Origins of laser damage in crystals of KDP

Abstract

The ability of optical materials to withstand high power ultraviolet (UV) laser irradiation without sustaining irrevocable damage is critically important in two areas central to LLNL: laser fusion and UV lithography. In particular, the output fluence of the National Ignition Facility (NIF) is limited by the 351 nm laser damage thresholds of the KH{sub 2}PO{sub 4} (KDP) frequency conversion crystals. The ability to increase the laser output would maximize the odds of achieving ignition, allow target physicists to assess target performance at higher drives, and provide higher temperature-density conditions for studies of the physics of stellar interiors. Moreover, in order to meet the current design criteria for fusion laser systems, KDP crystals must be conditioned by illumination with low fluence laser irradiation to increase the damage threshold by about a factor of two. Over the past two decades, LLNL generated an extensive data base on laser damage and conditioning in KDP and DKDP crystals. While the damage thresholds have improved over time--primarily in response to better filtration of growth solutions--they are still far below what is expected from the band structure of the perfect crystal. Thus these empirical studies have shown that damage in KDP, like the other NIF opticalmore » materials, is caused by extrinsic defects. The purpose of this project was to perform the basic science needed to understand the process of damage or conditioning and identify the defects responsible for damage. In addition, we sought to develop time resolved spectroscopy and imaging tools that would be generally applicable to investigations of laser-materials interactions.« less