Received: 12 August 2018 Revised: 24 September 2018 Accepted: 25 September 2018 Published on: 16 October 2018 DOI: 10.1002/ctpp.201800107 ORIGINAL ARTICLE Effects of the evolution of two cross-focused Gaussian laser beams on the terahertz generation in thermal collisional plasma Mohammad Reza Jafari Milani 1,2* Somayeh Rezaei 2 Mohammad Jafar Jafari 2 1 Photonics and Quantum Technologies Research School, Nuclear Science and Technology Research Institute, Tehran, Iran 2 Plasma Physics Research School, NSTRI, Tehran, Iran *Correspondence Mohammad Reza Jafari Milani, Photonics and Quantum Technologies Research School, Nuclear Science and Technology Research Institute, Tehran, Iran. Email: mrj.milani@gmail.com Generation of the terahertz (THz) radiation based on the beating of two cross-focused high intensity Gaussian laser beams in a warm rippled density plasma is numerically investigated, taking into account the ponderomotive force, Ohmic heating, and col- lisional nonlinearities. The beat ponderomotive force as a result of cross-focusing of beams induces a vertical velocity component that by coupling with the rippled density gives rise to a nonlinear current deriving THz radiation. The effect of laser beams spot size evolution and plasma parameters on the THz generation is studied. It was found that there exist special electron temperature and laser intensity ranges with “turning points” where the generation of THz radiation reaches its maximum value and outside of these ranges, it disappears. The results also indicated that increasing the background electron density as well as taking into account the collision frequency help THz generation. Moreover, the maximum yield of THz radiation occurs when the beat wave frequency approaches the plasma frequency. KEYWORDS laser-plasma interaction, self-focusing, THz generation 1 INTRODUCTION In the last few decades, considerable attention has been paid to generation of terahertz (THz) radiation and detection soon after developing ultrashort high-power laser technology. Electromagnetic radiation with spectrum 0.1–100THz has a wide range of applications in particular for medical imaging, material characterization, spectroscopy, and in the communication industry. [1–4] A high-power THz field can be emitted through different ways. Mostly, it is generated as a result of relativistic acceleration of electrons in the conventional accelerators [5] ; however; problems due to huge device transfer are unavoidable. Other schemes including photoconduction and optical rectification [6,7] may also be used as THz wave sources where high-power laser pumps are applied. The main challenge for the optical materials is damage threshold that is a limitation for the higher level of input laser power and consequently for THz efficiency. The plasma-based THz generation technique instead provides an ionized medium, which avoids the material breakdown. Therefore, recently significant efforts have been focused on studying the effects of different laser and plasma parameters on THz generation in the laser-plasma interaction scheme. [8] Nonlinearity emerging through the high-intensity laser-plasma interactions such as filamentation (as a result of ponderomotive force) can be the base and origin of THz emission. [9–12] In fact, the ponderomotive force of laser pulse imparts an oscillatory velocity to the electrons so that as a result of proper phase matching with periodically modulated density plasma, THz waves will be emitted. [13,14] So far, different laser and plasma conditions have been considered to obtain high-power and efficient THz radia- tion. Coherent sub-THz radiation as a result of femtosecond infrared filaments in air was detected experimentally by Tzortzakis et al. [9] An efficient THz emission was recorded by optimizing the specifications of pump pulses by Wang et al. [15] Their group measured a quite high-energy pulse, 0.57 J, for 5.5 THz radiation and also proposed a new design to improve conversion energy. Accurate theoretical and simulation investigations of plasma filament created by ultrashort laser pulses interaction with Contrib. Plasma Phys. 2019;59:292–303 www.cpp-journal.org © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 292