Abstract

Laser-produced plasmas created by intense femtosecond laser pulses have been demonstrated to be subpicosecond soft-x-ray sources.¹ The use of terawatt femtosecond lasers will permit the generation of more intense x-ray sources; however, increased target reflectivity at high fluence will result in less efficient coupling of laser energy into the plasma. We demonstrate experimentally that structured targets can dramatically increase this coupling. Our targets were lithographically produced SiO diffraction gratings with a 150 nm depth and a 300 nm period. An incident laser pulse of 3 mJ in 150 fs at 620 nm was focused to 10¹⁶ W/cm² on the target. For polarization perpendicular to the groove structure, an effective-index approximation shows that the laser light penetrates the grooves; a reflectivity of only 8% was observed experimentally. The x-ray yield for 30 eV to >1 keV increased by >11 times. This increased absorption is a result of the enhanced electric field in the voids between the grooves, resulting in an enhanced energy deposition, a hotter plasma, and a more intense x-ray emission.

© 1990 Optical Society of America