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, 4H-SiC 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.

An analytical and numerical study has been carried out for the generation of terahertz (THz) radiation by beating of two intense cosh-Gaussian laser beams (decentered Gaussian beams) in the rippled density magnetized plasma under the relativistic–ponderomotive regime. In this process, both laser beams exert a relativistic–ponderomotive force on plasma electrons at the beat frequency and impart them an oscillatory velocity in the presence of a static magnetic field. Due to coupling between this nonlinear oscillatory velocity with density ripple, nonlinear current is generated that excites the THz radiation at the different frequency. Higher-order paraxial-ray approximation (non-paraxial theory) has been used in this study. The effects of the decentered parameter, magnetic field, and density ripple on the THz radiation generation in ripple density magnetized plasma have been investigated. Further, the effect of beating of laser beams on the THz field amplitude and the efficiency of THz radiation have been studied. The amplitude and efficiency of the emitted radiation are found to be highly sensitive to the decentered parameter, magnetic field, and density ripple. It has been found that the amplitude and efficiency of the generated THz radiation increase significantly with increasing the values of decentered parameter, magnetic field, and density ripple.

A scheme of phase-matched terahertz generation by beating two co-propagating lasers in magnetized plasma, in the presence of a magnetosonic wave (MSW), is developed. The beat frequency ponderomotive force of the lasers imparts an oscillatory drift to electrons. The electron drift velocity couples with the electron density perturbation associated with the MSW to produce an irrotational nonlinear current $\left(\nabla \times {\vec J}\;{}^{\rm NL}\ne 0\right)$. The beat current density resonantly excites a THz (Terahertz) radiation when the phase-matching conditions are satisfied. The MSW mediates the phase matching. At 9.6 and 10.6 µm wavelengths, and background magnetic field of 285 kG, one may achieve normalized THz wave amplitude of the order of 10−3 and one obtains the ratio of THz power to pump power of the order of 10−6.

We propose a scheme for terahertz (THz) radiation generation by non-linear mixing of two cosh-Gaussian laser beams in axially magnetized plasma with spatially periodic density ripple where electron-neutral collisions have been taken into account. The laser beams exert a non-linear ponderomotive force due to spatial non-uniformity in the intensity. The plasma electrons acquire non-linear oscillatory velocity under the influence of ponderomotive force. This oscillatory velocity couples with preformed density ripples (n′ = n0αeiαz) to generate a strong transient non-linear current that resonantly derives THz radiation of frequency ~ωh (upper hybrid frequency). Laser frequencies (ω1 and ω2) are chosen such that the beat frequency (ω) lies in the THz region. The periodicity of density ripple provides phase-matching conditions (ω = ω1 − ω2 and $\vec k = \vec k_1 - \vec k_2 + {\rm \vec \alpha} $) to transfer maximum momentum from laser to THz radiation. The axially applied external magnetic field can be utilized to enhance the non-linear coupling and control various parameters of generated THz wave. The effects of decentered parameters (b), collisional frequency (νen), and magnetic field strength (B0 = ωcm/e) are analyzed for strong THz radiation generation. Analytical results show that the amplitude of THz wave enhances with decentered parameters as well as with the magnitude of axially applied magnetic field. The THz amplitude is found to be highly sensitive to collision frequency.

A one-dimensional (1D) numerical model for studying enhanced terahertz (THz) radiation generation by mixing of ordinary and extraordinary modes of two-color laser pulses propagating in magnetized plasma has been presented. The direction of the static external magnetic field is such that one of the two laser pulses propagates in the extraordinary mode, while the other pulse propagates in the ordinary mode, through homogeneous plasma. A transverse electromagnetic wave with frequency in the THz range is generated due to the presence of the external magnetic field. It is observed that larger amplitude THz radiation can be generated by mixing of the ordinary and extraordinary modes of the two-color laser pulses as compared with the single laser pulse propagating in the extraordinary mode. Further, 2D simulations using the XOOPIC code show that the fields obtained via simulation study are compatible with those obtained from the numerical model.

This paper presents a theoretical model for efficient terahertz (THz) radiation by self-focused amplitude-modulated laser beam in preformed ripple density plasma. The density of plasma is modified due to ponderomotive nonlinearity which arises because of the nonuniform spatial profile of the laser beam in magnetized plasma and leads to the self-focusing of the laser beam. The rate of self-focusing depends on the intensity of the amplitude-modulated beam as well as on the externally applied magnetic field strength. The electron also experiences time-dependent ponderomotive force by the laser beam at modulated frequency. A nonlinear current at THz frequency arises on account of the coupling between the ripple density plasma and nonlinear oscillatory velocity of the electrons. The yield of the generated THz radiation enhances with enhancement in self-focusing of the laser beam and applied magnetic field.

We develop a theoretical model for terahertz (THz) radiation generation, when an intense short laser pulse (ω1, k1) is mixed with its frequency shifted second harmonic (ω2, k2), where ω2 = 2ω1 + ωT and ωT is in the THz range in the plasma. The lasers exert a ponderomotive force on the electrons and drive density perturbations at (2ω1, 2k1) and (ω2 − ω1, k2 − k1). These density perturbations couple with the oscillatory velocities of the electron due to the lasers and produce a nonlinear current at (ω2 − 2ω1, k2 − 2k1). This current acts as an antenna to produce the THz radiation. The THz power depends upon the square of plasma density and $I_1^2 {I_2}$, where I1 and I2 are the intensities of fundamental and second harmonic laser. The radiation is mainly along the forward direction. Two-dimensional particle-in-cell simulations are used to study the near-field radiation properties.

With the development of ultraintense terawatt (TW) and petawatt (PW) laser systems, powerful terahertz (THz) radiation from laser–plasma interactions has been reported. Plasma-based THz systems, which are usually operated at extremely low repetition rates, call for single-shot diagnostics. In this paper, various state-of-the-art single-shot detection methods are introduced or designed for measurements and applications involved in high-power plasma-based THz sciences.

The first experimental measurements of intense (${\sim }7\times 10^{19}~ {\rm W}~ {\rm cm}^{-2}$) laser-driven terahertz (THz) radiation from a solid target which is preheated by an intense pulse of laser-accelerated protons is reported. The total energy of the THz radiation is found to decrease by approximately a factor of 2 compared to a cold target reference. This is attributed to an increase in the scale length of the preformed plasma, driven by proton heating, at the front surface of the target, where the THz radiation is generated. The results show the importance of controlling the preplasma scale length for THz production.