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

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 pseudodensity gradient, leading to a superluminal wakefield that emits Cherenkov-like radiation with radial polarization. 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 ultra-narrow spectral features, further enhancing the frequency tunability of the radiation mechanism for different applications.