To Terahertz Radiation Research Articles

Ab initio study of ultrafast demagnetization of elementary ferromagnets by terahertz versus optical pulses

We present an ab initio time-dependent density functional theory study of the ultrafast demagnetization of elemental ferromagnets subject to terahertz (THz) laser pumping compared to optical laser pumping. We find that THz radiation can cause as much demagnetization as optical radiation for significantly lower-energy transfer to the sample. We show that the demagnetization efficiency of the THz pulse is related to a coherent coupling of the radiation to the electron charge dynamics. In fact, our simulations reveal that the low-frequency THz electric field enforces in-phase electron charge oscillations around the atomic centers, leading to a highly efficient spin-orbit torque switching of the magnetization. Unlike for optical pumping, at the end of the pulse a major fraction of the excited electrons is driven back to equilibrium acquiring a thermalized Fermi-Dirac distribution.

Coulomb drag and plasmonic effects in graphene field-effect transistors enable resonant terahertz detection

We analyze the response of lateral n+-i-n-n+ graphene field-effect transistors (GFETs) to terahertz (THz) radiation. The nonlinearity due to the Coulomb drag of quasi-equilibrium carriers by injected ballistic carriers accompanied by plasmonic oscillations in a GFET channel enables a resonantly strong response. This effect can be used for effective resonant detection of THz radiation.

E. coli aggregation and impaired cell division after terahertz irradiation

In this study we demonstrated that exposure of Escherichia coli (E. coli) to terahertz (THz) radiation resulted in a change in the activities of the tdcABCDEFGR and matA–F genes (signs of cell aggregation), gene yjjQ (signs of suppression of cell motility), dicABCF, FtsZ, and minCDE genes (signs of suppression of cell division), sfmACDHF genes (signs of adhesin synthesis), yjbEFGH and gfcA genes (signs of cell envelope stabilization). Moreover, THz radiation induced E. coli csg operon genes of amyloid biosynthesis. Electron microscopy revealed that the irradiated bacteria underwent increased aggregation; 20% of them formed bundle-like structures consisting of two to four pili clumped together. This could be the result of changes in the adhesive properties of the pili. We also found aberrations in cell wall structure in the middle part of the bacterial cell; these aberrations impaired the cell at the initial stages of division and resulted in accumulation of long rod-like cells. Overall, THz radiation was shown to have adverse effects on bacterial populations resulting in cells with abnormal morphology.

Fluorescent bacterial biosensor E. coli/pTdcR-TurboYFP sensitive to terahertz radiation.

A fluorescent biosensor E. coli/pTdcR-TurboYFP sensitive to terahertz (THz) radiation was developed via transformation of Escherichia coli (E. coli) cells with plasmid, in which the promotor of the tdcR gene controls the expression of yellow fluorescent protein TurboYFP. The biosensor was exposed to THz radiation in various vessels and nutrient media. The threshold and dynamics of fluorescence were found to depend on irradiation conditions. Heat shock or chemical stress yielded the absence of fluorescence induction. The biosensor is applicable to studying influence of THz radiation on the activity of tdcR promotor that is involved in the transport and metabolism of threonine and serine in E. coli.

Study on the effects of terahertz radiation on gene networks of Escherichia coli by means of fluorescent biosensors.

Three novel fluorescent biosensors sensitive to terahertz (THz) radiation were developed via transformation of Escherichia coli (E. coli) cells with plasmids, in which a promotor of genes matA, safA, or chbB controls the expression of a fluorescent protein. The biosensors were exposed to THz radiation from two sources: a high-intensity pulsed short-wave free electron laser and a low-intensity continuous long-wave IMPATT-diode-based device. The threshold and dynamics of fluorescence were found to depend on radiation parameters and exposure time. Heat shock or chemical stress yielded the absence of fluorescence induction. The biosensors are evaluated to be suitable for studying influence of THz radiation on the activity of gene networks related with considered gene promoters.

Single-pixel terahertz imaging: a review

This paper is devoted to reviewing the results achieved so far in the application of the single-pixel imaging technique to terahertz (THz) systems. The use of THz radiation for imaging purposes has been largely explored in the last twenty years, due to the unique capabilities of this kind of radiation in interrogating material properties. However, THz imaging systems are still limited by the long acquisition time required to reconstruct the object image and significant efforts have been recently directed to overcome this drawback. One of the most promising approaches in this sense is the so-called "single-pixel" imaging, which in general enables image reconstruction by patterning the beam probing the object and measuring the total transmission (or reflection) with a single-pixel detector (i.e., with no spatial resolution). The main advantages of such technique are that i) no bulky moving parts are required to raster-scan the object and ii) compressed sensing (CS) algorithms, which allow an appropriate reconstruction of the image with an incomplete set of measurements, can be successfully implemented. Overall, this can result in a reduction of the acquisition time. In this review, we cover the experimental solutions proposed to implement such imaging technique at THz frequencies, as well as some practical uses for typical THz applications.

Microwave probing of relaxation oscillations related to terahertz power detection in superconducting hot electron bolometers

Relaxation oscillations related to terahertz (THz) power detection in superconducting niobium nitride hot electron bolometers (HEBs) have been studied using microwaves as a probe. The reflected microwave signal, which is related to the impedance changes of the HEB irradiated by an incident THz signal, can be used to characterize relaxation oscillations. The frequency of relaxation oscillation switching between the superconducting and resistive states increases linearly with incident THz power but nonlinearly with bias voltage or bath temperature. The observed strong periodic pulse trains of relaxation oscillations have a maximum duty cycle of ∼50%. These strong switching pulses indicate that DC Joule heating, THz radiation or thermal energy is absorbed in the bi-stable region of the HEB intermittently. In addition to relaxation oscillations, we also see weaker oscillations when the device is in the resistive state, which we call intrinsic oscillations. These have a constant frequency at the order of 10 kHz which is independent of external stimuli. The weak periodic pulse trains at a constant frequency can be divided into mainly two types—one is the pulse train with a small duty cycle and the other is the quasi-square wave. Measurement of these oscillations in HEBs has some practical applications. Digital THz power measurement with frequencies below and above the gap frequency can be made based on the linearity between relaxation oscillation frequency and incident THz power. The relaxation oscillation frequency can be used to estimate the maximum speed of a THz direct detection system especially for a multi-pixel HEB array. In particular, extra noise induced by relaxation oscillations and intrinsic oscillations can be determined for the HEB operating as a THz direct detector.

New approach to terahertz diagnostics of human psychoemotional state

A new approach to terahertz (THz) diagnostics of the human psychoemotional state is proposed, based on the analysis of the THz contribution to the total signal while simultaneously recording the IR and THz emissions from the human body (hereinafter referred to as the IR – THz image). The developed image-processing algorithm allows extraction of the informative contribution determined by the THz radiation from the total signal perceived by the recording system. Simultaneous registration of the IR – THz images of the human face and the psychophysiological indicators are carried out in situations of physical stress (short-term intense physical exercise – squats functional test), electrical stimulation, and information stress (cognitive load – simple arithmetic mental calculations). The obtained data are compared with those of similar measurements at rest. It is shown that using cluster analysis of IR – THz images it is possible to divide the test subjects into classes according to the type of reaction of the circulatory system under stressful conditions.

Terahertz imaging using ahigh-Tc superconducting Josephson junction detector

A high-Tc superconducting (HTS) detector based on aYBa2Cu3O7−x (YBCO) step-edge Josephson junction has been developed and applied to terahertz (THz)detection. The detector was coupled to a ring-slot antenna designed for operation at600 GHz, and used for THz imaging. The results suggest that the characteristic voltageand frequency of our HTS step-edge junctions can be readily optimized for thechosen THz frequency range at easily achievable temperatures. The images alsoclearly demonstrate some of the unique properties of THz radiation, including thesensitivity to water content and the ability to penetrate packaging materials.

Impact of High-Field Charge Transport on Terahertz Emission From Semiconductor Devices

We present an experimental study that provides insight into the charge carrier dynamics that lead to terahertz (THz) emission. We perform time-resolved THz experiments on semiinsulating GaAs structures with periodically poled surface electrodes. The individual charge carrier dynamics that contribute to the THz emission process are identified by their dependence on the applied field. Electron transport in inhomogeneous fields is found to dominate the emission of THz radiation. Other dynamics that become visible by their specific THz emission are the Dember effect, the intervalley transfer of electrons, and hole currents at high field strengths.

Nondestructive evaluation of cork enclosures using terahertz/millimeter wave spectroscopy and imaging

Natural cork enclosures, due to their cell structure, composition, and low moisture are fairly transparent to terahertz (THz) and millimeter waves enabling nondestructive evaluation of the cork's surface and interior. It is shown that the attenuation coefficient of the defect-free cork can be modeled with a Mie scattering model in the weakly scattering limit. Contrast in the THz images is a result of enhanced scattering of THz radiation by defects or voids as well as variations in the cork cell structure. The presence of voids, defects, and changes in grain structure can be determined with roughly 100-300 microm resolution.

Emission and detection of terahertz pulses from a metal-tip antenna

We investigate the antenna characteristics of a metal tip coupled to terahertz (THz) pulses generated from a photoconductive switch. Enhanced terahertz pulse emission is observed with the metal tip in contact with one of the electrodes of the photoconductive switch. Measurements of the angular dependence of the emitted THz radiation show that the metal tip acts as a highly directional antenna with radiation patterns well described by the theory for long-wire traveling-wave antennas. Similar behavior is observed for the metal tip acting as a THz pulse receiver, in accordance with the reciprocity principle. Effects related to the broadband nature of the THz pulses are discussed.