SEDL / STP / STP954-EB / STP23127S

Time Resolved Raman Studies of Laser Induced Damage in TiO² Optical Coatings

Exarhos, GJ
Battelle Pacific Northwest Laboratory, Richland, Washington

Morse, PL
Battelle Pacific Northwest Laboratory, Richland, Washington

Pages: 10    Published: Jan 1986

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Abstract

Molecular information available from Raman scattering measurements of sputter deposited TiO² on silica substrates has been used to characterize crystalline phases, thickness, and surface homogeneity. A two laser technique is described for investigating transient molecular changes in both coating and substrate which result from pulsed 532 nm laser irradiation. Single layer and multilayer coatings of both anatase and rutile phases of TiO² have been probed by Raman spectroscopy immediately following the damage pulse (nanoseconds) and at longer times. Transient measurements are designed to follow surface transformation/relaxation phenomena; measurements at longer times characterize the equilibrium damage state. The anatase coatings were found to be more susceptible to laser damage than rutile coatings and exhibited irreversible changes during the damage process. Rutile coatings exhibited a reversible phase transition to the anatase form followed by recrystallization to the rutile phase (milliseconds). Loss of coating material from all damaged surfaces is inferred from decreased Raman scattering intensity. Substrate damage characterized by laser induced scrambling of polarized light was also observed. Raman results and molecular structural information are used to formulate a possible damage mechanism for TiO² coatings.