[en] Photoluminescence (PL) microscopy and spectroscopy under 266 nm and 355 nm laser excitation are explored as a means of monitoring defect populations in laser-modified sites on the surface of fused silica and their subsequent response to heating to different temperatures via exposure to a CO₂ laser beam. Laser-induced temperature changes were estimated using an analytic solution to the heat flow equation and compared to changes in the PL emission intensity. The results indicate that the defect concentrations decrease significantly with increasing CO₂ laser exposure and are nearly eliminated when the peak surface temperature exceeds the softening point of fused silica (∼1900K), suggesting that this method might be suitable for in situ monitoring of repair of defective sites in fused silica optical components.

[en] Optical components within high energy laser systems are susceptible to laser-induced material modification when the breakdown threshold is exceeded or damage is initiated by pre-existing impurities or defects. These modifications are the result of exposure to extreme conditions involving the generation of high temperatures and pressures and occur on a volumetric scale of the order of a few cubic microns. The response of the material following localized energy deposition, including the timeline of events and the individual processes involved during this timeline, is still largely unknown. In this work, we investigate the events taking place during the entire timeline in both bulk and surface damage in fused silica using a set of time-resolved microscopy systems. These microscope systems offer up to 1 micron spatial resolution when imaging static or dynamic effects, allowing for imaging of the entire process with adequate temporal and spatial resolution. These systems incorporate various pump-probe geometries designed to optimize the sensitivity for detecting individual aspects of the process such as the propagation of shock waves, near-surface material motion, the speed of ejecta, and material transformations. The experimental results indicate that the material response can be separated into distinct phases, some terminating within a few tens of nanoseconds but some extending up to about 100 microseconds. Overall the results demonstrate that the final characteristics of the modified region depend on the material response to the energy deposition and not on the laser parameters.

[en] Fused silica irradiated with ∼3-ns 1064-, 355-, and 266-nm laser pulses as well as with ∼120-fs 825-nm pulses is studied by a combination of photoluminescence (PL) and Raman scattering spectroscopies. Results show that, for laser fluences above the laser-induced breakdown threshold, in all the cases studied, irradiation results in the formation of four defect-related PL bands centered on ∼1.9 (655), 2.2 (565), 2.7 (460), and 4.3 eV (290 nm). Bands centered on 1.9, 2.7, and 4.3 eV are attributed to nonbridging oxygen hole centers (1.9 eV) and oxygen-deficiency defects (2.7 and 4.3 eV). However, defects giving rise to a broad band at ∼2.2 eV are unknown. For all the laser-modified samples studied, Raman spectroscopy reveals a dramatic increase in the intensity of D₁ and D₂ lines, associated with in-phase breathing motions of oxygen atoms in puckered four- and planar three-membered ring structures, respectively. This indicates laser-induced material densification. Based on these results, we discuss physical processes occurring during the catastrophic laser-induced material breakdown, leading to material densification and the formation of point defects

[en] This work is an experimental investigation to evaluate the potential of fluorescence microscopy as a tool to detect surface contamination as well as reveal surface damage precursors on DKDP and SIO₂ optics. To achieve these technical objectives, microscopic imaging systems were built that also incorporated in-situ damage testing capabilities. Fluorescence imaging experiments were performed using 351-nm laser excitation while damage testing was performed at relatively high laser fluences. The experimental results demonstrated the potential of this technique to address the aforementioned technical issues

[en] The evaluation of optical components in various laser systems, with regard to their resistance to laser-induced damage, has often relied on measuring damage threshold fluences. For large-aperture laser systems a small amount of damage in optics does not impede performance. This necessitates the development of damage testing instrumentation that can directly provide information regarding beam obscuration. The number and size of damage scattering sites for a specific laser fluence, wavelength, and pulse duration determine overall beam losses due to damage. We present a design for rapid quantitative characterization of bulk damage performance of optical materials for use in large-aperture laser systems

[en] The spectral characteristics of the internal (PO₄ tetrahedron) modes of fast-grown KH₂PO₄ crystals under sub-damage threshold, 10 ns, 355 nm laser irradiation have been investigated. Pump-and-probe Raman spectroscopy indicates transient changes of the intensity of the 915cm^-1, endash PO₄ internal mode. This change is attributed to a transient increase of the absorption due to generation by the 355 nm pump pulse of electronic defects in the bulk of the crystal. copyright 1998 American Institute of Physics

[en] A microscopic fluorescence imaging system is used to detect optically active centers located inside a transparent dielectric crystal. Defect centers in the bulk of KH₂PO ₄ crystals are imaged based on their near-infrared emission following photoexcitation. The spatial resolution of the system is 1 μm in the image plane and 25 μm in depth. The experimental results indicate the presence of a large number of optically active defect clusters in different KH₂PO ₄ crystals, whereas the concentration of these clusters depends on the crystal sector and growth method. copyright 1999 Optical Society of America

[en] Employing ab initio calculations we have determined the structural parameters of the paraelectric and ferroelectric phases of KH₂PO₄, which are in good agreement with experiment. The calculations reveal that the O-O bond length and the coordinated motion of the P and K atoms have a large effect on the double-well potential for H, controlling the distance, δ, between the two equilibrium positions of the H along the O-O bond in the paraelectric phase. The calculations provide new evidence that the ferroelectric phase transition in H-bonded crystals has also a displacive feature. (author). Letter-to-the-editor

[en] The structure of KH₂PO₄ single crystals (so-called KDP) irradiated with ∼3 ns, 355 nm laser pulses with fluences above the laser-induced breakdown threshold is studied by a combination of Raman scattering, photoluminescence, and soft x-ray absorption spectroscopies. We compare spectra from the as-grown material, surface and bulk laser-induced damage sites, as well as from KPO₃ references. Results show that irradiation with fluences above the laser-induced breakdown threshold leads to decomposition of KDP at surface damage sites but not at bulk damage sites. New spectroscopic features are attributed to dehydration products. For the laser irradiation conditions used in this study, the decomposed near-surface layer absorbs photons at ∼3.4 eV (364 nm). These results may explain the recently reported fact that surface laser damage sites in KDP crystals tend to grow with subsequent exposure to high-power laser pulses, while bulk damage sites do not

[en] Optical absorption and electron paramagnetic resonance (EPR) techniques have been used to characterize the production and thermal decay of point defects in undoped single crystals of KD₂PO₄ grown at Lawrence Livermore National Laboratory. A crystal was irradiated at 77 K with x rays, and then warmed to room temperature. Immediately after irradiation, broad optical absorption bands were observed to peak near 230, 390, and 550 nm. During warming, these absorption bands thermally decayed in the 80-140 K range. Another absorption band peaking near 450 nm appeared as the three bands disappeared. This last band decayed between 140 and 240 K. Correlations with EPR data suggest that the band at 230 nm is associated with interstitial deuterium atoms, the two bands at 390 and 550 nm are associated with self-trapped holes, and the band at 450 nm is associated with holes trapped adjacent to deuterium vacancies. Additional EPR spectra from several oxygen-vacancy centers and a silicon-associated hole center were observed as the crystal was warmed. All the electron and hole traps reported in this study are expected to participate in the room-temperature response of KD₂PO₄ crystals to pulsed high-power ultraviolet laser beams