Generation of terahertz radiation in collisional plasma by beating of two dark hollow laser beams FARHAD BAKHTIARI, SHOLE GOLMOHAMMADY, MASOUD YOUSEFI, FATEMEH D. KASHANI, AND BIJAN GHAFARY Photonics Lab, Physics Department, Iran Universityof Science & Technology, Heydarkhani, Tehran, Iran (RECEIVED 15 February 2015; ACCEPTED 15 April 2015) Abstract This paper presents a scheme of terahertz radiation generation based on beating of two dark hollow laser beams with different frequencies, the same electric field amplitudes, in actual plasma with spatially periodic density that electron–neutral collisions have taken into account. The main feature of considered hollow laser beams is, having the same power at different beam orders. Because of special distribution in beam intensity gradient in dark hollow laser beam, the produced terahertz radiation has special field profile. The effects of laser and plasma parameters on terahertz radiation generation are investigated analytically. It can be deduced that by increasing beating frequency, efficiency of terahertz generation decreases which can be compensated by manipulating density ripple magnitudes and dark-size adjusting parameter. The intensity of the emitted radiations is found to be highly sensitive to the beam order. Based on the results of this paper, optimization of laser and plasma parameters can increase the efficiency of terahertz radiation generation strongly. Keywords: Beating; Collisional plasma; Dark hollow beam; Terahertz generation 1. INTRODUCTION The terahertz (THz), which its spectrum is altered between microwave and infrared termed (0.1–10 THz) remained un- explored until two decades ago due to lack of high-power ra- diation sources in this region. The topic of THz radiation generation has attracted great interest with deep concepts in both fundamental and applied sciences. THz radiation sources have a number of applications in spectroscopy, sub millimeter astronomy, manufacturing, quality control, process monitoring, chemical and material characterization, security screening, medical imaging, three-dimensional (3D) imaging of teeth, to- mography, topography, remote sensing, etc., (Beard et al., 2002; Ferguson & Zhang, 2002; Shen et al., 2005; Pickwell & Wallace, 2006; Zheng et al., 2006). Driven by the above mentioned applications, several new theoretical and experi- mental mechanisms to generate THz radiation are proposed (Savage et al., 1992; Gildenburg & Vvedenskii, 2007; Singh & Sharma, 2013). Using electro-optic crystal, such as ZnSe, GaP, LiNbO3, or photo conductive antenna, super luminous la- ser–pulse interaction with large band gap semiconductors and dielectric is formal mechanism of the generation of THz sources (Budiarto et al., 1996; Hashimshony et al., 2001; Shi et al., 2002; Holzman & Elezzabi, 2003; Jiang et al., 2011; Wang et al., 2011; Al-Naib et al., 2013). Material breakdown in high- power laser pulses, low conversion efficiency, and narrow bandwidth of emitted THz radiation are disadvantage of these types of THz sources (Budiarto et al., 1996). Due to these dis- advantages, a lot of investigations have been done for introduc- ing new schematic of generation of THz sources. Since plasma is impervious to material breakdown when subjected to high intensity lasers, high-power terahertz radia- tion generation based on laser–plasma interaction has attracted great interest (Hamster et al., 1993; Yoshii et al., 1997; Yugami et al., 2002; Kostin & Vvedenskii, 2010). Various phenomenas in Laser–plasma couplings like self-focusing property of laser, possibility of second-harmonic and many others are affected by the presence of magnetic fields (Brodin & Lundberg, 1998; Gupta & Sharma, 2002; Jha et al., 2007; Hur et al., 2008). The effects of externally applied static magnetic fields on wake excitation and nonlinear evolution of laser pulses have been reported (Ren & Mori, 2004). Moreover, the effects of different Laser beam profiles such as Gaussian, super Gaussian, Cosh–Gaussian, etc., in the in- teraction with plasmas by various distributions, on THz 1 Address correspondence and reprint requests to: Farhad Bakhtiari, Pho- tonics Lab, Physics Department, Iran University of Science & Technology, Heydarkhani, Tehran, Iran. E-mail: fbakhtiari@physics.iust.ac.ir Laser and Particle Beams, page 1 of 10, 2015. © Cambridge University Press, 2015 0263-0346/15 doi:10.1017/S026303461500049X