Coupling between a laser and a prestructured target with an arbitrary structure period

The coupling between a laser and a prestructured target with an arbitrary structure period is investigated with the help of two-dimensional (2D) particle-in-cell (PIC) simulations. It is shown that new electromagnetic (e.m.) waves will be generated after the coupling. Since the coupling is a resonant process, strong surface currents will be generated, which result in the generation of strong quasistatic magnetic fields. The frequencies which the newly excited e.m. waves contain are harmonics of the laser frequency and the frequencies can be controlled by the structure period. Also, the propagation directions of the newly excited e.m. waves are well controlled by the ratio between the structure period and the laser wavelength, which means e.m. waves excited by lasers with different frequencies have different propagation directions. As a result, the prestructured target can act as a new kind of optical gratings which can be used to split superintense laser pulses. The controlling of both the harmonic frequencies and the propagation directions can be explained by matching condition of the coupling, which is a formula resembling but physically different from the diffraction grating equation. The quasistatic magnetic fields are on the target front surface and as strong as hundreds of teslas, but the amplitude will be decreased by the newly excited e.m. waves when they are propagating along the target surface. Since the propagation directions are controlled by the structure period, with an optimal structure period, the prestructured target can also act as a source of strong quasistatic magnetic fields.