Electromagnetic Terahertz Research Articles

Vertical versus planar pulsed photoconductive antennas that emit in the terahertz regime

The design process of a photoconductive antenna (PCA), which emits efficiently in the electromagnetic terahertz range, demands some considerations that are discussed through this work. In this work, several essential characteristics of a photoconductive antenna made with LT-GaAS are studied by means of well established commercial software (COMSOL 5.3 [1]). An approach to the efficiency is also made through the study of geometry, the laser illumination position, the substrate doping distribution, the direction of the bias applied to the semiconductor, the matching impedance at the laser operating frequency and, finally, the plasmonics effects or penetration laser enhancement due to the use of nano antennas. We study and compare two kinds of structures, one which is quasi-bidimensional or planar and the other which is vertical. Additionally, the photoconductive antennas are also modeled by using a simplified equivalent circuit which helps to understand the antennas’ performance. Therefore, some fundamental parameters, like the transient capacitance between the metal contacts are also studied. Furthermore, we introduce an optimized vertical design which achieves the best results.

Terahertz Focusing and Polarization Control in Large-Area Bias-Free Semiconductor Emitters

We show that, when large-area multiplex terahertz semiconductor emitters, that work on diffusion currents and Schottky potentials, are illuminated by ultrashort optical pulses they can radiate a directional electromagnetic terahertz pulse which is controlled by the angular spectrum of the incident optical beam. Using the lens that focuses the incident near-infrared pulse, we have demonstrated THz emission focusing in free space, at the same point where the optical radiation would focus. We investigated the beam waist and Gouy phase shift of the THz emission as a function of frequency. We also show that the polarization profile of the emitted THz can be tailored by the metallic patterning on the semiconductor, demonstrating radial polarization when a circular emitter design is used. Our techniques can be used for fast THz beam steering and mode control for efficiently coupling to waveguides without the need for THz lenses or parabolic mirrors.

Terahertz Imaging and Electromagnetic Model of Axon Demyelination in Alzheimer's Disease

We investigate the utility of terahertz (THz) spectroscopy in identifying Alzheimer's disease in human brain tissue. Using reflection-mode time-domain THz spectroscopy, two-dimensional images of formalin-fixed and paraffin-embedded tissue samples of the hippocampus area were recorded in the 60 GHz-2 THz band. The THz images were compared with microscopic images of the same samples after hematoxylin and eosin staining. We demonstrate that the THz reflection spectra, particularly from white matter, reveal detectable differences between postmortem brain tissues exhibiting Alzheimer's disease and normal controls. The THz reflectivity of white matter areas was on average 4.2% higher than that of gray matter regions in tissue samples with known Alzheimer's history, whereas for normal control samples the contrast was only 2%. Additional studies further enhance this hypothesis, and the demyelination of white matter in Alzheimer's tissues suggests a possible cause for the differences in the THz reflection spectra. We also compare the THz response of the tissues with conventional Luxol-fast blue staining to demonstrate the correlation between the two methods for predicting demyelination. In addition, we present a simplified electromagnetic model of white matter axons exhibiting various degrees of demyelination to further support this hypothesis through full-wave electromagnetic simulations. This study offers, for the first time, proof of concept for the feasibility of detecting Alzheimer's disease using THz spectroscopy on ex vivo samples.

A Theoretical Description for Elliptical Plasma Antenna Response in Presence of Terahertz Electromagnetic Plane Wave Based on Surface Plasmon Concept

Excitation of THz surface plasmons due to the scattering of transverse magnetic (TM) wave by an elliptical plasma antenna consisting of a metallic rod with dielectric coating is investigated. The incident, transmitted, and scattered electromagnetic waves are expressed in terms of Mathieu and modified Mathieu functions. The geometrical structure and its effect on the scattering cross-section and resulting resonance frequencies are studied and analyzed based on theory of Fabry-Perot resonator. It is shown that the resonance frequency can be varied by changing the geometrical structure and material used between the surfaces of this structure.

Stimulation of the Filling of Bone Defects by Terahertz Electromagnetic Irradiation

Studies in an experimental group of 10 mongrel dogs addressed the stimulation of tibial defect filling by terahertz electromagnetic irradiation using an Orbita EHF therapy apparatus. Animals of the control group (n = 6) did not undergo this physical treatment procedure. These studies demonstrated stimulation of bone repair, apparent as inhibition of osteolytic processes leading to a significant (p = 0.017) decrease in the time to union in dogs of the experimental group.

Solution of Sine-Gordon Equations with Weak and Moderate Inductive Couplings under Bias Current and Dissipations

It is found that coherent electromagnetic (EM) radiation in terahertz (THz) band takes place when a single crystal of cuprate high-Tc superconductor Bi2Sr2CaCu2O8+δ (BSCCO) is biased by a dc voltage. In this work we study the inductively coupled sine-Gordon equations which compose a good model for intrinsic Josephson junctions (IJJs) realized in BSCCO due to the strong layer structure. We derive a general solution for the coupled sine-Gordon equations valid for weak and moderate inductive couplings, which can enhance injection of dc energy into IJJs and convert it to THz EM radiation. This solution evolves into the π phase kink state known before for strong inductive coupling.

Photoinduced pseudospin effects in silicene beyond the off-resonant condition

We study the photoinduced manipulation of charge carriers in monolayer silicene subject to intense electromagnetic terahertz radiation. Considering the Dirac cone approximation and going beyond the off-resonant condition for large frequencies of the radiation field, where only virtual photon processes are allowed, we present the exact zero-momentum pseudospin polarization and numerical results for the quasienergy band structure and time-averaged density of states. We find that resonant processes, due to real photon emission and absorbtion processes, induce a band inversion that qualitatively modifies the quasienergy spectrum. These band-structure changes manifest themselves as an inversion of the averaged pseudospin polarization. Through the analysis of the time-averaged density of states we find that effective photoinduced gap manipulation can only be achieved in the intermediate and strong matter-radiation coupling regime where the off-resonant approximation breaks down.

Wireless Underground Sensor Networks: Channel Modeling and Operation Analysis in the Terahertz Band

Wireless underground sensor networks (WUSNs) are networks of sensor nodes operating below the ground surface, which are envisioned to provide real-time monitoring capabilities in the complex underground environments consisting of soil, water, oil, and other components. In this paper, we investigate the possibilities and limitations of using WUSNs for increasing the efficiency of oil recovery processes. To realize this, millimeter scale sensor nodes with antennas at the same scale should be deployed in the confined oil reservoir fractures. This necessitates the sensor nodes to be operating in the terahertz (THz) range and the main challenge is establishing reliable underground communication despite the hostile environment which does not allow the direct use of most existing wireless solutions. The major problems are extremely high path loss, small communication range, and high dynamics of the electromagnetic (EM) waves when penetrating through soil, sand, and water and through the very specific crude oil medium. The objective of the paper is to address these issues in order to propose a novel communication channel model considering the propagation properties of terahertz EM waves in the complex underground environment of the oil reservoirs and to investigate the feasible transmission distances between nodes for different water-crude-oil-soil-CO2compositions.

Laser beat wave excitation of terahertz radiation in a plasma slab

Terahertz (THz) radiation generation by nonlinear mixing of lasers, obliquely incident on a plasma slab is investigated. Two cases are considered: (i) electron density profile is parabolic but density peak is below the critical density corresponding to the beat frequency, (ii) plasma boundaries are sharp and density is uniform. In both cases, nonlinearity arises through the ponderomotive force that gives rise to electron drift at the beat frequency. In the case of inhomogeneous plasma, non zero curl of the nonlinear current density gives rise to electromagnetic THz generation. In case of uniform plasma, the sharp density variation at the plasma boundaries leads to radiation generation. In a slab width of less than a terahertz wavelength, plasma density one fourth of terahertz critical density, laser intensities ∼1017 W/cm2 at 1 μm, one obtains the THz intensity ∼1 GW/cm2 at 3 THz radiation frequency.

Effect of Continuous Irradiation with Terahertz Electromagnetic Waves of the NO Frequency Range on Behavioral Reactions of Male Albino Rats under Stress Conditions

We studied the effect of terahertz waves (NO frequency range, 150.176-150.664 GHz) on stress-induced variations in behavioral reactions of male albino rats during hypokinetic stress. THz irradiation was followed by partial or complete normalization of behavioral reactions of male albino rats after hypokinetic stress. The most significant effect was observed after continuous irradiation for 30 min.

Взаємодія терагерцової електромагнітної хвилі з плазмонною системою “металічна ґратка–електронний 2D-газ”. Аналіз особливостей ближнього поля

Взаємодія терагерцової електромагнітної хвилі з плазмонною системою “металічна ґратка–електронний 2D-газ”. Аналіз особливостей ближнього поля

Patterns of biological effects of electromagnetic terahertz waves at frequencies of active cellular metabolites of post stressed changes in hemostasis

There had been studied the influence of electromagnetic waves at terahertz frequencies of active cellular metabolites (nitric oxide 150.176-150.664 GHz and 129.0 GHz atmospheric oxygen) on the changed parameters of homeostasis in experimental animals. It is shown that in the 15 minute exposure mode terahertz waves at frequencies of nitric oxide 150.176-150.664 GHz observed partial recovery of the changed parameters of homeostasis in stressed animals, and at 30 minute mode, the impact of these waves there is complete recovery of homeostatic parameters of male rats in a state of acute and prolonged immobilization stress.

Manipulating terahertz electromagnetic induced transparency through parallel plate waveguide cavities

To mechanically manipulate of electromagnetic induced transparency (EIT) in terahertz asymmetric parallel-plate waveguide cavities, the influence of waveguide spacing on the transmission response has been studied experimentally. After setting the appropriate shifting length between two cavities, we found with mechanically increasing the waveguide spacing, the symmetric resonance shows degeneracy when its wavelength is smaller than the waveguide spacing. An on-to-off modulation for symmetric resonance appeared in EIT can be observed with destructive interference broken. This control mechanism of EIT will open a door to design the tunable EIT devices.

Hemodynamic changes caused by exposure of animals with acute immobilization stress to continuous terahertz radiation with frequencies equal to absorption and emission frequencies of nitrogen oxide and atmospheric oxygen

The aim was to study the effects of exposure of albino rats to continuous terahertz radiation with frequencies equal to absorption and emission frequencies of nitrogen oxide (150.176-150.664 GHz) and atmospheric oxygen (129.0 ± 0.75 GHz) during their immobilization stress on their blood flow rate. Methods – The group of 120 male non-pedigree albino rats with average weight of 180-220 g was chosen as a test subject. Simulation of hemodynamic disorders was achieved by incurring active immobilization stress. All rats were exposed to electromagnetic terahertz radiation equal to absorption and emission frequencies of nitrogen oxide (150.176-150.664 GHz) and atmospheric oxygen (129.0 ± 0.75 GHz) for 5, 15 and 30 minutes. Results – Experimental simulation of hemodynamic disorders during acute immobilization stress has shown that exposure to continuous terahertz radiation with frequencies equal to absorption and emission frequencies of nitrogen oxide (150.176-150.664 GHz) and atmospheric oxygen (129.0 ± 0.75 GHz) for 5, 15 and 30 minutes allows to revert post-stress hemodynamic changes in great vessels. Conclusion – This allows using terahertz electromagnetic radiation with frequencies equal to absorption and emission frequencies of nitrogen oxide (150.176-150.664 GHz) and atmospheric oxygen (129.0 ± 0.75 GHz) to treat hemodynamic disorders accompanying some of pathologic diseases.

Nonperturbative Interband Response of a Bulk InSb Semiconductor Driven Off Resonantly by Terahertz Electromagnetic Few-Cycle Pulses

Intense multiterahertz pulses are used to study the coherent nonlinear response of bulk InSb by means of field-resolved four-wave mixing spectroscopy. At amplitudes above 5 MV/cm the signals show a clear temporal substructure which is unexpected in perturbative nonlinear optics. Simulations based on a model of a two-level quantum system demonstrate that in spite of the strongly off-resonant character of the excitation the high-field few-cycle pulses drive the interband resonances into a nonperturbative regime of Rabi flopping. The rotating wave approximation breaks down in this case and the system reaches a complete population inversion.

Correction of Microcirculatory Disturbances with Terahertz Electromagnetic Radiation at Nitric Oxide Frequencies in Albino Rats under Conditions of Acute Stress

Terahertz electromagnetic irradiation at frequencies corresponding to NO molecular emission and absorption spectrum 150.176-150.664 GHz corrects disturbances in peripheral circulation typical of acute stress reaction in rats during acute immobilization stress.

Radiation of Terahertz Electromagnetic Waves from Build-In Nano Josephson Junctions of Cuprate High-Tc Superconductors

The nano-scale intrinsic Josephson junctions in highly anisotropic cuprate superconductors have potential for generation of terahertz electromagnetic waves. When the thickness of a superconductor sample is much smaller than the wavelength of electromagnetic waves in vacuum, the superconductor renders itself as a cavity. Unlike conventional lasers, the presence of the cavity does not guarantee a coherent emission because of the internal degree of freedom of the superconductivity phase in long junctions. We study the excitation of terahertz wave by solitons in a stack of intrinsic Josephson junctions, especially for relatively short junctions. Coherent emission requires a rectangular configuration of solitons. However such a configuration is unstable against weak fluctuations, contrarily solitons favor a triangular lattice corresponding to an out phase oscillation of electromagnetic waves. To utilize the cavity, we propose to use an array of stacks of short intrinsic Josephson junctions to generate powerful terahertz electromagnetic waves. The cavity synchronizes the plasma oscillation in different stacks and the emission intensity is predicted to be proportional to the number of stacks squared.

Terahertz spectrum analyzer based on frequency and power measurement

We demonstrate a terahertz (THz) spectrum analyzer based on frequency and power measurement. A power spectrum of a continuous THz wave is measured through optical heterodyne detection using an electromagnetic THz frequency comb and a bolometer and power measurement using a bolometer with a calibrated responsivity. The THz spectrum analyzer has a frequency precision of 1x10(-11), a frequency resolution of 1Hz, a frequency band up to 1.7THz, and an optical noise equivalent power of approximately 1 pW/Hz(1/2).

Effect of Terahertz Electromagnetic Irradiation at Nitric Oxide Frequencies on Concentration of Nitrites in Blood Serum of Albino Rats under Conditions of Immobilization Stress

The terahertz electromagnetic irradiation at NO emission and absorption spectrum frequencies elevated the concentration of nitrites in blood serum of albino rats subjected to acute immobilization stress.

Understanding the Nature of Ultrafast Polarization Dynamics of Ferroelectric Memory in the Multiferroic BiFeO3

The crystallographic anisotropy in electromagnetic terahertz radiation from multiferroic BiFeO3 (100), (110), and (111) films suggests the ultrafast depolarization of ferroelectric order as a new mechanism of terahertz emission. This implies that the spontaneous polarization in ferroelectric materials can be manipulated in time scale of picoseconds, thus paving the way to ultrafast data-storage devices based on nonvolatile ferroelectric memories.