Cherenkov-type coherent radiation from laser wakefields driven in nonuniform plasma

The evolution of the laser wakefields driven in a plasma slab with a longitudinal up-ramp density profile and the associated radiation are investigated theoretically and numerically by using two-dimensional particle-in-cell simulations. The radiation shows a conical distribution along the laser propagation direction, which is produced via the Cherenkov-type mechanism as the phase velocity of the wakefields is superluminal. The relationship between the radiation intensity and the laser focal spot size is found from simulation, which agrees with theory. At moderate laser intensity, the radiation shows a spectrum at harmonics of the electron plasma frequency. When the wakefields are excited in the highly nonlinear regime, the radiation spectrum shows a supercontinuum profile extending to a dozen times of the electron plasma frequency. Our results may be applied to optimize experimental parameters for enhanced radiation generation from the laser wakefields.