Polarized Terahertz Radiation Research Articles

Terahertz Radiation Driven Nonlinear Transport Phenomena in Two-Dimensional Tellurene.

Nonlinear electron transport induced by polarized terahertz radiation is studied in two-dimensional tellurene at room temperature. A direct current, quadratic in the radiation's electric field, is observed. Contributions sensitive to radiation helicity and polarization orientation as well as polarization independent current are found. We show that these contributions can be modified by the magnitude of the external gate potential. We demonstrate that this terahertz-driven electric current arises from the Berry curvature dipole and the side-jump microscopic mechanisms.

Generation of Circularly Polarized Strong-Field Terahertz Waves

Abstract Terahertz (THz) circular dichroism (TCD) spectroscopy has been widely used to study the chiral characteristics of biological macromolecules and various other materials. With the rapid development of strong-field THz generation and field-modulated techniques, the advancement of tunable strong-field TCD spectroscopy technology is of great significance. In this work, we built an integrated strong-field TCD spectroscopy system. We employed the tilted-pulse-front technique to generate linearly polarized strong-field THz radiation and a reflective metasurface for linear-to-circular conversion of the strong-field THz radiation. We obtained circularly polarized THz radiation with an ellipticity of over 0.9 in the frequency range from 0.38 THz to 0.61 THz, with a LTC conversion efficiency of over 90%. The peak electric field strength of the obtained strong-field circularly polarized THz radiation exceeded 100 kV/cm. This system is expected to be used to study the chiral characteristics of materials under strong-field conditions, as well as changes in the chiral characteristics under different field conditions.

Nonlinear helicity anomalies in the cyclotron resonance photoresistance of two-dimensional electron systems

Our studies of the cyclotron resonance (CR) photoresistance in GaAs-based two-dimensional electron systems (2DES) reveal an anomalously low sensitivity to the helicity of the incoming circularly polarized terahertz radiation. We find that this anomaly is strongly intensity dependent, and the ratio of the low-temperature photoresistance signals for the CR-active (CRA) and CR-inactive (CRI) polarities of magnetic field increases with lowering power, but, nevertheless, remains substantially lower than expected from conventional theory assuming interaction of the plane electromagnetic wave with the uniform 2DES. Our analysis shows that all data can be well described by the nonlinear CR-enhanced electron gas heating in both CRA and CRI regimes. This description, however, does not specify the source of anomalous CRI absorption that remains unclear. Published by the American Physical Society 2024

Coulomb potential determining terahertz polarization in a two-color laser field.

The orientation and ellipticity of terahertz (THz) polarization generated by a two-color strong field not only casts light on underlying mechanisms of laser-matter interaction, but also plays an important role for various applications. We develop the Coulomb-corrected classical trajectory Monte Carlo (CTMC) method to well reproduce the joint measurements, that the THz polarization generated by the linearly polarized 800 nm and circularly polarized 400 nm fields is independent on two-color phase delay. The trajectory analysis shows that the Coulomb potential twists the THz polarization by deflecting the orientation of asymptotic momentum of electron trajectories. Further, the CTMC calculations predict that, the two-color mid-infrared field can effectively accelerate the electron rapidly away from the parent core to relieve the disturbance of Coulomb potential, and simultaneously create large transverse acceleration of trajectories, leading to the circularly polarized THz radiation.

Strain-tuned spin polarization and optical conductivity in MoS2/EuS heterostructures

We investigate strain-tuned spin transport and optical conductivity in a monolayer (ML) $n$-type ${\mathrm{MoS}}_{2}/\mathrm{EuS}$ heterostructure under linearly polarized terahertz radiation using a low-energy effective Hamiltonian, which takes displacement of the Dirac point induced by strain into account. Through modifying the strain modulus, we find an adjustable relationship between the band edge energy and the strain. Thus the threshold and range of allowed angles are strongly dependent on the strength of strain $\ensuremath{\epsilon}$, whereas the angle $\ensuremath{\theta}$ along which the strain is applied only acts on the former. Due to spin-flipped scattering generated by Rashba spin-orbit coupling, the in-plane spin polarizations (i.e., ${P}_{X}$ and ${P}_{Y}$) can be achieved, and their intensity can be enhanced by increasing $\ensuremath{\epsilon}$. Interestingly, we find that the out-of-plane spin polarization (i.e., ${P}_{Z}$) is related to the synergy of $\ensuremath{\epsilon}$ and the Rashba parameter ${\ensuremath{\lambda}}_{R}$. Especially, ${P}_{Z}$ can approach $100%$ when $\ensuremath{\epsilon}=9%$ and ${\ensuremath{\lambda}}_{R}=2$ meV. Furthermore, the optical transition between the spin-splitting subbands of the conduction band under the strain is studied. It is shown that by varying $\ensuremath{\epsilon}$ we can control the magnitude of both absorption peaks and absorption valleys in optical conductivity. Moreover, the presence of ${\ensuremath{\lambda}}_{R}$ can be used to further enhance the absorption peaks and valleys because ${\ensuremath{\lambda}}_{R}$ can tune the energy difference between the spin-splitting subbands within a specific valley and thus affect the optical transition channels. Our findings demonstrate that the strain can influence the tunneling behavior and valley Hall conductivity in a ML $n$-type ${\mathrm{MoS}}_{2}/\mathrm{EuS}$ heterostructure, and these features are applicable to other two-dimensional ML transition metal dichalcogenides, which is promising for terahertz and valleytronic devices.

Terahertz spectroscopy of spin excitations in magnetoelectric LiFePO4 in high magnetic fields

Magnetoelectric multiferroics feature crosscoupling between magnetic and charge order. Here, the authors study the absorption of electromagnetic radiation by spin waves in magnetoelectric LiFePO${}_{4}$ in the terahertz (THz) spectral range. Using linearly polarized THz radiation, they establish that there are spin waves that interact with magnetic $a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d$ electric components of radiation. This is unusual for electron spins since they are susceptible to magnetic fields only. Another striking observation is that the number of spin-wave modes in the THz spectrum exceeds four times the number of modes expected for a four-spin system. Using the mean-field model and the magnetic field dependence of the THz spectra, the authors identify four conventional spin-wave modes in LiFePO${}_{4}$. It is proposed that other THz-active spin-wave modes are more exotic quadrupolar excitations, such as spin-stretching modes and two-magnon excitations.

Measurement and Control of Radially Polarized THz Radiation from DC-Biased Laser Plasma Filaments in Air.

Detection and manipulation of radially polarized terahertz (THz) radiation is essential for many applications. A new measurement scheme is proposed for the diagnosis of radially polarized THz radiation from a longitudinal dc-biased plasma filament, by introducing a movable metal mask. The amplitude and spectrum of the radially polarized THz beam was measured with a <110>-cut ZnTe crystal, where the THz beam pattern was modulated by the mask. Based on this measurement scheme, it was demonstrated that the amplitude and spectrum of the radially polarized THz radiation from the longitudinal dc-biased filament could be manipulated by controlling the strength and the location of the dc-biased field.

Terahertz radiation induced circular Hall effect in graphene

We report on the observation of the circular transversal terahertz photoconductivity in monolayer graphene supplied by a back gate. The photoconductivity response is caused by the free carrier absorption and reverses its sign upon switching the radiation helicity. The observed dc Hall effect manifests the time inversion symmetry breaking induced by circularly polarized terahertz radiation in the absence of a magnetic field. For low gate voltages, the photosignal is found to be proportional to the radiation intensity and can be ascribed to the alignment of electron momenta by the combined action of THz and static electric fields as well as by the dynamic heating and cooling of the electron gas. Strikingly, at high gate voltages, we observe that the linear-in-intensity Hall photoconductivity vanishes; the photoresponse at low intensities becomes superlinear and varies with the square of the radiation intensity. We attribute this behavior to the interplay of the second- and fourth-order effects in the radiation electric field which has not been addressed theoretically so far and requires additional studies.

Terahertz Chiral Metamaterials Enabled by Textile Manufacturing.

Easy-to-fabricate, large-area, and inexpensive microstructures that realize control of the polarization of terahertz (THz) radiation are of fundamental importance to the development of the field of THz wave photonics. However, due to the lack of natural materials that can facilitate strong THz radiation-matter interactions, THz polarization components remain an undeveloped technology. Strong resonance-based responses offered by THz metamaterials have led to the recent development of THz metadevices, whereas, for polarization control devices, micrometer-scale fabrication techniques including aligned photolithography are generally required to create multilayer microstructures. In this work, leveraging a two-step textile manufacturing approach, a chiral metamaterial capable of exhibiting strong chiroptical responses at THz frequencies is demonstrated. Chiral-selective transmission and pronounced optical activity are experimentally observed. In sharp contrast to smart-clothing-related devices (e.g., textile antennas), the investigated chiral metamaterials gain their THz properties directly from the yarn-twisting enabled microhelical strings. It is envisioned that the interplay between meta-atom designs and textile manufacturing technology will lead to a new family of metadevices for complete control over the phase, amplitude, and polarization of THz radiation.

Experimentally exploring the terahertz radiation in a long filament by a two-color laser field

We have experimentally investigated the propagation effect of terahertz (THz) radiation from a long filament by checking the evolution of THz temporal waveform through shifting the longitudinal position of a short filament. The results show that the relative delay and carrier-envelope phase of the THz waveform change linearly with the shifted distance of the filament. After measuring the longitudinal intensity distributions of laser fields in long filament, we reconstruct successfully the process of the polarization-controlled THz radiation in a long filament. Besides, we also discuss the efficiency and polarization of THz radiation from a long filament excited by a circularly polarized two-color laser field.

Coherent Elliptically Polarized Terahertz Wave Generation in WSe2 by Linearly Polarized Femtosecond Laser Excitation.

Coherent polarization control of terahertz (THz) wave radiation in both the time-domain and the frequency-domain is significant in information technology, material science, and spectroscopic analysis. Elliptically polarized THz radiation is generally limited to chiral materials induced by circularly polarized light excitation. Herein, we demonstrate the coherent elliptically polarized THz radiation from few-layer tungsten diselenide (WSe2) in both the time-domain and the frequency-domain under linearly polarized femtosecond laser excitation. This coherent elliptical THz radiation is mainly dominated by in-plane anisotropic shift current and out-of-plane drift current, which is verified by the THz radiation dependence on the pump laser polarization angles, incident angles, and sample azimuthal angles systematically. The ellipticity and major axis direction of the elliptical THz wave can be efficiently controlled by either pump light polarization or sample azimuthal angle due to the controllable amplitudes and phases of two coherent orthogonal THz wave components. Our finding provides a method to distinguish drift and shift photocurrents in different directions and offers a unique design concept for elliptical THz generation with two-dimensional (2D) material physics.

THz polarization-dependent response of antenna-coupled HgCdTe photoconductors under an external constant electric field

The photoresponse of antenna-coupled Hg1-x Cd x Te (x ≈ 0.201) narrow-gap epitaxially grown photoconductors with normal band ordering having large spin–orbit coupling and being irradiated by the normal to the photoconductor surface terahertz (THz) radiation (linearly or circularly polarized), is investigated at T = 80 and 300 K. The oscillations of polarized photoresponse in biased and unbised structures were revealed. In experiments, Hg1-x Cd x Te layers (point group ) grown on (013) GaAs substrates were used. In biased structures, to the ordinary charge current and Rashba spin splitting current in zero electric field, an additional polarized current, which grows with the external electric field strength, arises. Presumably, the oscillating photoresponses under polarized THz radiation in these unbiased and biased structures are connected with the circular and linear photogalvanic effects without and with an external electric field. The observed features of the polarized dependent photoresponse can be important for spintronics applications at elevated temperatures.

Generation of chirp-controllable circularly polarized terahertz radiation in magnetized plasma

We propose a promising way to generate chirp-controllable circularly polarized terahertz waves from a magnetized Ar plasma driven by a linearly polarized laser. With an inhomogeneous static magnetic field, the local electrons are subjected to different strengths of the Lorentz force and emit terahertz waves with different cyclotron frequencies. In addition, the chirp rate and the temporal waveform of this chirped terahertz field can be adjusted by varying the gradient of the static magnetic field and the gas-density distribution.

Terahertz optical Hall effect in p-type monolayer hexagonal boron nitride on fused silica substrate.

We demonstrate for the first time, to the best of our knowledge, that the optical Hall effect (OHE) can be observed in p-type monolayer (ML) hexagonal boron nitride (hBN) on a fused silica substrate by applying linearly polarized terahertz (THz) irradiation. When ML hBN is placed on fused silica, in which the incident pulsed THz field can create local and transient electromagnetic dipoles, proximity-induced interactions can be presented. The Rashba spin-orbit coupling can be enhanced, and the in-plane spin component can be induced, along with the lifting of valley degeneracy. Thus, in the presence of linearly polarized THz radiation, the nonzero transverse optical conductivity (or Hall conductivity) can be observed. We measure the THz transmission through ML hBN/fused silica in the temperature range from 80 to 280 K by using THz time-domain spectroscopy in combination with an optical polarization examination. The Faraday ellipticity and rotation angle, together with the complex longitudinal and transverse conductivities, are obtained. The temperature dependence of these quantities is examined. The results obtained from this work indicate that ML hBN is a valleytronic material, and proximity-induced interactions can lead to the observation of OHE in the absence of an external magnetic field.

Generation of terahertz radiation by short laser pulses propagating in obliquely magnetized plasma

This paper presents an analytical and simulation study of terahertz (THz) radiation generation using short, circularly polarized laser pulses propagating in plasma embedded in arbitrarily oriented magnetic field. Perturbation technique is used to obtain generated electric and magnetic wakefields within and behind the laser pulse. Coupling of components of the obliquely applied magnetic field with transverse and axial plasma electron velocities leads to the generation of linearly as well as elliptically polarized transverse electromagnetic radiation oscillating at THz frequency, under appropriate conditions. The amplitude of these fields and ellipticity of the elliptically polarized THz radiation can be varied with the help of the obliqueness of the external magnetic field. Analytical results are validated using VSim PIC simulation code.

Generation of elliptically polarized terahertz radiation from black phosphorus crystallites

Polarized THz radiation is increasingly being investigated and applied in the new interdisciplinary field “TeraNano.” We report the generation of terahertz (THz) radiation by a linearly polarized femtosecond optical pulse in black phosphorus (BP) crystallites grown by chemical vapor deposition. Characterization of THz radiation is performed by time-resolved THz spectroscopy. It was found that BP emits elliptically polarized THz radiation with an ellipticity of ε = 0.77 ± 0.03. A saturation was found in the dependence of the THz intensity on the intensity of the incident optical pump for the latter higher than 6 mJ / cm2. The results of our work can provide new fundamental knowledge needed to create detectors and generators based on BP in the THz range.

Circularly polarized terahertz pulse generation in a plasma channel created by a uv high-intense laser pulse in the presence of a static magnetic field.

This paper is devoted to the problem of obtaining high-intense circularly polarized terahertz radiation in a nonequilibrium xenon plasma channel in the presence of an external static magnetic field. The physical mechanism is based on the well-known cyclotron resonance effect which enables to provide effective plasma-electromagnetic wave interaction that can increase the gain in plasma proposed earlier by the authors. According to the suggested scheme, a static magnetic field is imposed on a gas target along the propagation direction of a femtosecond ultraviolet laser driver. Numerical simulations demonstrate that, by varying a magnetic field, one can control the range of amplified frequencies in plasmas at atmospheric and lower pressures. Moreover, a pressure decrease in the nonequilibrium rare gas plasma channel allows one to obtain a larger value of the gain factor. Thus, controlling the central frequency and bandwidth of the amplified terahertz signals leads to producing both ultrashort and rather long high-intense pulses at the output of nonequilibrium plasma channel.

Radially polarized terahertz (THz) generation by frequency difference of Hermite Cosh Gaussian lasers in hot electron-collisional plasma

We report a scheme to generate radially polarized THz radiation by frequency difference of Hermite Cosh Gaussian lasers in hot electron, collisional and density modulated plasma. The effect of laser (spatial field distribution) and plasma parameters (plasma density structure, electron-neutral collisions and electron temperature) on emitted THz field profile is investigated. We found that THz field profile varies with spatial intensity profile of pump lasers. The THz amplitude increases with the pump laser intensity gradient. Our investigations show that electron collisions attenuate the THz amplitude without affecting the THz field profile. The highest THz field and conversion efficiency are obtained at resonance condition i.e. ω1−ω2≈ωp. Off-resonant excitation of THz radiation is also observed with less THz amplitude. Our investigations also reveal that the value of THz- fieldpeak and efficiency increase monotonically with electron thermal velocity (electron temperature). A high THz field (~ GV/m) and conversion efficiency ~ 0.08 can be obtained for optimized pump laser and plasma parameters.

Terahertz optical Hall effect in monolayer MoS2 in the presence of proximity-induced interactions

The effect of proximity-induced interactions such as Rashba spin-orbit coupling (SOC) and exchange interaction on the electronic and optical properties of n-type monolayer (ML) ${\mathrm{MoS}}_{2}$ is investigated. We predict and demonstrate that the Rashba SOC can induce an in-plane spin splitting with terahertz (THz) energy, while the exchange interaction lifts the energy degeneracy in different valleys. Thus, spin polarization can be achieved in an n-type ML ${\mathrm{MoS}}_{2}$ and valley Hall or optical Hall effect can be observed using linearly polarized THz radiation. In such a case, the transverse optical conductivity ${\ensuremath{\sigma}}_{xy}(\ensuremath{\omega})$ results from spin-flip transition within spin-split conduction bands and from the fact that contributions from electrons with different spin orientations in different valleys can no longer be canceled out. Interestingly, we find that for fixed effective Zeeman field (or exchange interaction) the lowest spin-split conduction band in ML ${\mathrm{MoS}}_{2}$ can be tuned from one in the $K$ valley to another one in the ${K}^{\ensuremath{'}}$ valley by varying the Rashba parameter ${\ensuremath{\lambda}}_{R}$. Therefore, by changing ${\ensuremath{\lambda}}_{R}$ we can turn the sign of the spin polarization and $\mathrm{Im}{\ensuremath{\sigma}}_{xy}(\ensuremath{\omega})$ from positive to negative. Moreover, we find that the dominant contribution of the selection rules to ${\ensuremath{\sigma}}_{xx}(\ensuremath{\omega})$ is from electrons in the $K$ valley and to ${\ensuremath{\sigma}}_{xy}(\ensuremath{\omega})$ is from electrons in the ${K}^{\ensuremath{'}}$ valley. These important and interesting theoretical findings can be helpful to experimental observation of the optical Hall effect in valleytronic systems using linearly polarized THz radiation fields.

Symmetry breaking and circular photogalvanic effect in epitaxial CdxHg1−xTe films

We report on the observation of symmetry breaking and the circular photogalvanic effect in Cd$_x$Hg$_{1-x}$Te alloys. We demonstrate that irradiation of bulk epitaxial films with circularly polarized terahertz radiation leads to the circular photogalvanic effect (CPGE) yielding a photocurrent whose direction reverses upon switching the photon helicity. This effect is forbidden in bulk zinc-blende crystals by symmetry arguments, therefore, its observation indicates either the symmetry reduction of bulk material or that the photocurrent is excited in the topological surface states formed in a material with low Cadmium concentration. We show that the bulk states play a crucial role because the CPGE was also clearly detected in samples with non-inverted band structure. We suggest that strain is a reason of the symmetry reduction. We develop a theory of the CPGE showing that the photocurrent results from the quantum interference of different pathways contributing to the free-carrier absorption (Drude-like) of monochromatic radiation.