FDTD simulation of high-intensity, ultrashort laser pulses for X-ray generation

Subpicosecond x-ray pulses are needed as sources for time-resolved x-ray scattering experiments. Ultrashort x-ray pulses can be generated by ultrashort, high-intensity laser pulses impinging on a solid target. A major limitation of this method is the low laser energy to x-ray energy conversion efficiency, This is due primarily to the large reflectivity of light on a solid target. A substantial improvement is realized by employing a grating at the surface of the solid. However, this method leaves numerous parameters to be determined in order to establish the most efficient configuration, such as spacing and depth of the grating, as well as angle of incidence and polarization of the impinging light. A simulation procedure using the finite-difference time-domain (FDTD) method is being developed to simulate this interaction. This utilizes a three dimensional simulation of a plane wave impinging on a surface with gratings of various sizes and depths. The illuminating plane wave can be generated at arbitrary angles and polarizations. It is also possible to simulate the interactions with various materials. As the laser pulse impinges on the solid, a high density plasma is generated, so while the laser amplitude varies, the dielectric properties of the medium must be dynamically altered.