Ultra-fast photoluminescence in fused silica surface flaws susceptible to laser damage
- PDF / 190,195 Bytes
- 5 Pages / 432 x 648 pts Page_size
- 22 Downloads / 148 Views
Ultra-fast photoluminescence in fused silica surface flaws susceptible to laser damage Ted A. Laurence1, Jeff D. Bude1, and Nan Shen1 1 Condensed Matter and Materials Division, Lawrence Livermore National Laboratory Livermore, CA 94550, U.S.A. ABSTRACT Using high-sensitivity confocal time-resolved photoluminescence (PL) techniques, we found an ultrafast PL (40 ps-5 ns) from impurity-free surface flaws on fused silica. This PL is excited by the single-photon absorption of sub-band gap light. Regions which exhibit this PL are strongly absorptive well below the band gap, as evidenced by a propensity to damage with 3.5 eV nanosecond-scale laser pulses. Very high defect densities are needed to explain the damage thresholds observed. For such high defect densities, significant interactions between defects may strongly affect the temporal characteristics of the emission of electronic excitations. We propose that the distribution in lifetimes observed is not simply due to a large variety of defect states, but due to a variety of energy transfer interactions between defect states. INTRODUCTION The goal of the National Ignition Facility at Lawrence Livermore National Laboratory is to obtain nuclear fusion in a laboratory setting using laser-based inertial confinement fusion. Laser pulses from 192 beams implode a fuel capsule, increasing the temperature and pressure to the regime where nuclear fusion can occur. Such high energy laser pulses can cause damage in the final focusing optics. This laser damage from nanosecond-scale pulses occurs in a regime with important differences from femtosecond laser damage. For nanosecond scale pulses, the electronic excitations have sufficient time to transfer their energy to the lattice [1]. This fact plays a key role in the development of laser-assisted absorption fronts [2], which lead to the large damage sites caused by temperature-activated absorption. Another important point is that, for the ns-scale pulses at the focusing optics, the intensities in pulses for which damage occurs is only near 2 GW/cm2, which is much lower than intensities at which multiphoton absorption Ȥ(3) processes are observed. In order to explain this laser damage, high defect densities are required. We previously estimated that absorptivities of at least Į=1000 cm-1 are required to explain laser damage of silica surface flaws [3]. For example, if the absorption cross-section ı of a defect is very high at 10-16 cm-2, the defect density required is n = Į / ı = 1019 cm-3. For smaller crosssections, n would have to be greater. We recently discovered a novel photoluminescence with distributed fluorescence lifetimes in surface flaws in fused silica that are associated with laser damage by high fluence, ns-scale lasers [3]. This PL exhibits a broad distribution in lifetimes and emission and excitation spectra; some of the spectral characteristics of this PL were observed previously [4]. We have only found it associated with surfaces and nano-structured materials, particularly surfaces created by fractures [5]. The surfa
Data Loading...
