To Terahertz Research Articles

Investigation of Dielectric Properties of Polymers and their Discrimination Using Terahertz Time-Domain Spectroscopy with Principal Component Analysis.

Polymers are among the most commonly used materials in our everyday life. They are generally transparent to terahertz (THz) radiation, but are quite difficult to differentiate using optical techniques as few or no characteristic features exist in the spectral range of <2.0 THz for small and portable radiation systems. In this work, we report experimental measurement of refractive indices and absorption coefficients of styrene acrylonitrile (SAN) and Bakelite in the spectral range of 0.2-2.0 THz for the first time. Additionally, we demonstrate that by combining principle component analysis (PCA) with THz time-domain spectroscopy one can differentiate such polymers. In this analysis, the first three principle components PC1, PC2, and PC3 depict >94% variance with a distribution of 72.45%, 11.52%, and 9.38%, respectively.

The spectral analysis of fuel oils using terahertz radiation and chemometric methods

The combustion characteristics of fuel oils are closely related to both engine efficiency and pollutant emissions, and the analysis of oils and their additives is thus important. These oils and additives have been found to generate distinct responses to terahertz (THz) radiation as the result of various molecular vibrational modes. In the present work, THz spectroscopy was employed to identify a number of oils, including lubricants, gasoline and diesel, with different additives. The identities of dozens of these oils could be readily established using statistical models based on principal component analysis. The THz spectra of gasoline, diesel, sulfur and methyl methacrylate (MMA) were acquired and linear fittings were obtained. By using chemometric methods, including back propagation, artificial neural network and support vector machine techniques, typical concentrations of sulfur in gasoline (ppm-grade) could be detected, together with MMA in diesel below 0.5%. The absorption characteristics of the oil additives were also assessed using 2D correlation spectroscopy, and several hidden absorption peaks were discovered. The technique discussed herein should provide a useful new means of analyzing fuel oils with various additives and impurities in a non-destructive manner and therefore will be of benefit to the field of chemical detection and identification.

Tunable Graphene Metasurfaces with Gradient Features by Self‐Assembly‐Based Moiré Nanosphere Lithography

Patterned arrays of graphene nanostructures, also referred as graphene metasurfaces, have proven to be capable of efficiently coupling with incident light by surface plasmon resonances. In this work, a new type of graphene metasurfaces with moiré patterns using cost‐effective and scalable moiré nanosphere lithography (MNSL) is demonstrated. A large gradient in feature size (i.e., from sub‐200 nm to 1.1 μm) of the graphene nanostructures exists in single metasurfaces. The in‐plane quasi‐periodic arrangement of the graphene nanostructures can be easily tuned to form a variety of moiré patterns. The experimental measurement and numerical simulations show that the graphene moiré metasurfaces support tunable and multiband optical responses due the size and shape dependences of surface plasmon resonance modes of graphene nanostructures. It is also demonstrated that the multiband optical responses of graphene moiré metasurfaces can be tuned from mid‐infrared (MIR) to terahertz (THz) regimes by choosing polystyrene spheres of different sizes for MNSL. These findings provide a cost‐effective and scalable strategy to achieve ultrathin functional devices, including multiband light modulators, broadband biosensors, and multiband photodetectors, which feature tunable and multiband responses in wide range of wavelengths from MIR to THz.

Low-Loss and Low-Dispersion Transmission Line Over DC-to-THz Spectrum

Transmission lines or waveguides are the most fundamental building blocks of all electronic and photonic circuits and systems. Efforts have been made to incrementally evolve and improve existing transmission line structures to meet the increasingly stringent demands for signal transmission bandwidth and performance. However, a potentially revolutionary scheme or disruptive concept is required in support of future technological needs and bridging the gap between electronics and photonics. In this paper, we report on a fully integrated transmission line with simple structure that overcomes the long-standing bottleneck problems of high attenuation, strong dispersion, and low mode confinement in the guided-wave signal transmission from dc to terahertz (THz). This so-called mode-selective transmission line (MSTL) supports super-broadband and/or ultrafast pulse signal propagation, making it a disruptive solution for building future high-performance analog and digital integrated electronics and photonics. To demonstrate this scheme, an MSTL on fused silica substrate is designed, fabricated, and experimentally measured from near-dc to 0.5 THz, showing less than 0.35 dB/mm attenuation and low dispersion characteristics over the entire frequency range.

THz electromagnetic radiation driven by intense relativistic electron beam based on ion focus regime

The simulation study finds that the relativistic electron beam propagating through the plasma background can produce electromagnetic (EM) radiation. With the propagation of the electron beam, the oscillations of the beam electrons in transverse and longitudinal directions have been observed simultaneously, which provides the basis for the electromagnetic radiation. The simulation results clearly show that the electromagnetic radiation frequency can reach up to terahertz (THz) wave band which may result from the filter-like property of plasma background, and the electromagnetic radiation frequency closely depends on the plasma density. To understand the above simulation results physically, the dispersion relation of the beam-plasma system has been derived using the field-matching method, and the dispersion curves show that the slow wave modes can couple with the electron beam effectively in THz wave band, which is an important theoretical evidence of the EM radiation.

Longgu (Fossilia Ossis Mastodi) alters the profiles of organic and inorganic components in Keishikaryukotsuboreito.

Longgu (Fossilia Ossis Mastodi) is a non-botanical crude drug, defined as "the ossified bone of large mammal" in the Japanese Pharmacopoeia sixteenth edition (JP16). It is a non-reproducible drug and is now facing the threat of exhaustion. To solve this problem, we aimed to clarify the role of longgu in Kampo prescriptions, which has not yet been scientifically ascertained. In this study, we focused on decoction of Keishikaryukotsuboreito (KRB). The profile of inorganic and organic components in the extract was analyzed by inductively coupled plasma mass spectrometry (ICP-MS) and gas chromatography flame ionization detection (GC/FID), respectively. Twenty-five elements were detected by ICP-MS in KRB and longgu-free KRB (KB) decoctions. However, 23 elements were detected in unadultrated longgu (R) decoctions, and their total amount was 30 times lower than those of the KRB and KB decoctions were. No organic compounds were detected in R decoctions by GC/FID, though many were detected in KRB and KB decoctions. KRB decoctions were distinguished from KB decoctions by multivariate analysis. The only difference in the crude drugs was the presence of longgu, and therefore the difference in the profiles was attributed to the effect of longgu. Longgu was submitted to terahertz (THz) wave spectrometry after the decocting process. The THz spectra indicated that longgu adsorbed compounds during the KRB decoction. These results suggested that longgu not only releases its components, but also adsorbs ingredients from other crude drugs during decoction.

Classification of THz pulse signals using two-dimensional cross-correlation feature extraction and non-linear classifiers

This work provides a performance comparison of four different machine learning classifiers: multinomial logistic regression with ridge estimators (MLR) classifier, k-nearest neighbours (KNN), support vector machine (SVM) and naïve Bayes (NB) as applied to terahertz (THz) transient time domain sequences associated with pixelated images of different powder samples. The six substances considered, although have similar optical properties, their complex insertion loss at the THz part of the spectrum is significantly different because of differences in both their frequency dependent THz extinction coefficient as well as differences in their refractive index and scattering properties. As scattering can be unquantifiable in many spectroscopic experiments, classification solely on differences in complex insertion loss can be inconclusive. The problem is addressed using two-dimensional (2-D) cross-correlations between background and sample interferograms, these ensure good noise suppression of the datasets and provide a range of statistical features that are subsequently used as inputs to the above classifiers. A cross-validation procedure is adopted to assess the performance of the classifiers. Firstly the measurements related to samples that had thicknesses of 2mm were classified, then samples at thicknesses of 4mm, and after that 3mm were classified and the success rate and consistency of each classifier was recorded. In addition, mixtures having thicknesses of 2 and 4mm as well as mixtures of 2, 3 and 4mm were presented simultaneously to all classifiers. This approach provided further cross-validation of the classification consistency of each algorithm. The results confirm the superiority in classification accuracy and robustness of the MLR (least accuracy 88.24%) and KNN (least accuracy 90.19%) algorithms which consistently outperformed the SVM (least accuracy 74.51%) and NB (least accuracy 56.86%) classifiers for the same number of feature vectors across all studies. The work establishes a general methodology for assessing the performance of other hyperspectral dataset classifiers on the basis of 2-D cross-correlations in far-infrared spectroscopy or other parts of the electromagnetic spectrum. It also advances the wider proliferation of automated THz imaging systems across new application areas e.g., biomedical imaging, industrial processing and quality control where interpretation of hyperspectral images is still under development.

Spin-polarized oscillations and 100% spin polarization plateau in an InAs quantum dot subjected to terahertz irradiation and a magnetic field

Spin polarization in an InAs quantum dot subjected to terahertz (THz) irradiation and external magnetic fields is theoretically discussed. Spin-polarized oscillations and spin polarization plateaus are obtained, depending on the relative magnitude between the external magnetic field strength and the THz photon energy. Furthermore, we show that the oscillation of spin polarization is also related to the THz irradiation amplitude. These features may be beneficial for low magnetic field detectors, ultrasensitive THz detectors or spin-selective applications.

Modeling of Magnetized Graphene From Microwave to THz Range by DGTD With a Scalar RBC and an ADE

This paper presents a discontinuous Galerkin time-domain (DGTD) method for the transient analysis of magnetized graphene from the microwave to terahertz (THz) frequencies. By considering the atom thick graphene layer as an infinitely thin conductive sheet with finite surface conductivity, a frequency-dependent anisotropic resistive boundary condition (RBC) is obtained. Based on this RBC, the direct volumetric discretization of graphene layer is avoided. Instead of directly deriving the numerical flux for DGTD considering the presence of this anisotropic and dispersive RBC, an auxiliary surface polarization current governed by a first-order time-dependent partial differential equation (PDE) is introduced over the graphene with the purpose to obtain an isotropic and simultaneously nondispersive RBC. In this way, the new formulated numerical flux expression derived from the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Rankine–Hugoniot jump relations</i> is isotropic, and no time-domain convolution is involved in the finalized matrix equations. To verify the applicability and accuracy of the proposed algorithm, the Faraday rotation and the surface plasmon resonance of a plane wave through magnetically biased graphene are investigated. For open-region scattering problems, a hybrid DGTD and time-domain boundary integral (TDBI) method is applied to rigorously truncate the computational domain.

Sub-terahertz and terahertz microstrip resonant-tunneling-diode oscillators

We present a theoretical analysis of traveling-wave microstrip resonant-tunneling-diode (RTD) oscillators. Such oscillators are similar to terahertz (THz) quantum-cascade lasers (QCLs) with a metal-metal waveguide and with just the active part of a single QCL period (an RTD) as their active core. Assuming realistic parameters of RTDs, we show that the microstrip RTD oscillators should be working at sub-THz and THz frequencies. Contrary to the contemporary THz QCLs, RTD microstrips are room-temperature oscillators. The major loss- and gain-enhancement mechanisms in RTD microstrips are identified.

Deep-subwavelength Guiding and Superfocusing of Spoof Surface Plasmon Polaritons on Helically Grooved Metal Wire

Deep-subwavelength guiding and superfocusing of spoof surface plasmon polaritons (SSPPs) realized on a helically grooved metal wire at microwave frequencies are presented in this paper. Two smooth bridges with gradient helical grooves decorated on the cylindrical and conical metal wire are proposed and designed, respectively. High-efficiency and broadband mode conversion from the traditional guided waves to the SSPPs and superfocusing of SSPPs are reported. Numerical simulations quantitatively show that the amplitudes of electric field at the tip of the conical wire with gradient helical grooves can be magnified 50 times more than that of the input signal in broadband. Moreover, the second transition structure ensures that the depth of helical groove can be tuned flexibly and arbitrarily, making it compatible with all kinds of N-type coaxial connectors. Strong field concentration and superfocusing of these two structures can be easily extended to terahertz (THz) frequencies by tuning the geometrical parameters and can find important applications in sensing, spectroscopy and near-field imaging in the microwave and THz frequencies.

Electrically controlling the polarizing direction of a graphene polarizer

We theoretically demonstrate a polarizer with an electrically controllable polarizing direction in the far infrared range using two orthogonal periodic arrays of graphene ribbons, which have different widths and are supported on a dielectric film placed on a thick piece of metal. The operation mechanism originates from the polarization-dependent resonant absorption of the two orthogonal graphene ribbons, which can be respectively controlled with different external bias voltages. The operation wavelength can be expanded to terahertz (THz) radiation.

Liquid-sheet jets for terahertz spectroscopy.

We investigated liquid-sheet jets with controllable thickness for application to terahertz (THz) spectroscopy. Slit-type and colliding-jet nozzles were used to generate optically flat liquid jets. The thickness of the liquid sheet was determined precisely by spectral interference and THz time-domain-spectroscopy methods. By adjusting the collision angle of the colliding-jet nozzle, we could control the thickness of the liquid sheet from 50 to 120 μm.

Antenna Array Excited by Spoof Planar Plasmonic Waveguide

This letter presents an efficient and simple way to excite a microstrip patch antenna array in K-band. This array is formed by a spoof planar plasmonic waveguide for excitation and several circular patches for radiation. The optimized proposed 2 × 4 patch array, operating in 18.1 ~ 19.4 GHz, features an average gain of 15.2 dBi with a maximum of 16.5 dBi at 18.2 GHz. An experiment was performed, and the measured results prove validity of the design. This method opens up a new kind of feeding method for patch antenna array, which can be broadly engineered in millimeter-waves (MMW) region even to terahertz (THz) region.

Terahertz generation and detection with InGaAs-based large-area photoconductive devices excited at 1.55 μm

We report on scalable large-area terahertz emitters and detectors based on In0.53Ga0.47As/In0.52Al0.48As heterostructures for excitation with 1.55 μm radiation. Different geometries involving three different electrode gap sizes are compared with respect to terahertz (THz) emission, bias field distribution, and Joule heating. The field distribution becomes more favorable for THz emission as gap size increases, while Joule heating exhibits the opposite dependence. Devices with three different gap sizes, namely 3 μm, 5 μm, and 7.5 μm, have been investigated experimentally, the emitter with a gap size of 7.5 μm showed the best performance. The scalable devices are furthermore employed as detectors. The scalable electrode geometry enables spatially integrated detection, which is attractive for specific applications, e.g., where an unfocused THz beam has to be used.

Prediction of oven-dry density of wood by time-domain terahertz spectroscopy

Abstract Wood is relatively transparent to terahertz (THz) radiation with wavelengths in the submillimeter range. This radiation has a high potential for sensing and imaging wood with a good spatial resolution. THz is especially sensitive to moisture content, fiber alignment, and density – all of which are critical in the manufacturing of wood products. In this work, a systematic study was undertaken on 46 very different wood species by means of THz time-domain spectroscopy with density determination in focus. The dielectric response of wood was modeled based on the Maxwell-Garnett effective medium theory. The dielectric function of the cell wall material was found to be extremely consistent over this large number of species with very different properties. This renders possible to determine wood density by THz time-domain spectroscopy. A strong correlation between the measured and predicted densities has been observed for all the samples investigated.

Continuous narrow-band amplification tuning effect at terahertz frequencies in double-quantum-well resonant tunneling nanostructures

High-frequency response properties of single and double-quantum-well resonant tunneling diodes (RTD) are examined in a wide frequency range, up to terahertz (THz) frequencies, on the base of proposed quantum theory. Numerical solutions of time-dependent Schrödinger equations with open-system boundary conditions in an external electromagnetic field are performed. The numerical solutions take into account the influence of bias DC voltage on electronic states in RTD with finite height and width of barriers both for monoenergetic and for Fermi-distributed electrons in emitter and collector parts of structures. We show that the presence of an additional level in double-quantum-well structures breaks the response symmetry and leads to selective narrow-band frequency amplification, as well as to the effect of amplification frequency tuning at THz frequencies by variation of applied bias voltage. These phenomena predict an increase of gain coefficient and open new perspectives for engineering of novel types of THz oscillators and other high-frequency units.

A MEMS Photoacoustic Detector of Terahertz Radiation for Chemical Sensing

Abstract A piezoelectric Microelectromechanical system (MEMS) cantilever pressure sensor was designed, modeled, fabricated, and tested for sensing the photoacoustic response of gases to terahertz (THz) radiation. The sensing layers were comprised of three thin films; a lead zirconate titanate (PZT) piezoelectric layer sandwiched between two metal contact layers. The sensor materials were deposited on the silicon device layer of a silicon-on-insulator (SOI) wafer, which formed the physical structure of the cantilever. To release the cantilever, a hole was etched through the backside of the wafer and the buried oxide was removed with hydrofluoric acid. Devices were then tested in a custom made THz vacuum test chamber. Cantilever deflection was observed with a laser interferometer in the test chamber and preliminary data indicates the signals were caused by the photoacoustic effect. Future device data will also include the piezoelectric voltage signal analysis.

Wavelength-Independent Optical Polarizer Based on Metallic Nanowire Arrays

We propose a design of high-performance wavelength-independent optical polarizer for ultraviolet/visible to terahertz (THz) spectral region based on metallic nanowire (NW) arrays on ultrathin substrates, which can reduce the influence of the substrate material. This NW-based polarizer is anticipated to have far-reaching impact on numerous existing devices including polarimetric imagers in the mid-infrared and THz domains, complementary metal-oxide semiconductor (CMOS) imagers, and DVD/CD players.

Gas-Phase Synchrotron FTIR Spectroscopy of Weakly Volatile Alkyl Phosphonate and Alkyl Phosphate Compounds: Vibrational and Conformational Analysis in the Terahertz/Far-IR Spectral Domain

The high brilliance of the AILES beamline at the SOLEIL synchrotron facility has been exploited for the study of the gas-phase vibrational spectra of weakly volatile organophosphorous compounds. The propagation of the synchrotron radiation in long path length gas cells allowed improvements in the sensitivity limits and spectral coverage compared with a previous study, performed by our group with conventional thermal sources. A ppm level detection in the entire IR domain up to terahertz (THz) frequencies has been realized for dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), triethyl phosphate (TEP), and diethyl (2-methylallyl)phosphonate (DEMaP). In the present study, the assignment of the gas-phase vibrational and the conformational analysis of the two most stable conformers of DMMP and TMP have been extended to the torsional THz spectra in the 20-120 cm(-1) range. The improvement of the S/N ratio below 600 cm(-1) has permitted for the first time a gas-phase conformational analysis of the two weakly volatile and highly flexible TEP and DEMaP compounds. The experimental far-infrared (FIR)/THz spectra have been studied taking into account four low-energy conformers determined by means of high level of theory quantum chemistry calculations. Finally, due to its particularly low vapor pressure, the detection of gas-phase tributyl phosphate (TBP) in the FIR domain was unsuccessful. Nevertheless, the mid-IR/near-IR spectra of TBP recorded in a multipass cell heated to 355 K have been assigned with the harmonic vibrational predictions of the most stable conformer.