Prepulse-controlled beat wave terahertz generation and the influence of carbon target geometry

Terahertz (THz) generation via two-color beat wave excitation in plasma is investigated using particle-in-cell simulations. A controlled prepulse is introduced to modify plasma conditions before the arrival of the main laser pulses, and its influence on THz emission is examined. The prepulse reshapes the plasma density profile and induces a temporal delay in the emitted THz radiation. The dependence of THz generation on prepulse intensity, duration, and number of pulses is also analyzed. To enhance the emission, structured carbon targets are introduced into the interaction region. The ring geometry is considered because its boundaries provide strong field confinement and support surface plasma wave excitation. Optimization of the ring target thickness, position, and radius yields a THz enhancement exceeding 40% at certain frequencies. Using the same parameters, the study is extended to other carbon target geometries, including disk and strip configurations. A comparison reveals that horizontal strip targets produce the strongest emission, owing to their on-axis placement and multiple laser–plasma interaction regions that boost the nonlinear current. Analysis of the THz energy density further shows that the prepulse delays the peak energy accumulation, while optimized target geometries significantly increase the overall yield.