Abstract-Generation of terahertz radiation by beating of two color lasers in hot clustered plasma with step density profile

Shivani Vij,  Niti Kant

https://iopscience.iop.org/article/10.1088/2516-1067/ab2359/meta

A theoretical model of terahertz generation is presented using nonlinear mixing of two lasers in clustered plasma with step density profile. The cluster is used as a target to enhance the optical to THz conversion efficiency. The lasers are incident obliquely to the clustered plasma surface and exert a ponderomotive force on cluster and plasma electrons. The ponderomotive force has a transverse component that drives nonlinear current and produces THz radiation in the reflected component. The enhancement in the efficiency of THz radiation generation occurs due to cluster plasmon resonance and by coupling between plasma and THz wave. The amplitude of generated THz wave is maximum when the plasma frequency approaches to THz frequency and laser frequency is equal to , where ω pe is cluster plasma frequency. Also, the amplitude is enhanced at an optimum angle of incidence. The dependence of efficiency of THz radiation generation on laser intensity, cluster radius and electron thermal velocity is also studied. We report the normalized THz wave amplitude ~0.044 from Ar cluster at 0.89 μm and 0.44 μm wavelengths of incident lasers with intensity = 7 × 1014 W cm−2.

Abstract-Two color laser self focusing and terahertz generation in multi-ion species plasma

Nafis Ahmada,  Saleh T. Mahmouda, Gunjan Purohitb,  Firoz Khan

https://www.sciencedirect.com/science/article/pii/S0030402617317904


An analytical formalism of self focusing of two co-propagating laser beams, fundamental (ω₁)$\left({}_{\mathrm{}}\right)$ and close to second harmonic (ω₂≈2ω₁)$\left({}_{\mathrm{}}\mathrm{}{}_{\mathrm{}}\right)$, in a multi-ion species plasma and resonant terahertz radiation generation has been developed. The nonlinearity arises due to ponderomotive force on electrons by both the beams and subsequent ambipolar redistribution of the plasma. The ions of higher charged state are strongly depleted from the axial region due to space charge field effects and electron charge is largely compensated by the ions of lower charge. When the power of the strong beam equals the critical power for its self-guiding, the plasma provides an oscillatory waveguide for the weak beam; the weak beam shrinks and rebounds when its frequency is lower than that of the strong beam, and it swallows and rebounds when the frequency is higher. When both beams are strong the nonlinearities have cumulative effect on both the beams. The cross coupling of the beams gives rise to nonlinear current giving rise to terahertz radiation at ω=ω₂−2ω₁${}_{\mathrm{}}\mathrm{}{}_{\mathrm{}}$ frequency. An axial density ripple aids phase matching and gives rise to higher power conversion efficiency. Self focusing leads to enhancement in conversion efficiency.

Abstract-Excitation-wavelength dependent terahertz wave polarization control in laser-induced filament

Liangliang Zhang, Shijing Zhang, Rui Zhang, Tong Wu, Yuejin Zhao, Cunlin Zhang, and X.-C. Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-26-32346

We examine the terahertz (THz) emission from air filament driven by two-color lasers with relatively longer wavelengths than 800 nm. The THz energy dependence on the input laser energy increases more rapidly with a longer laser wavelength, and the scaling laws of THz energy as a function of fundamental wavelength vary for different optical powers, which is theoretically validated by considering the optical wavelength-dependent ionization rate. Furthermore, the THz polarization undergoes a continuous rotation as a function of the laser wavelength, since the relative phase and polarization of the two pulses are adjusted through changing the excitation wavelength in the frequency doubling crystal. Our results contribute to the understanding of THz wave generation in a femtosecond laser filament and suggest a practical way to control the polarization of terahertz pulses for potential applications.

© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Observation of Terahertz Radiation via the Two-Color Laser Scheme with Uncommon Frequency Ratios

L.-L. Zhang, W.-M. Wang, D. Wu, R. Zhang, S.-J. Zhang, C.-L. Zhang, Y. Zhang, Z.-M. Sheng, X.-C. Zhang

(Submitted on 19 Oct 2017)

https://arxiv.org/abs/1710.06998

In the widely-studied two-color laser scheme for terahertz (THz) radiation from a gas, the frequency ratio of the two lasers is usually fixed at ω2/ω1=1:2. We investigate THz generation with uncommon frequency ratios. Our experiments show, for the first time, efficient THz generation with new ratios of ω2/ω1=1:4 and 2:3. We observe that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization and the polarization adjustment becomes inefficient by rotating the other laser polarization; the THz energy shows similar scaling laws with different frequency ratios. These observations are inconsistent with multi-wave mixing theory, but support the gas-ionization model. This study pushes the development of the two-color scheme and provides a new dimension to explore the long-standing problem of the THz generation mechanism.

Abstract-Terahertz emission driven by two-color laser pulses at various frequency ratios

W. -M. Wang, Z. -M. Sheng, Y. -T. Li, Y. Zhang, and J. Zhang

https://journals.aps.org/pra/accepted/e3073NbeQfc1301fb53673d6f1b3f282e85d2c8b0

We present a simulation study of terahertz radiation from a gas driven by two-color laser pulses in a broad range of frequency ratios \omega_1/\omega_0. Our particle-in-cell simulation results show that there are three series with \omega_1/\omega_0=2n, n+1/2, n\pm1/3 (n is a positive integer) for high-efficiency and stable radiation generation. The radiation strength basically decreases with the increasing \omega_1 and scales linearly with the laser wavelength. These rules are broken when \omega_1/\omega_0