Generation Of Terahertz Radiation Research Articles

Terahertz Wave Generation From Two-Color Laser-Excited Air Plasma Modulated by Bichromatic Laser Fields

Terahertz (THz) waves were generated using a two-color femtosecond laser with a fundamental wavelength of 1500 nm; the waves were modulated using two-color modulation plasma with a fundamental wavelength of 800 nm in a collinear configuration. The properties of the THz waves including intensity, polarization, and spectrum, were systematically investigated by changing the energy ratio, relative phase, and the polarization direction of the pump and modulation beams. The results demonstrate that the THz intensity has a superimposed effect when the two beams simultaneously ionize the ambient air. When the energy ratio of the beams was changed, the intensity ratio of the high- and low-frequency components of the THz spectrum changed continuously, and the THz polarization gradually transitioned from elliptical to linear. Moreover, the center frequency of the THz spectrum can be blue shifted by shortening the optical length of the modulation beam. An arbitrary elliptically polarized THz wave can be obtained by controlling the relative polarization angle of the two-color modulation lasers. The findings of this study will facilitate practical applications involving the generation and selection of specific THz radiation.

Correlation of fast electron ejections, terahertz waves, and harmonics emitted from plasma mirrors driven by sub-relativistic ultrashort laser pulse

When an ultrashort laser pulse incidents onto a plasma mirror, there exist fast electron ejections, terahertz (THz) radiation, and harmonic generation simultaneously. We investigated the correlation of these three emission phenomena at a preplasma density gradient scale length of (0.05–1)λ and sub-relativistic laser intensity (a0 = 0.4) via particle-in-cell simulation. It is shown that THz radiation is positively correlated with fast electron ejections. As the gradient scale length increases, both enhance first, reach a maximum at 0.4λ, and then degrade at a longer scale length. Harmonic generation, on the other hand, presents the strongest radiation at a sharp surface of 0.05λ and then decays continuously at a softer gradient, indicating that it has an anti-correlation with the fast electron ejections at first (<0.4λ) but turns into a positive correlation at a softer gradient. We find that the laser energy absorption mechanism plays a vital role in the correlation among these emission phenomena. At a sharp boundary of <0.4λ gradient scale length, the Brunel mechanism is dominated, and the absorption rate increases gradually with the increasing gradient scale length. However, at the softer boundary of >0.4λ, the absorption rate decreases continuously according to stochastic heating, and the dependence on laser polarization is eventually lost. The transition of laser absorption mechanisms alters the correlation among fast electrons, THz driven by ejected fast electrons via coherent transition radiation, and harmonics excited by bounded electrons.

Terahertz Pulse Generation from GaAs Metasurfaces.

Ultrafast optical excitation of select materials gives rise to the generation of broadband terahertz (THz) pulses. This effect has enabled the field of THz time-domain spectroscopy and led to the discovery of many physical mechanisms behind THz generation. However, only a few materials possess the required properties to generate THz radiation efficiently. Optical metasurfaces can relax stringent material requirements by shifting the focus onto the engineering of local electromagnetic fields to boost THz generation. Here we demonstrate the generation of THz pulses in a 160 nm thick nanostructured GaAs metasurface. Despite the drastically reduced volume, the metasurface emits THz radiation with efficiency comparable to that of a thick GaAs crystal. We reveal that along with classical second-order volume nonlinearity, an additional mechanism contributes strongly to THz generation in the metasurface, which we attribute to surface nonlinearity. Our results lay the foundation for engineering of semiconductor metasurfaces for efficient and versatile THz radiation emitters.

Data Mining for Terahertz Generation Crystals.

A data mining approach to discover and develop new organic nonlinear optical crystals that produce intense pulses of terahertz radiation is demonstrated. The Cambridge Structural Database is mined for non-centrosymmetric materials and these structural data are used in tandem with density functional theory calculations to predict new materials that efficiently generate terahertz radiation. This enables us to (in a relatively short time) discover, synthesize, and grow large, high-quality crystals of four promising materials and characterize them for intense terahertz generation. In a direct comparison to the current state-of-the-art organic terahertz generation crystals, these new materials excel. The discovery and characterization of these novel terahertz generators validate the approach of combining data mining with density functional theory calculations to predict properties of high-performance organic materials, potentially for a host of exciting applications.

Local OAM manipulation of a terahertz wave from the air filament by chirping the few-cycle vortex pump laser

We propose a scheme to manipulate the local orbital angular momentum (OAM) of the ultra-broadband (0.1-30 THz) terahertz (THz) waves from the laser-induced short air filament via chirping the few-cycle vortex laser pump. The simulation results show that either the THz vortex pulses with linear azimuth-dependent phases or the THz angular accelerating vortex beams (AAVBs) with nonlinear azimuth-dependent phases can be produced by tuning the chirp parameter of the pump. Thus, the dominant physical mechanism for THz generation can be determined. The THz temporal and transverse spatial distributions can be also controlled by the chirp parameter. Furthermore, their local OAM density distributions present very complex structures because most of the modulated azimuthal intensity and the corresponding local angular helicity distributions are not able to cancel out completely. Via analyzing the simulated THz results at the different pump intensities, we classify the initial pump intensity into three cases. For the low intensity case, the Kerr effect comes into prominence, so the generated THz radiation shall be vortex pulses. While for the high intensity case, the leading plasma effect dominates. In contrast, when the pump intensity is at the medium level, the Kerr nonlinearity and the plasma effect may be comparable and competitive. Basically, THz AAVBs are generated for both high and medium intensity cases. Our study will provide the possibility for studying the optically induced rotation technology more intuitively from the perspective of angular momentum transfer.

THz generation by optical rectification of femtosecond laser pulses in a liquid crystal

In this study, we propose the generation of broadband terahertz (THz) radiation from a liquid crystal with large birefringence in the THz range, pumped by 800 nm femtosecond laser pulses based on optical rectification. Our measurements revealed that the THz amplitude depends on the orientation of the liquid crystal relative to the polarization of the femtosecond pump. Additionally, it was found that the THz peak intensity is linearly dependent on the pump fluence. Furthermore, an elliptically polarized THz wave pattern was observed. The results may lead to a new type of tunable THz source.

Coherent terahertz radiation generation by a flattened Gaussian laser beam at a plasma–vacuum interface

Coherent terahertz radiation generation by a flattened Gaussian laser beam at a plasma–vacuum interface

Optical generation of strong-field terahertz radiation and its application in nonlinear terahertz metasurfaces

Abstract Extremely nonlinear terahertz (THz)-matter interactions and applications have positioned themselves as the next frontier in quantum information, nonlinear optics, and particle acceleration. However, the absence of free-space highly intense THz sources and the diffraction limit, which prevents THz waves from being concentrated to the nanoscale scale, are inhibiting the growth of extreme THz. To address this difficulty, suitably extremely concentrated THz sources are being produced, while (non-)resonant artificial metastructures are being widely used to enhance local fields, resulting in deep-subwavelength (<λ/103) confinement of highly enhanced THz fields in micro-/nano-gaps. We discuss solid-state stable sources of intense THz radiation generated by femtosecond lasers in this Review, with a special emphasis on the lithium niobate-based tilted pulse front approach and the nonlinear THz metasurfaces allowed by it. Finally, we forecast the field’s future directions in extreme THz research.

Soliton mode of terahertz radiation generation using picosecond laser pulses with tilted wave fronts

The soliton-like mode for highly efficient generation of terahertz radiation in a quadratically nonlinear medium by means of optical pulses with tilted wave fronts is investigated. For optical and terahertz pulses, a system of the Zakharov–Boussinesq type equations describing the process of generation at arbitrary angles of the tilt of wave fronts is derived. Various cases of the soliton generation are analyzed. It is shown that the tilt of wave fronts is fundamental for the formation of the optical-terahertz solitons of a new type, which we call dispersionless solitons.

Terahertz Plasmonic Quantum Cascade Lasers: a Comprehensive Physics Study

Generation of terahertz (THz) radiation has been a hot research topic in recent years. Plasmonic quantum cascade lasers (QCLs) are among the most compact and efficient sources to generate THz radiation. In this paper, we comprehensively study plasmonic QCLs designed based on the antenna-feedback structure to generate efficient radiation about the center frequency of 3 THz. By changing the geometric structure of the plasmonic cavity and using two-dimensional simulation, a minimum loss less than 5.9 cm−1 is achieved at the lasing frequency. It is also possible to control the orientation of the output beam either vertically or tilted by changing the geometry of the antenna design via chirped or non-chirped grating scheme. Moreover, the output characteristics of the QCL are simulated based on the three-level rate equations through which the dynamics of the laser, as well as the P-I curve, are investigated. Also, the gain spectra for two laser designs (with chirped and non-chirped gratings) are simulated and compared to each other. The results of this paper may provide deep insight into designing efficient laser sources in the THz region.

Terahertz annular antenna driven with a short intense laser pulse

Generation of terahertz radiation by an oscillating discharge, excited by short laser pulses, may be controlled by geometry of the irradiated target. In this work, an annular target with a thin slit is considered as an efficient emitter of secondary radiation when driven by a short intense laser pulse. Under irradiation, a slit works as a diode, which is quickly filled by dense plasmas, closing the circuit for a traveling discharge pulse. Such a diode defines the discharge pulse propagation direction in a closed contour, enabling its multiple passes along the coil. The obtained oscillating charge efficiently generates multi-period quasi-monochromatic terahertz waves with a maximum along the coil axis and controllable characteristics.

High-peak-power orthogonally-polarized dual-wavelength Nd:YLF/BaWO<sub>4 </sub>Raman laser

Orthogonally-polarized dual-wavelength laser has significant practical applications in various fields, such as precision metrology, terahertz radiation generation, differential radar, spectral analysis. The Nd:YLF crystal has two orthogonally-polarized emission peaks with comparable emission cross sections, high-energy storage capability and relatively weak thermal lens effect. Owing to these properties, it has been recognized as a suitable gain medium for generating orthogonally-polarized dual-wavelength laser. In this paper, the Nd:YLF crystal with low doping concentration is employed as a laser gain medium to produce 1047 nm and 1053 nm dual-wavelength fundamental lasers with orthogonal polarizations, and the risk of thermal cracking of Nd:YLF crystal is reduced by appropriately increasing the pump spots. Using the intracavity Raman frequency shift in BaWO<sub>4</sub> crystal, orthogonally-polarized dual-wavelength Raman lasers at 1159.9 nm and 1167.1 nm are achieved to have high peak power. Under the total incident pump power of 40 W and a pulse repetition rate of 5 kHz, the maximum dual-wavelength Raman output power is obtained to be 2.67 W. The corresponding total optical conversion efficiency is 6.7%. For 1159.9 nm and 1167.1 nm Raman laser, their maximum average output power values are 1.31 W and 1.36 W, respectively. Their narrowest pulse widths are 1.50 ns and 1.53 ns, and the corresponding peak power values are as high as 174.7 kW and 177.8 kW, respectively. The results show that the problem of thermal cracking of Nd:YLF crystal at high pump power can be solved by reducing the doping concentration and increasing the pump spot. The Nd:YLF/BaWO<sub>4</sub> is a promising crystal combination for realizing orthogonally-polarized dual-wavelength Raman laser.

Generation of Intense Near and Mid-Infrared Femtosecond Radiation (1.2–2.4 μm) with the Use of the Broadband Parametric Down-Conversion in a Type-II BBO Crystal Pumped by a Ti:Sapphire Laser and Its Application for the Generation of Terahertz Radiation in Organic Crystals

Generation of Intense Near and Mid-Infrared Femtosecond Radiation (1.2–2.4 μm) with the Use of the Broadband Parametric Down-Conversion in a Type-II BBO Crystal Pumped by a Ti:Sapphire Laser and Its Application for the Generation of Terahertz Radiation in Organic Crystals

Terahertz emission from LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterostructures pumped with femtosecond laser

Since the discovery of the ultrafast demagnetization of the ferromagnetic metal, the spin degree of electrons is gradually used to generate terahertz radiation. The terahertz radiation generated by the inverse Rashba-Edelstein effect was confirmed first at the interface of Ag/Bi. However, the spin-to-charge conversion efficiency of the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface is one order of magnitude lager than that of the Ag/Bi interface under equilibrium or quasi-equilibrium condition. Whether the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterostructures can be used to convert spin current to generate terahertz radiation remains to be systemically studied. In this work, we fabricate the NiFe/LaAlO<sub>3</sub>//SrTiO<sub>3</sub> heterostructures and investigate the generation of terahertz radiation by femtosecond laser pumping and its dependence of the magnetic field direction. We change the thickness of the LaAlO<sub>3</sub> to show the applicability of the superdiffusive spin transport model and optical transmission model. We find the multireflections at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface weaken the terahertz radiation intensity. This work provides experimental and theoretical support for further optimizing the generation of terahertz electromagnetic waves.

Anisotropic photoconductivity excited in a semiconductor by two-frequency optical radiation

Anisotropic photoconductivity at a difference frequency, excited in a semiconductor by linearly polarized two-frequency optical radiation, is considered. The anisotropy of the photoconductivity arises due to the optical alignment of photoexcited electrons momenta and dependence of their effective mass and momentum relaxation time on energy. It is shown that the contribution of the anisotropic photoconductivity to the photocurrent at the difference frequency lying in the terahertz frequency range can be comparable with that of the isotropic photoconductivity. This effect can manifest itself in photoconductive antennae, devices used to generate terahertz radiation. Keywords: Anisotropic photoconductivity, photomixing, terahertz radiation, photoconductive antenna.

Анизотропная фотопроводимость, возбуждаемая в полупроводнике двухчастотным оптическим излучением

Anisotropic photoconductivity at a difference frequency, excited in a semiconductor by linearly polarized two-frequency optical radiation, is considered. The anisotropy of the photoconductivity arises due to the optical alignment of photoexcited electrons momenta and dependence of their effective mass and momentum relaxation time on energy. It is shown that the contribution of the anisotropic photoconductivity to the photocurrent at the difference frequency lying in the terahertz frequency range can be comparable with that of the isotropic photoconductivity. This effect can manifest itself in photoconductive antennaе – devices used to generate terahertz radiation.

Efficiency of Photoconductive Terahertz Generation in Nitrogen-Doped Diamonds

The efficiency of the generation of terahertz radiation from nitrogen-doped (∼0.1–100 ppm) diamonds was investigated. The synthetic polycrystalline and monocrystalline diamond substrates were pumped by a 400 nm femtosecond laser and tested for the photoconductive emitter operation. The dependency of the emitted THz power on the intensity of the optical excitation was measured. The nitrogen concentrations of the diamonds involved were measured from the optical absorbance, which was found to crucially depend on the synthesis technique. The observed correlation between the doping level and the level of the performance of diamond-based antennas demonstrates the prospects of doped diamond as a material for highly efficient large-aperture photoconductive antennas.

Terahertz wave amplification by a laser-modulated relativistic electron beam

Generation of tunable terahertz radiation based on the interaction of a frequency-chirped low power terahertz wave with a relativistic electron beam is proposed. The electron beam is first modulated temporally due to the ponderomotive force induced by the beat wave of two copropagating intense laser beams of slightly different frequencies. Then the electron beam interacts with a low-amplitude driver wave at terahertz frequency with a linear frequency chirping. A nonlinear transverse current density is driven that emits terahertz radiation at an up-shifted frequency with a relatively enhanced amplitude. The modulation process is analyzed theoretically by a complete set of fluid equations and considering the effects of both the electrons space-charge potential and the beat wave ponderomotive force. The initial density and energy of the electron beam are both optimized in order to improve the terahertz amplification process. The effect of the driver's frequency chirping, as well as the effects of the initial electron beam density and energy on the emitted terahertz radiation are investigated numerically. It is shown that the frequency chirping of the driver wave plays a key role in terahertz amplification. It is shown that there is an optimum chirp value for which the terahertz radiation is amplified significantly compared with the case without chirping. Moreover, transverse effects of the laser beams and the driver wave on the emitted terahertz radiation are also investigated.

Photoconductive terahertz generation in nitrogen-doped single-crystal diamond.

The generation of terahertz radiation in a photoconductive emitter based on nitrogen-doped single-crystal diamond was realized for the first time. Under 400 nm femtosecond laser pumping, the performance of diamond antennas with different dopant levels was investigated and compared with a reference ZnSe antenna. Terahertz waveforms and corresponding spectra were measured. A low saturation level for high-nitrogen-containing diamond substrate was revealed. The results indicate the prospects of doped diamond as a material for high-efficiency large-aperture photoconductive antennas.

Hat-Top Beams for Generating Tunable THz Radiations Using a Medium of Conducting Nanocylinders

There are a large number of studies for terahertz (THz) radiation generation, but tunable THz sources are still a challenge since it is difficult to tune frequency, focus and intensity of the radiation simultaneously. The present work proposes the THz generation by the interaction of two hat-top laser beams with a host medium of argon gas containing graphite nanocylinders, as these beams result in highly nonlinear interaction because of a smooth dip in their peak intensity and a fast rise and fall in the overall intensity pattern. Such an interaction produces nonlinear current (6.7 × 108 A/m2) because of the electron cloud of the nanocylinders, which can be modulated by the laser and medium properties for realizing tunable THz radiation. The orientation of basal planes of nanocylinders is shown to be important for this mechanism, though it may be challenging for the experimentalists. The resonant excitation takes place when the plasmon frequency matches the beating frequency of the laser beams, and in the proposed mechanism one can have longitudinal surface plasmon resonance (~12 THz) and transverse surface plasmon resonance (~40 THz), leading to frequency-tunable THz radiation. The role of height and inter particle distance between the adjacent nanocylinders on the THz field amplitude and the efficiency of the mechanism is uncovered by controlling the aspect ratio in the nanocylinders. For example, reducing the inter particle distance from 180 nm to 60 nm leads to the enhancement of THz field from 1 × 108 V/m to 5.48 × 108 V/m. The profile of the emitted THz radiation is investigated in detail under the effect of various parameters in order to prove the practicality of the proposal. The proposed design and mechanism would be attractive for electromagnetic and communication societies which are dealing with millimeter-waves and THz components in addition to its medical application.