undetectable to the observer.

Design and performance of reflective terahertz air-biased-coherent-detection for time-domain spectroscopy

When a sample being tested is optically opaque or has a high absorption coefficient, a reflective measurement is often more suitable than a transmission measurement. We report the design and evaluation of a reflective terahertz time-domain spectroscopy (R-THz-TDS), using air as THz wave emitter and sensor, together with air-biased-coherent-detection (ABCD) method for the first time. With an 85 fs pulse amplified laser, we demonstrate a usable bandwidth from 0.5 THz to 12 THz, together with a peak dynamic range (DR) better than 2000:1 and a peak THz electrical field greater than 30 kV/cm. With a 32 fs pulse amplified laser, the usable bandwidth is remarkably expanded to a continuous 35 THz. Several far-infrared optical properties in various samples are reported. Furthermore, the time-resolved optical pump-THz probe experiment is performed. Finally, the uniqueness and advantage of this spectrometer are comprehensively compared with traditional THz-TDS and Fourier transform infrared (FTIR) spectroscopy.

Measurement of Refractive Index for High Reflectance Materials with Terahertz Time Domain Reflection Spectroscopy

A method to measure the refractive index for high reflectance materials in the terahertz range with terahertz time domain reflection spectroscopy is proposed. In this method, the THz waveforms reflected by a silicon wafer and high reflectance sample are measured respectively. The refractive index of the silicon wafer, measured with the THz time domain transmission spectroscopy, is used as a reference in the THz time domain reflective spectroscopy. Therefore, the complex refractive index of the sample can be obtained by resorting to the known reflective index of the silicon and the Fresnel law. To improve the accuracy of the phase shift, the Kramers–Kronig transform is adopted. This method is also verified by the index of the silicon in THz reflection spectroscopy. The bulk metal plates have been taken as the sample, and the experimentally obtained metallic refractive indexes are compared with the simple Drude model.

Studies on 2,4-DNT Mixtures Using Reflection Terahertz Time Domain Spectroscopy for Explosives Detection

Absorption spectra (0.2–1.8 THz) of the mixtures of explosive 2,4-DNT and polyethylene(PE) at different ratios are obtained using reflection terahertz time domain spectroscopy (THz-TDS). The pronounced absorption peak of 2,4-DNT at 1.08 THz is always observed for the mixtures with 2,4-DNT ratios above 20%. Experimental results demonstrate that more applicable and realistic THz-TDS in reflection geometry can be used to distinguish explosive mixed with other material having no THz fingerprints, and has a high potential in the detection of explosives.

The Impact of Reflections From Stratified Building Materials on the Wave Propagation in Future Indoor Terahertz Communication Systems

In order to derive reliable propagation models for future terahertz indoor pico-cellular communication systems, accurate reflectivity data of building materials is necessary. Here we present reflection terahertz time domain spectroscopy (THz-TDS) measurements and matching transfer matrix simulations of the frequency dependent reflection coefficient of multi layer building materials in the frequency range from 100 to 500 GHz for a set of angles, both in TE- and TM-polarization. Two prominent stratified structures, a double pane window and white paint on plaster are investigated as they usually account for large areas in indoor environments. Communication systems located above 100 GHz are expected to be strongly affected by the variations of the reflectivity over the frequency and incident angle of such stratified materials as they will rely both on line of sight (LOS) and non line of sight (NLOS) propagation. We discuss this impact on the power distribution in a sample scenario employing the ray-tracing method.

Investigation of aqueous alcohol and sugar solutions with reflection terahertz time-domain spectroscopy

We give a detailed analysis of a general realization of reflection terahertz time-domain spectroscopy. The method is self-referenced and applicable at all incidence angles and for all polarizations of the incident terahertz radiation. Hence it is a general method for the determination of the dielectric properties of especially liquids in environments where transmission measurements are difficult. We investigate the dielectric properties in the 0.1-1.0 THz frequency range of liquids using reflection terahertz time-domain spectroscopy. We apply the technique for the determination of alcohol and sugar concentration of commercial alcoholic beverages and liquors. The special geometry of the experiment allows measurement on sparkling beverages.

Numerical phase correction method for terahertz time-domain reflection spectroscopy

We propose a numerical method for the misplacement phase error correction in terahertz time-domain reflection spectroscopy (THz-TDRS). The developed algorithm is based on the maximum entropy principle and can be readily implemented into data processing, allowing one to reveal material parameters of the opaque materials from the THz reflection measurements. The method resolves the phase retrieval problem in the THz-TDRS and dramatically simplifies the experimental procedure.

Measurement of optical properties of highly doped silicon by terahertz time domain reflection spectroscopy

Optical properties of doped silicon wafers have been measured by means of terahertz time domain reflection spectroscopy. A method is proposed to obtain the relative phase by reflection accurately. By using this method, the relative phase is obtained within an error of less than 10 mrad at 1 THz. The experimentally obtained complex conductivity of relatively high-doped silicon (ρ=0.136 Ω cm) in the terahertz region agrees with the simple Drude model.