Enhanced terahertz radiation generation by phase-controlled two-color laser pulses interacting with an under-dense plasma

We investigate terahertz (THz) radiation generation at the vacuum-plasma interface driven by the oblique incidence of s-polarized Gaussian laser pulse(s) on a semi-infinite underdense plasma. Extending beyond the conventional single-frequency bipolar pulse (B-pulse), this work focuses on leveraging two-color mixed-frequency pulse- (M-pulse) excitation to enhance THz performance in terms of strength and broadening that can be tuned through controlled amplitude and phase of the constituent pulses of the M-pulse. A new expression for the ponderomotive force (PF)-which acts as the main driver of THz radiation at the vacuum-plasma boundary-is derived to capture the hitherto unexplored effects of phase asymmetry intrinsic to the M-pulse, in contrast to a single B-pulse where its phase is irrelevant. This PF formulation captures the underlying cycle-to-cycle symmetry-breaking for the M-pulse field, responsible for efficient THz emission. We demonstrate analytically that such M-pulses of the same total energy as a B-pulse may generate significantly enhanced PF, leading to THz yields several orders of magnitude higher. With a judicious choice of low-frequency to high-frequency ratio, the M-pulse configuration is shown to emerge as a highly efficient, phase-controllable driver of THz radiation and offers a promising route for optimizing THz source design via tailored two-color laser-plasma interactions. Particle-in-cell simulations performed with a finite-size plasma corroborate the main findings of the analytical model.