Control of Strong-Laser-Field Coupling to Electrons in Solid Targets with Wavelength-Scale Spheres

Irradiation of a planar solid by an intense laser pulse leads to fast electron acceleration and hard x-ray production. We have investigated whether this high field production of fast electrons can be controlled by introducing dielectric spheres of well-defined size on the target surface. We find that the presence of spheres with a diameter slightly larger than half the laser wavelength leads to Mie enhancements of the laser field which, accompanied by multipass stochastic heating of the electrons, leads to significantly enhanced hard x-ray yield and temperature.

Figure 1

Scanning electron microscope image of $1\mu \mathrm{m}$ diameter polystyrene spheres arrayed on a Cu substrate. The apparent dimples on the spheres are an artifact of the SEM measurement.

Figure 2

a)  Si K-alpha spectra integrated over 200 shots. The spectrum from a Si wafer coated with 260 nm spheres is compared to spectra from polished flat Si surfaces and from Si surfaces with sub-micron-scale random roughness collected when the Si targets were irradiated at 45 degrees with p-polarized 400 nm light. b) Hard x-ray signal through various filters as a function of the $1/e^2$ cutoff of each filter. The x-ray spectra from silica targets coated with 260 nm spheres (open squares from $55°$irradiation and closed squares from normal irradiation) are compared to spectra derived from targets coated with 100 nm spheres (normal incidence) and from flat targets (irradiated at $55°$ with p-polarization). Error bars were determined from the scatter in data from multiple laser shots.

Figure 3

a) Si K-alpha yield as a function of sphere size coated on Si wafers. b) Hard x-ray yield through two different filters as a function of sphere size. c) Hot electron temperature derived from hard x-ray spectra as a function of sphere size.

Figure 4

a) Calculated field distribution around spheres on a surface arrayed in a hexagonal pattern for spheres with 260 nm diameter illuminated by 400 nm light. b) Calculated maximum enhancement of the square of the electric field when compared to vacuum fields as a function of sphere size irradiated. The dashed line illustrates the field that arises from 100% reflection from a planar surface.