Narrowband THz radiation from laser wakefields driven in uniform plasma via tight laser focusing

Abstract We propose a mechanism for generating narrowband terahertz (THz) radiation from laser wakefields via laser defocusing in uniform plasma. The defocused laser pulse induces a positive pseudo-density gradient, leading to a superluminal wakefield that emits Cherenkov-like radiation at harmonics of the electron plasma frequency. The radiation is radially polarized, typically with a bandwidth around 1%. Simulation results show that the THz energy scales with the fourth order of the laser intensity for weakly relativistic intense lasers and is inversely proportional to the beam waist. Larger focal spots reduce radiation power but improve its directivity. Under highly relativistic intensity, nonlinear wakefields can generate harmonic THz structures with narrow spectra, further enhancing the frequency tunability of the radiation for different applications.