Low-frequency Terahertz Research Articles

Computational study of IR, Raman, and NMR spectra of 4-methylmethcathinone drug.

Molecular electronic structure, IR, UV, and NMR spectra of the most popular cathinone, known as mephedrone or 4-methylmethcathinone (4-MMC), is studied thoroughly by quantum chemical calculation in terms of the density functional theory (DFT). Geometry optimization of 4-MMC and its hydrochloride complex is performed with the B3LYP functional, and all vibrational frequencies are analyzed in all details. On this background, the IR and Raman spectra are interpreted. The importance of low-frequency terahertz and Raman spectra is stressed for distinguishing of various MMC isomers. The UV spectrum is calculated by time-dependent DFT method which allows complete interpretation of intense absorption bands at 270 and 210nm as combinations of various ππ*, nπ*, and charge transfer excitations in amino-phenyl moieties. Very informative analysis of UV absorption and NMR spectra provides useful details on the structure-activity relationship for mephedrone molecule.

Wide temperature range studies of the low frequency THz spectrum of benzoic acid using FTIR spectroscopy

Fourier Transform Infrared Spectroscopy was employed to measure the low frequency terahertz transmission spectrum of benzoic acid over a wide temperature range. One broad absorption peak was observed at room temperature but more peaks appeared as the sample was cooled. All the peaks shifted to higher frequencies with the decrease in temperature and they displayed a rate of change in the range of (2–5.6) x 10−2cm−1K−1. An anomalous behavior was observed in the peak width and the peak intensity for some absorption peaks and this was attributed to the peaks being composite. There is a good agreement between the model of benzoic acid crystal and the experiment. The QUANTUM ESPRESSO code utilizing the PBEsol functional proved to be slightly better than the GAUSSIAN 03 method which employed NC-PBE functional in predicting the absorption peaks. A comparison was made of these results and the previous studies done using the THz-TDS. These results proved the ability of the FTIR technique in performing low frequency THz measurements.

Long propagating velocity-controlled Einstein's mirror for terahertz light conversion.

We show that Einstein's relativistic mirror with long (hundreds of µm) propagation distance and controllable propagation velocity can be implemented in the form of a dense free carrier front generated by multiphoton absorption of tilted-pulse-front femtosecond laser pulses in a dielectric or semiconductor medium. The velocity control is achieved by varying the pulse front tilt angle. Simulations demonstrate that such fronts can serve as efficient Doppler-type converters of terahertz pulses. In particular, the pulse reflected from a front, generated by three-photon absorption of a Ti:sapphire laser in ZnS, can exhibit strong (up to more than an order of magnitude) pulse compression and spectrum broadening without a noticeable amplitude change. The proposed technique may be used to convert strong low-frequency terahertz pulses, generated by optical rectification of tilted-pulse-front laser pulses, to desirable temporal and spectral characteristics for a variety of applications.

Efficient Wide-Band Large-Angle Refraction and Splitting of a Terahertz Beam by Low-Index 3D-Printed Bilayer Metagratings

Near-perfect anomalous reflection and refraction have been demonstrated using passive lossless metasurfaces and metagratings operating at microwave, infrared, and visible frequencies, while related studies at terahertz frequencies are lacking. Here we propose low-index (with a refractive index of 1.57) 3D-printed dielectric metagratings for efficient wide-band diffraction engineering at low terahertz frequencies. A simplified analytical model reveals that the number of propagating waveguide modes inside the grating is a key factor in diffraction engineering, and is insufficient in a low-index design regardless of the detailed dimensions in a period. Additional waveguide modes are introduced in asymmetric bilayer and trilayer metagratings, providing sufficient degrees of freedom for efficient large-angle anomalous refraction and beam splitting. These metagratings are inherently wide-band, benefiting from low dispersion of the waveguide modes. Three metagratings are designed, 3D printed, and tested experimentally at 0.14 THz for ${70}^{\ensuremath{\circ}}$ refraction, ${80}^{\ensuremath{\circ}}$ refraction, and $\ifmmode\pm\else\textpm\fi{}{70}^{\ensuremath{\circ}}$ beam splitting. The measured efficiency shows good agreement with the design. The proposed metagratings, with simple structures and large feature sizes, can be easily scaled to applications at higher terahertz frequencies.

Assessing the Performance of Density Functional Theory Methods on the Prediction of Low-Frequency Vibrational Spectra

The low-frequency (terahertz) dynamics of condensed phase materials provide valuable insight into numerous bulk phenomena. However, the assignment and interpretation of experimental results require computational methods due to the complex mode types that depend on weak intermolecular forces. Solid-state density functional theory has been used in this regard with great success, yet the selection of specific computational parameters, namely the chosen basis set and density functional, has a profound influence on the accuracy of predicted spectra. In this work, the role of these two parameters is investigated in a series of organic molecular crystals, in order to assess the ability of various methods to reproduce intermolecular forces, and subsequently experimental terahertz spectra. Specifically, naphthalene, oxalic acid, and thymine were chosen based on the varied intermolecular interactions present in each material. The results highlight that unconstrained geometry optimizations can be used as an initial proxy for the accuracy of interatomic forces, with errors in the calculated geometries indicative of subsequent errors in the calculated low-frequency vibrational spectra, providing a powerful metric for the validation of theoretical results. Finally, the origins of the observed shortcomings are analyzed, providing a basic framework for further studies on related materials.

Design of an optical biosensor based on photonic crystal at THz frequency

We discuss the transmission characteristics of photonic crystal with two defect illuminated by a continuous wave of Transverse Electric (TE) mode at low terahertz frequency. To study its performance we solve numerically the corresponding Maxwell equations by means of finite difference frequency domain method. By varying the angle of incident, width of defect layer and refractive index in the second defect layer, it is found that the device exhibits a significant transmission enhancement at two wavelengths with typical changes. We also demonstrate its potential applications for biosensor platform

Low-Terahertz Transparent Graphene-Based Absorber.

A new, transparent, metal-free absorber, based on the use of multilayer graphene/dielectric laminates (GLs), is proposed for applications in the low-terahertz frequency range. The designed absorber has a total thickness of around 70 µm and consists of a front matching dielectric layer followed by a GL, a dielectric spacer and a back GL. The laminates are periodic structures constituted of graphene sheets separated by 50-nm-thick polyethylene terephthalate (PET) interlayers, while the matching layer and the spacer are one-quarter-wavelength thick and made of PET. The GLs are modeled as homogeneous-equivalent single layers (ESLs) characterized by their sheet resistances Rs. An innovative analytical method is proposed in order to select Rs values optimizing the electromagnetic wave absorption either in low-gigahertz or low-terahertz frequency range. The frequency spectra of the absorption, reflection and transmission coefficients are computed in the range up to 4 THz by using different values of Rs. Then, realistic Rs values of chemically doped graphene monolayers over PET substrates are considered. The designed absorbers are characterized by an absorption coefficient with a peak value of about 0.8 at the first resonant frequency of 1.1 THz, and a 1.4 THz bandwidth centered at 1.5 THz with reflection coefficient below - 10 dB. Moreover, the optical transmittance of the proposed absorbers are computed by means of the optical matrix theory and it is found to be greater than 86% in all the visible ranges.

Conical versus Gaussian terahertz emission from two-color laser-induced air plasma filaments.

In this Letter, we demonstrate that the far-field terahertz (THz) beam generated from a Ti:Sapphire two-color laser-induced filament can exhibit a conical or Gaussian distribution, depending on the filtering experimental conditions. Using both an incoherent Golay cell detector and a two-dimensional coherent electro-optic detection covering the 0.2-2.6 THz spectral range, in our experimental conditions, we provide evidence that the conical emission is due to photo-induced carriers in the silicon filter, typically used to block the remaining pump laser light. Moreover, the low-frequency THz beam retrieves an almost $ {{\rm TEM}_{00}} $TEM00 Gaussian spatial distribution when the silicon filter is preceded by a large bandgap ceramic filter, which stops the pump beam, thus preventing the carrier generation in the silicon filter.

Properties of omnidirectional gap and defect mode of one-dimensional graphene-dielectric periodic structures

We theoretically investigate the optical response of one-dimensional graphene based photonic crystal (1D-GPC) structures with and without defect layer in the low terahertz (THz) frequency region. The 1D-GPC is composed of alternated layers of isotropic dielectric material (SiO2) and graphene monolayer sheets. Using the transfer matrix method (TMM), the effects of several parameters such as the incidence angle, width, thickness, and refractive index of the defect layer and the chemical potential of graphene sheets on the device response will be explored in detail for both TE and TM polarizations. Our simulation results indicate that when a defect layer is introduced, the structure may support two kinds of defect modes. One kind is created in the Bragg gap and the other one is created in the graphene-induced photonic band gap (GIPBG). Our results reveal that a defect mode can appear within the GIPBG only for an optical thickness of the defect layer greater than that of the alternated dielectric layers of the 1D-GPC. While, the defect modes in the Bragg gap may be created by simply breaking of the periodicity of the dielectric lattice. In addition, the resonance frequency of defect mode in the GIPBG is found to be almost insensitive to the incident angle for both polarizations. Moreover, we show that an effective Brewster angle for our structure can be deduced from the variation of the amplitude of defect mode with the incident angle. Our analysis has shown an innovative idea for the realization of tunable broadband reflectors and narrowband filtering devices.

Six-Billion-Fold Amplification via a Two-Stage Terahertz Parametric Amplifier

We have been using the principles of injection-seeded terahertz parametric generation (TPG) to develop the terahertz parametric amplifier (TPA) since 2014. Although we previously achieved amplification of 55 dB at 1.6 THz, amplification of low-frequency terahertz waves (≤1 THz) or ultraweak inputs was difficult, because broadband terahertz waves (spontaneous TPG) were generated and interfered with amplification. Thus, we herein present a new technique that divides terahertz-wave amplification into two steps: preamplification and main amplification. In this configuration, the terahertz wave is upconverted to a near-infrared (NIR) beam. Then, the noise is removed under the NIR condition and converted again to terahertz waves by the main amplifier. This two-stage TPA affords an extremely high amplification factor (up to six-billion-fold; 98 dB) for an ultraweak input of 500 zJ (zJ = 10−21 J). This is the first amplifier that has such a high gain for sub-aJ order input in the terahertz region.

Structure-based tunable metamaterials for electromagnetically induced transparency windows in low terahertz frequency

The electromagnetically induced transparency (EIT) metamaterials with active or passive modulation have been extensively studied and applied in slow-light devices, light on-off, and light storage. However, the preparation complexity and characterization difficulties of the EIT metamaterials limit their optoelectronic applications. Here, we have employed a structure-based tunable design to fulfill the passive modulation of EIT metamaterial. We propose a simple but effective EIT structural model composed of a cutting wire and two circular split rings. Through theoretical analysis and computational simulation, it is found that the localized surface plasma and the inductive-capacitive resonance mode coincidently contribute to the EIT effect, giving rise to a narrow transparency window. Moreover, the amplitude of the transparency peak gradually decreases at the same resonance frequency with the increment of the lateral distance between the wire and split ring, while it decreases proportionally with the increment of the radius of the split ring. Interestingly, when the radius continues to increase, the asymmetry of the EIT window aggravates and the second EIT broad transparency window appears, indicating the existence of another intriguing coupling mechanism. Our work unveils a simple and practical modulation strategy for EIT-based multifunctional optical devices and shed light on its potential application on the optical devices.

Adaptive Multichannel Terahertz Communication by Space-Time Shared Aperture Metasurfaces

We propose a technique for adaptive multichannel communication in the low-terahertz frequency regime through simultaneous and independent multifrequency multibeam scanning via a single time-modulated metasurface consisting of graphene micro-patch antennas whose Fermi energy levels are modulated by radio-frequency biasing signals. For this purpose, we divide the metasurface aperture into interleaved orthogonally modulated sub-array antennas with distinct modulation frequencies, rendering a shared aperture in space-time. The higher-order frequency harmonics generated by the sub-arrays in such a space-time shared-aperture metasurface are mutually orthogonal as they do not yield an observable interference pattern. A distinct constant progressive modulation phase delay can be adopted in each sub-array to independently scan its corresponding higher-order frequency harmonics via dispersionless modulation-induced phase gradient with minimal sidelobe level and full angle-of-view over a wide bandwidth. The number of sub-arrays and distinct channels can be scaled easily without suffering from cross-talk due to orthogonality of the channels. The concept is established theoretically and verified through numerical simulations. We characterize active beam-scanning performance of the space-time shared aperture metasurface in terms of gain and half power beamwidth and their dependence to the array architecture. We consider both one-dimensional and two-dimensional interleaving for scanning the beams along elevation and azimuth angles. We also obtain the upper bounds for the number of independent channels attainable without compromising angle-of-view as 8 and 64 for one-dimensional and two-dimensional interleaving, respectively within the same physical aperture. Our results point toward high-capacity platforms with low size, weight and power for next generations of terahertz communication systems.

A Study of Low-Energy Processes in Polymethyl Methacrylate and Its Short-Chain Oligomers by Terahertz IR and Raman Spectroscopies

Based on the data of low-frequency (terahertz) IR and Raman spectroscopies, it is established that low-energy vibrational excitations in PMMA and its oligomers with a degree of polymerization n = 2, 7, 9, and 50 are due to correlated small-angle torsional–vibrational dynamics, which prepares the relaxational mobility of macromolecules. The minimum size of the chain segment (“torsional–vibrational segment”), the spectrum of which is identical to the spectrum of the high-molecular polymer, is determined.

Broadband Terahertz Polarizing Beam Splitter Based on a Graphene-Based Defective One-Dimensional Photonic Crystal

Seeking operative terahertz (THz) devices has always stimulated considerable attention. Of particular interest is the THz beam splitter. Here, a tunable THz polarizing beam splitter (PBS) is proposed based on a graphene-embedded quarter-wave stack with a central defect layer of air. The spectral performance of the structure is investigated by the transfer matrix method. It is found that the electromagnetic waves can be decomposed into two separate polarized waves at incident angles greater than the critical angle. Furthermore, it is shown that a new kind of Brewster angle is found at the low THz frequencies due to the existence of the graphene nano-layers. The appearance of this angle which we call it the graphene induced Brewster angle results in the separation of TM- and TE- polarized waves at the low THz frequencies (f 200 dB) for TM waves with frequency f <; 2 THz is achieved by increasing the chemical potential to 0.5 eV. Moreover, this structure can exhibit extremely high extinction ratio for TE waves with high THz frequencies. Finally, the degree of polarization equals to one is reported for the PBS proposed here. This structure offers the opportunity to realize a high-efficiency PBS with very high extinction ratios at the broadband THz frequency.

Low-frequency Vibrational Modes in Small Polypeptides of Essential Amino Acids

The length-dependent low-frequency terahertz absorption spectrum of the essential amino acid chains has been investigated. Since this special type of amino acids cannot be synthesized inside the human body, the feasibility of noninvasively monitoring their deficiency is of significant importance. Here, the considered chains consist of two to six identical essential amino acids forming homogeneous polypeptides. The terahertz vibrational modes are calculated for different chain sizes of these polypeptides. The low-frequency terahertz spectra show unique and specific absorption peaks. Several fascinating patterns for the chain-size-dependent position of the peaks are reported here that are feasible of being applied for their noninvasive detection. For instance, different shared and identical characteristics peaks are observed for the specific polypeptides which are independent of the chain-size. Furthermore, it is shown that the observed trends for the maximum populated peak in some of the essential polypeptides can be considered as a possible footprint for in the terahertz spectrum.

A finite-element-based coarse-grained model for global protein vibration

Protein mechanical vibrations play a pivotal role in biological activity. In particular, low-frequency (terahertz) modes are related to protein conformational changes, which represent the foundations for a correct protein functionality. Relying on the fact that such low-frequency motions involve large protein portions, thus modeling of local details is not necessary, coarse-grained models have proven their efficacy in capturing the essential dynamic behavior. In this paper, we show that a coarse-grained finite element space truss model is suitable for investigating protein vibrations. Hen egg-white lysozyme is selected as a case study and modal analysis is performed in order to investigate the protein dynamics; the influence of interaction cutoff values on optimal force constant, obtained vibrational frequencies and mode shapes is also explored. The validity of the structural model is demonstrated by comparing the calculated B-factors with the experimental ones. Moreover, from the methodology framework the truss model is shown to be consistent with the well-known anisotropic network model and this has been confirmed by the obtained results. The proposed truss model is then believed to be a simple yet powerful tool to investigate protein dynamics, and it could also be used to analyze conformational changes and protein stability from a Structural Mechanics viewpoint.

Complex Refractive Index of Strontium Titanate in the Terahertz Frequency Range

The recent progress in terahertz time-domain spectroscopy enables the accurate and reliable measurements of dielectric properties in comparison with the traditional far-infrared spectroscopy using an incoherent light source. The broadband THz-TDS is a powerful tool to determine the real and imaginary parts of a complex dielectric constant by the transmission which allows to detect the parameters of the soft modes in ferroelectrics. In this work, the terahertz time-domain spectroscopy was used to investigate the dependence of the complex refractive index of a single-crystal quantum paraelectric strontium titanate in the terahertz frequency range from 0.3 to 2 THz. It was shown that the low-frequency terahertz response of the material is determined by the soft phonon mode TO1. The measured experimental dependences showed a good agreement with the theoretical curves obtained from the analysis of the Lorentz oscillator model for the complex dielectric constant of strontium titanate. The obtained results are necessary for understanding the principle of possibility to manipulate the order parameter in ferroelectric materials and can be used to create energy-efficient memory devices with a speed of recording information close to the theoretical limit.

Terahertz Attenuation in Snow and Sleet

The attenuation of terahertz (THz) radiation by snow aggregates and sleet is theoretically investigated using the Mie theory approach for electromagnetic radiation. Differences in attenuation spectra between rain, sleet, and dry snow are identified, and the difficulty of measuring THz attenuation by dry snow aggregates is highlighted and discussed in detail. At lower THz frequencies (~ 0–0.4 THz), it is suggested that water vapor may be the leading attenuator for instances of dry snowfall. A comparison between theoretical predictions and recent measurements is presented.

High-sensitivity photoconductive detectors with wide dipole electrodes for low frequency THz wave detection

Photoconductive detectors (PCDs) with wide dipole electrodes were fabricated on low temperature-grown GaAs for observation of low frequency terahertz electromagnetic waves. The screening effect in PCDs caused by the space charge of photoexcited electron-hole pairs brings poor sensitivity in detection. This can be effectively reduced by widening the dipole electrodes in PCDs. The width dependence of terahertz (THz) wave detection properties of PCDs showed that not only the sensitivity but also the signal-to-noise ratio can be fairly improved by increasing the width, especially at frequencies lower than 1.2 THz.

Low frequency terahertz spectrum of acetylsalicylic acid over a wide temperature range investigated by FTIR spectroscopy.

The temperature dependence of the terahertz spectrum of acetylsalicylic acid over an extended temperature range as well as the theoretical modeling results are reported. Several absorption bands which were not observed at room temperature emerged as the sample was cooled down to 14.9 K. All the bands shifted to higher energies upon cooling. The bands which exhibited an anomalous behavior in the frequency shift have a rate of change in the range (4-5) × 10-3 cm-1 K-1-1 while bands which displayed an expected behavior have the rate of change in the range (1-3) × 10-2 cm-1 K-1-1. There is a fluctuation in both the intensity and the width of the bands implying composite bands. There is no fundamental correspondence between the frequency shifts, band widths and band intensities hence making analysis of all the three variables necessary to understand the extent to which the bands have been resolved. A good agreement was observed between the experiment and the model of acetylsalicylic acid crystal. The model of an acetylsalicylic acid crystal with PBEsol DFT functional proved to be better than the previously used model of an acetylsalicylic acid dimer using a B3LYP/6-31++G** DFT functional.