1 © 2020 Astro Ltd Printed in the UK Introduction The terahertz (THz) region in the electromagnetic spectrum lies between the microwaves and the infra-red region, nar- rowing the gap between electronics and optics. Previously, complications in effciently producing and controlling THz waves served as an obstruction for the region. However, due to continuous development in this area, THz radiation is a matter of recent interest. THz radiations are utilised in many felds such as remote sensing, communication technology [1, 2], environmental monitoring, and biological imaging etc [3, 4]. Due to the unique abilities of THz radiation, a vari- ety of ideas have emerged for several applications including security telecommunications, industrial non-destructive test- ing, health, etc. Recently, an experiment was conducted at the Free-electron LASer in Hamburg (FLASH) at Deutsches Elektronen-SYnchrotron (DESY), utilizing THz radiation. Azima et al [5] employed a single-shot THz streak-camera to measure the duration and spectral phase of the intense XUV pulses. However, for THz applications in the case of photo- conductive antennas and semiconductors, etc, low conversion effciency and the material breakdown effciency serve as major shortcomings. In order to overcome these concerns, plasma acts as an appropriate medium which overcomes the material damage related problems in the case of THz generation. Several tech- niques have been proposed experimentally [6–8] and theor- etically [9–14] for effcient THz generation by employing plasma as a medium. Kwon et al [15] have reported highly energetic, short duration bursts of coherent THz radiation from an embedded plasma dipole. Plasma dipole oscillations are produced due to the trapping of electrons in the potential Laser Physics Terahertz generation by beating of two chirped pulse lasers in spatially periodic density plasma Alka Mehta 1 , Jyoti Rajput 1 , K Kang 2 and Niti Kant 1 1 Department of Physics, Lovely Professional University, G.T. Road, Phagwara 144411, Punjab, India 2 Department of Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea E-mail: nitikant@yahoo.com Received 13 December 2019 Accepted for publication 27 January 2020 Published 27 February 2020 Abstract The present paper reports on the combined infuence of the chirp characteristics of pump pulses and spatially periodic density (density ripple) plasma, on terahertz (THz) radiation generation, by beating of two chirped pulses. The beating lasers exert a nonlinear ponderomotive force along the z-direction. A self-consistent feld is generated due to the nonlinear oscillations of plasma electrons; as a result of these linear and nonlinear forces the plasma electrons attain an oscillatory velocity that couples with the density ripple to generate a stronger transient transverse current, driving THz radiation. The importance of chirp parameter, amplitude and periodicity of density structure are discussed for emitted THz radiation. Our numerical simulations disclose that the variation of the chirp frequency parameter and ripple amplitude have considerable roles in improving the nonlinear oscillating current. By optimizing the chirp parameter and amplitude of the density ripple, a notable change in the magnitude of the terahertz feld amplitude is found. The present paper maybe useful for broadband THz pulses, for use in plasma diagnostics and time-domain spectroscopy. Keywords: terahertz radiation, density ripple, plasma, frequency chirp, ponderomotive force (Some fgures may appear in colour only in the online journal) Astro Ltd 1555-6611/20 /045402+5$33.00 https://doi.org/10.1088/1555-6611/ab7238 Laser Phys. 30 (2020) 045402 (5pp)