<title>Reflection of high intensity short laser pulse by plasma gratings</title>

Abstract

The generation of ultra-intense laser pulses is very important in a number of scientific and technical applications, such as plasma accelerators, x-ray lasers, etc. Present Nd:glass lasers are able to deliver sufficiently large pulse energies because a big diameter of their final amplifier aperture and enough high damage threshold of glass for nanosecond laser pulses. However, the damage threshold decreases with the laser pulse duration and, therefore, nanosecond multi kilo-joule laser pulses have to be compressed without a losses of energy and beam quality after the end amplifiers to obtain the required ultra-intense sub-picosecond pulses. The standard technique to reach maximal laser peak power by reducing its pulse length at a given energy is the CPA-scheme [1] in which the laser pulse is stretched, amplified, and compressed by dispersive linear optics. CPA-based optical systems have been able to produce petawatt (PW) laser pulses. The pulse energy is limited by the thermal damage of the optical elements, especially the compression gratings, which for kJ applications have to be very large and therefore extremely expensive. One possibility would be to develop less expensive large-size compound gratings or one could, perhaps, significantly increase the damage threshold of the gratings by multi-layer dielectrics [2] or even using plasma gratings [3]. In the last case back reflection of a short, intense laser pulses at oblique incidence on solid targets is explained with a model where a periodic electron density modulation acts as a diffraction grating. The pump and reflected electromagnetic waves drive through the ponderomotive force the grating and the overall system becomes parametrically unstable. The instability is shown to saturate at some level, because the higher harmonics in the electron density modulation turn the diffraction more diffuse thus reducing both the sustaining ponderomotive force and the back reflection coefficient. The calculated reflection coefficient value is close to the experimental one at the same conditions [4]. We considered the conversion of pump laser long pulse energy into seed short pulse energy on surface plasma gratings. The optimal conditions for maximal conversion efficiency into a back reflected pulse are found. The analytical model and numerical code, which simulate and explain the processes were developed. The result of calculations show that at short plasma length and the optimal parameters of plasma grating the diffraction efficiency can be enough high and such gratings can be used for laser pulse compression.