Generation of high order spatially coherent harmonics from solid targets by femtosecond laser pulses

Summary form only given. Laser-matter interaction at high intensity provides a variety of new schemes for the production of ultrashort pulses of short wavelength radiation. Rapid progress has been made in the generation of optical harmonics of high order in noble gases, and a few hundred harmonic orders have been observed. More recently, harmonic generation of laser light upon reflection from the plasma-vacuum boundary has attracted considerable attention both in experiment and in theory. High conversion efficiencies and no high-frequency limit are predicted for harmonic generation at the intensities approaching relativistic level. A sharp plasma-vacuum interface is essential for this type of harmonic generation. With the use of relatively long pumping pulses, when the hydrodynamic expansion of the target was counteracted by the pondermotive forces, up to 75 harmonic has been observed. However the harmonics were generated not in the specular direction, but distributed in a wide angle. The paper describes new results on the observation of high order harmonics using femtosecond pulses, when the target hydrodynamic expansion during the laser pulse can be neglected and spatially coherent harmonics are expected.