Optimized Terahertz Generation in Unmagnetized Plasma with Cubically Chirped Hermite-Cosh-Gaussian Lasers

Theoretical investigation is presented for the generation of terahertz (THz) radiation through the nonlinear interaction of a cubic frequency chirped Hermite-Cosh-Gaussian (HChG) laser beam with an underdense plasma. The structured intensity profile of the HChG beam, characterized by its mode index, beam width parameter, decentered parameter, and Cosh envelope, enables enhanced spatial confinement and field gradients, which are very significant for efficient THz generation. The incorporation of a cubic frequency chirp modulates the temporal phase of the laser pulse, increasing the interaction duration between the laser and plasma electrons. This leads to a stronger ponderomotive force, which drives a transient nonlinear current density at the beat frequency, resulting in the emission of THz radiation. Analytical expressions for the nonlinear current and THz field amplitude are derived, and parametric analysis reveals that the THz output is highly sensitive to the chirp coefficient, laser intensity, mode index and plasma density profile. We found that higher-order spatial modes and carefully adjusted beam positioning (decentering) further enhance THz output. The best results were achieved at a lower chirp value (b = 0.00099), which outperformed traditional linear and quadratic chirping. The study demonstrates that cubic chirping significantly enhances THz conversion efficiency compared to linear and quadratic chirps, offering a tunable and high-intensity THz source for applications in spectroscopy, imaging, wireless communication and ultrafast material characterization and medical diagnostics.