Abstract
Single high−intensity (106−109 W/cm2) pulses of 1.06−μ radiation emitted from a Q−switched Nd : YAG laser have been used to study reflectivity behavior of single−crystal copper and tantalum samples. Metal reflectivity during irradiation undergoes a significant drop to values much lower than can be attributed to high−temperature effects. This reflectivity drop under high radiation intensity has two characteristic features. First, if vaporization is avoided, then during the rising portion of the laser pulse the reflectivity drops continuously until it reaches a constant low value. For copper this occurs at an intensity of 3×108 W/cm2 and has a reflectivity value of ≈0.64. The corresponding values for tantalum are I≈2.5×107 W/cm2 and R≈0.41. For still higher intensities a further drop in reflectivity beyond the plateau region is observed. This drop in reflectivity is always accompanied by heavy melting and followed by some vaporization. As suggested by Prokhorov et al. this further drop in reflectivity could be associated with dielectriclike behavior of the molten zone.