Manuscript Received Accumulated Surface Damage on ZnS Crystals Produced by Closely Spaced Pairs of Picosecond Laser Pulses* L. L. Chase and H. W. H. Lee Lawrence Livermore National Laboratory Excitation of a transparent ZnS crystal by repetitive picosecond dye laser pulses causes an accumulated surface modification leading to optical damage. The onset of the damage is detected by an abrupt increase in the emission of neutral Zn (and possibly S2) from the surface. Comparison of the neutral emission thresholds with pulse-pair and single-pulse excitation shows that linear absorption is the dominant laser-surface interaction. In general, this measurement technique shows considerable promise for investigating the possible influence of nonlinear absorption or excitation processes on damage mechanisms. The data suggest that heating of small absorbing regions produces the surface modification that leads to the observed surface ablation. The nature of the damage observed at fluences above the threshold suggests that it is caused by heating of a relatively large (~10 - 100 μm) surface region that has been modified by the accumulation pulses. Key words: surface damage; accumulation; picosecond; neutral species emission 1. Introduction The interaction of intense laser beams with the surfaces of transparent optical materials can involve linear and nonlinear absorption caused by surface states, defects and impurities. Direct measurement and characterization of these interactions is very difficult. It is known, however, that laser excitation of many materials causes the emission of charged and neutral surface constituents and impurities. The characteristics of the emitted products, such as the identity of the emitted species, velocity distributions, absolute and relative yields, and internal excitations, provide useful information regarding the laser-surface interaction[1,2]. In previous work on ZnS with nanosecond lasers at wavelengths above and below the interband absorption threshold, it was found that Zn, S, and possibly S2 were emitted at laser fluences below the threshold for observable optical damage[1]. In this work we measured the yield of neutral Zn and S from surfaces of ZnS single crystals caused by multiple-pulse excitation with a picosecond dye laser at a wavelength of 580 nm, which is much longer than the interband absorption threshold of about 330 nm. This wavelength is, however, well within the region of two-photon bulk absorption[3]. In this circumstance two photon absorption involving bulk, surface, or defect levels may be a significant factor in optical damage mechanisms, particularly for picosecond pulses, which have high peak intensities at the damage threshold. 2. Experimental Procedure The experimental setup is shown in figure 1. The sample was irradiated in a vacuum system at a pressure of about 10-9 torr using a picosecond dye laser operating at 10 Hz with a pulsewidth of 1.5 ps and a pulse energy of up to 300 μJ. The laser was focused to a spot diameter of about 150 μm, yielding fluences of up to about 1.3 J/cm2. A quadrupole mass spectrometer was used with a reduced mass resolution to detect the emitted Zn at masses 64 - 68 amu. It is also possible that S2 (mostly mass 64 amu and some at 66 amu) is detected, although its presence in the emission is not established with certainty. because of its mass overlap with Zn. The output of the mass spectrometer was sampled by a gated integrator for a time interval of 1 ms following the laser pulse. The polished surface of the crystal was cleaned in methanol and heated in vacuum to 300 C to desorb contaminants. Data were collected using multiple excitation on each of a grid of spots separated by about 1 mm on the sample surfaces. *Work performed under the auspices of the Division of Materials Sciences of the Office of Basic Energy Sciences, U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48. |