High-Order Spatial FDTD Solver of Maxwell’s Equations for Terahertz Radiation Production

We applied a spatial high-order finite-difference-time-domain (HO-FDTD) scheme to solve 2D Maxwell’s equations in order to develop a fluid model employed to study the production of terahertz radiation by the filamentation of two femtosecond lasers in air plasma. We examined the performance of the applied scheme, in this context, we implemented the developed model to study selected phenomena in terahertz radiation production, such as the excitation energy and conversion efficiency of the produced THz radiation, in addition to the influence of the pulse chirping on properties of the produced radiation. The obtained numerical results have clarified that the applied HO-FDTD scheme is precisely accurate to solve Maxwell’s equations and sufficiently valid to study the production of terahertz radiation by the filamentation of two femtosecond lasers in air plasma.

Numerical Simulation for the Efficiency of the Produced Terahertz Radiation by Two Femtosecond Laser Pulses: Above-Threshold-Ionization

The tunneling ionization (TI) is the most dominated ionization process in the production of terahertz radiation by two femtosecond lasers, although its validity above the ionization threshold of some gases is uncertain. In the present research, we employ a 1D fluid code to simulate the efficiency of the produced terahertz radiation by two femtosecond laser beams in air plasma. Two ionization models in the context of the TI process which are the Ammosov-Delone-Krainov (ADK) for noble gases and its developed molecular ADK (MO-ADK) model for molecular gases are intrinsically used to conduct this study. The main target of the present research is to examine the validity of these models Above-Threshold-Ionization (ATI) of these gases. For this purpose, we simulated the ionization rate and the power spectrum of the produced radiation, in addition we numerically evaluated the efficiency of the produced radiation as function of the input beams intensity at particular energy fraction factor, relative phase and initial pulse duration of these beams. These calculations conducted for a selected noble gas at varying energy levels and a chosen molecular air plasma gas at different quantum numbers. Numerical results near and above the ionization threshold of the selected gases have clarified that the ADK and MO-ADK model are successful valid to study the efficiency of the produced THz radiation at low energy levels and small quantum numbers of the selected gases, meanwhile, with any further increase in the energy level and the quantum number values of these gases, both of the ADK and MO-ADK are failed to correctly analyze the efficiency process and estimate its fundamental parameters.

Effect of magnetic field on the terahertz radiation detection in high electron mobility transistors

In this paper, we make a theoretical investigation of the plasma-wave instability mechanism in a two-dimensional electron fluid in a high electron mobility transistor （HEMT） driven by the terahertz radiation in the presence of a perpendicular magnetic field. It is found that the resonant peaks of the gate-to-source/drain admittances and detection responsivity depend on the strength of the external magnetic field. Such phenomena can be used to produce a desired effect by adjusting the intensity of the magnetic field.

Nonlinear dynamics in wurtzite InN diodes under terahertz radiation

We carry out a theoretical study of nonlinear dynamics in terahertz-driven n＋nn＋ wurtzite InN diodes by using time-dependent drift diffusion equations. A cooperative nonlinear oscillatory mode appears due to the negative differential mobility effect, which is the unique feature of wurtzite InN aroused by its strong nonparabolicity of the I＂1 valley. The appearance of different nonlinear oscillatory modes, including periodic and chaotic states, is attributed to the competition between the self-sustained oscillation and the external driving oscillation. The transitions between the periodic and chaotic states are carefully investigated using chaos-detecting methods, such as the bifurcation diagram, the Fourier spectrum and the first return map. The resulting bifurcation diagram displays an interesting and complex transition picture with the driving amplitude as the control parameter.

Tunable terahertz radiation from arbitrary profile dielectric grating coated with graphene excited by an electron beam

In this paper, the enhanced terahertz radiation transformed from surface plasmon polaritons, excited by a uniformly moving electron bunch, in a structure consisting of a monolayer graphene supported on a dielectric grating with arbitrary profile is investigated. The results show that the grating profile has significant influence on the dispersion curves and radiation characteristics including radiation frequency and intensity. The dependence of dispersion and radiation characteristics on the grating shape for both the symmetric and asymmetric gratings is studied in detail. Moreover, we find that, for an asymmetric grating with certain profile, there exist two different diffraction types, and one of the two types can provide higher radiation intensity comparing to the other one. These results will definitely facilitate the practical application in developing a room-temperature, tunable, coherent and miniature terahertz radiation source.

Excitonic optical absorption in semiconductors under intense terahertz radiation

The excitonic optical absorption of GaAs bulk semiconductors under intense terahertz （THz） radiation is investigated numerically. The method of solving initial-value problems, combined with the perfect matched layer technique, is used to calculate the optical susceptibility. In the presence of a driving THz field, in addition to the usual exciton peaks, 2p replica of the dark 2p exciton and even-THz-photon-sidebands of the main exciton resonance emerge in the continuum above the band edge and below the main exciton resonance. Moreover, to understand the shift of the position of the main exciton peak under intense THz radiation, it is necessary to take into consideration both the dynamical Franz-Keldysh effect and ac Stark effect simultaneously. For moderate frequency fields, the main exciton peak decreases and broadens due to the field-induced ionization of the excitons with THz field increasing. However, for high frequency THz fields, the characteristics of the exciton recur even under very strong THz fields, which accords with the recent experimental results qualitatively.

Resonant detection of terahertz radiation utilizing plasma waves in high-electron-mobility transistors

This paper theoretically studies the high-electron-mobility transistor （HEMT） driven by the terahertz radiation. It calculates the gate-to-source/drain admittance and the detection responsivity of the HEMT as a function of the signal frequency. It finds that the peaks of the admittances and the responsivity show redshift, and the heights of the peaks decrease with increasing the lengths of the source-to-gate and gate-to-drain spacing. Such phenomena may be useful in designing different HEMTs by utilizing the effects associated with the plasma oscillations excitation.

A new horizon for neuroscience:terahertz biotechnology in brain research

Terahertz biotechnology has been increasingly applied in various biomedical fields and has especially shown great potential for application in brain sciences.In this article,we review the development of terahertz biotechnology and its applications in the field of neuropsychiatry.Available evidence indicates promising prospects for the use of terahertz spectroscopy and terahertz imaging techniques in the diagnosis of amyloid disease,cerebrovascular disease,glioma,psychiatric disease,traumatic brain injury,and myelin deficit.In vitro and animal experiments have also demonstrated the potential therapeutic value of terahertz technology in some neuropsychiatric diseases.Although the precise underlying mechanism of the interactions between terahertz electromagnetic waves and the biosystem is not yet fully understood,the research progress in this field shows great potential for biomedical noninvasive diagnostic and therapeutic applications.However,the biosafety of terahertz radiation requires further exploration regarding its two-sided efficacy in practical applications.This review demonstrates that terahertz biotechnology has the potential to be a promising method in the field of neuropsychiatry based on its unique advantages.

Intense Cherenkov-type terahertz electromagnetic radiation from ultrafast laser-plasma interaction

A Cherenkowtype terahertz electromagnetic radiation is revealed, which results efficiently from the collective effects in the time-domain of ultrafast pulsed electron current produced by ultrafast intense laser plasma interaction. The emitted pulse waveform and spectrum, and the dependence of laser pulse parameters on the structure of the radiation field are investigated numerically. The condition of THz radiation generation in this regime and Cherenkov geometry of the radiation field are studied analytically.

Experimental study on terahertz radiation

In this letter, we describe a coherent subpicosecond terahertz (THz) spectroscopy system based on non-resonant optical rectification for the generation of THz radiation. We studied the two-photon absorption (TPA) of ZnTe induced by femtosecond laser pulses via THz generation, and its influence on the generation of THz radiation. Experimental results demonstrated that the intensity of pump beam against TPA must be traded off to get an optimum generation of THz radiation. As an example, we measured absorption spectrum of water vapor by time-domain spectroscopy (TDS) in the frequency range from 0.5 to 2.5 THz with a high overall accuracy.

Intense low-noise terahertz generation by relativistic laser irradiating near-critical-density plasma

Low-noise terahertz(THz)radiation over 100 MV/cm generation by a linearly-polarized relativistic laser pulse interacting with a near-critical-density(NCD)plasma slab is studied by theory and particle-in-cell(PIC)simulations.A theoretical model is established to examine the dipole-like radiation emission.The THz radiation is attributed to the singlecycle low-frequency surface current,which is longitudinally constrained by the quasi-equilibrium established by the laser ponderomotive force and the ponderomotively induced electrostatic force.Through theoretical analysis,the spatiotemporal characteristics,polarization property of the THz radiation,and the relation between the radiation strength with the initial parameters of driving laser and plasma are obtained,which are in good consistence with the PIC simulation results.Furthermore,it is found by PIC simulations that the generation of thermal electrons can be suppressed within the appropriate parameter regime,resulting in a clear THz radiation waveform.The appropriate parameter region is given for generating a low-noise intense THz radiation with peak strength reaching 100 MV/cm,which could find potential applications in nonlinear THz physics.

Intense terahertz radiation: generation and application

Strong terahertz(THz)radiation provides a powerful tool to manipulate and control complex condensed matter systems.This review provides an overview of progress in the generation,detection,and applications of intense THz radiation.The tabletop intense THz sources based on Ti:sapphire laser are reviewed,including photoconductive antennas(PCAs),optical rectification sources,plasma-based THz sources,and some novel techniques for THz generations,such as topological insulators,spintronic materials,and metasurfaces.The coherent THz detection methods are summarized,and their limitations for intense THz detection are analyzed.Applications of intense THz radiation are introduced,including applications in spectroscopy detection,nonlinear effects,and switching of coherent magnons.The review is concluded with a short perspective on the generation and applications of intense THz radiation.

Efficient Terahertz Photoconductive Emitters with Improved Electrode Structures

We present the design, fabrication, and characterization of two new types of terahertz photoconductive emitters. One has an asymmetric four-contact electrode structure and the other has an arc-shaped electrode structure, which are all modified from a traditional strip line antenna. Numerical simulations and real experiments confirm the good performance of the proposed antennas. An amplitude increase of about 40% is experimentally observed for the terahertz signals generated from the new structures. The special electrode structure and its induced local bias field enhancement are responsible for this radiation efficiency improvement. Our work demonstrates the feasibility of developing highly efficient terahertz photoconductive emitters by optimizing the electrode structure.

Electromagnetic Emission from Laser Wakefields in Magnetized Underdense Plasmas

A wakefield driven by a short intense laser pulse in a perpendicularly magnetized underdense plasma is studied analytically and numerically for both weakly relativistic and highly relativistic situations. Owing to the DC magnetic field, a transverse component of the electric fields associated with the wakefield appears, while the longitudinal wave is not greatly affected by the magnetic field up to 22 Tesla. Moreover, the scaling law of the transverse field versus the longitudinal field is derived. One-dimensional particle-in-cell simulation results confirm the analytical results. Wakefield transmission through the plasma-vacuum boundary, where electromagnetic emission into vacuum occurs, is also investigated numerically. These results are useful for the generation of terahertz radiation and the diagnosis of laser wakefields.

Optical rectification in 4H-SiC:paving the way to generate strong terahertz fields with ultra-wide bandwidth

The 4H-SiC crystal is found to have great potential in terahertz generation via nonlinear optical frequency conversion due to its extremely high optical damage threshold,wide transparent range,etc.In this paper,optical rectification(OR)with tilted-pulse-front(TPF)setting based on the 4H-SiC crystal is proposed.The theory accounts for the optimization of incident pulse pre-chirping in the TPF OR process under high-intensity femtosecond laser pumping.Compared with the currently recognized LiNbO3-based TPF OR,which generates a single-cycle terahertz pulse within 3 THz,4HSiC demonstrates a significant advantage in producing ultra-widely tunable(up to over 14 THz,TPF angle 31◦–38◦)terahertz waves with high efficiency(~10–2)and strong field(~MV/cm).Besides,the spectrum characteristics,as well as the evolution from single-to multi-cycle terahertz pulses can be modulated flexibly by pre-chirping.The simulation results show that 4H-SiC enables terahertz frequency extending to an unprecedent range by OR,which has extremely important potential in strong-field terahertz applications.

Carrier-Envelope Phase-Dependent Phase Matching of Terahertz Emission in Laser Plasma

We investigate off-axis phase-matched terahertz(THz)radiation in laser plasma pumped by few-cycle laser pulses.We find that the THz amplitude and angular distributions in the far field are sensitively dependent on the pump pulse’s focal carrier-envelope phase(CEP).Ring-like profiles of THz radiation are obtained at CEP values of 0.5πand 1.5π,due to the inversely symmetric local THz waveforms emitted before and after laser focus.Off-axis phase-matched THz radiation offers a tool to accurately measure the CEP of few-cycle pulses at the center of a medium.

On the Potential Application of the Wrinkled SiGe/SiGe Nanofilms

Terahertz radiation (THzR) consists of electromagnetic waves within the band of frequencies from 0.3 to 3 terahertz with the wavelengths of radiation in the range from 0.1 mm to 1 mm, respectively. The technology for generating and manipulating THzR is still in its initial stage. Herein, we demonstrate that the wrinkled Si1–xGex/Si1–yGey films can be used as radiation sources, which emit electromagnetic waves (EMW) in a very wide range of the frequencies including the terahertz band from 0.3 to 3 THz and far IR from 3 THz to 20 THz. These findings provide the theoretical foundation for the wrinkled nanofilm radiation emission and may allow, to some extent, to fill the terahertz gap.

Design of broadband terahertz vector and vortex beams: I. Review of materials and components

In this paper,we review the existing approaches for vortex and vector beam shaping and generation in the terahertz frequency range.The particular focus of this review is on the possibility of homogeneous topological charge formation in the ultra-wide spectral interval inherent to ultrashort terahertz pulses.We review the available materials and components,analyse proposed and potentially possible solutions for broadband terahertz vortex and vector beam shaping,compare all developed approaches,and put forward a unified concept for constructing passive shapers of such beams from the existing component base.

Design of broadband terahertz vector and vortex beams: II. Holographic assessment

In this paper,we demonstrate the capabilities of the terahertz pulse time-domain holography in visualisation,simulation,and assessment of broadband THz vortex beam formation dynamics upon its shaping by elements of beam converter,and further propagation and manipulation.By adding Jones matrix formalism to describe broadband optical elements,we highlight the differences in the spatio-spectral and spatio-temporal structure of the formed vortex and vector beams dependence on the modulator used and visualise their modal features.The influence of diffraction field structure from each element in the broadband vortex modulator is revealed in numerical simulation and the formed beams are analysed against the simplified Laguerre-Gaussian beam model.