Generation of ion acoustic wave in terahertz range by a femtosecond laser pulse in a plasma

The generation of terahertz (THz) ion acoustic waves (IAWs) via the decay instability of femtosecond laser-driven Langmuir waves in a plasma is investigated theoretically. A high-intensity femtosecond laser pulse incident on a plasma at an oblique angle excites large-amplitude Langmuir waves near the critical layer, which subsequently decay into IAWs and secondary plasma waves. We derive analytical expressions for the laser electric field components, incorporating spatial and temporal Gaussian profiles, and model the energy transfer dynamics between waves and electrons. Landau damping and resonance absorption are identified as critical mechanisms governing the heating of electrons, with the damping rate $\Gamma$ derived in normalised form, revealing an exponential dependence on the normalised wave frequency. The temporal evolution of electron temperature is quantified, showing a direct relationship with the laser field intensity and plasma parameters. Furthermore, the inhomogeneous Airy equation formalism is employed to solve for the electric field structure near resonance layers, highlighting the role of plasma scale length and thermal velocity in wave localisation. Our results demonstrate efficient THz IAW generation under optimised laser and plasma conditions, with the damping rate minimised at specific frequency ratios, enabling sustained wave propagation. This work advances the understanding of laser–plasma interactions for THz wave generation, offering insights for applications in high-frequency radiation sources and plasma diagnostics.