Damage Threshold Measurements Research Articles

Optical ablation by high-power short-pulse lasers

Laser-induced damage threshold measurements were performed on homogeneous and multilayer dielectrics and gold-coated optics at 1053 and 526 nm for pulse durations τ ranging from 140 fs to 1 ns. Gold coatings were found, both experimentally and theoretically, to be limited to 0.6 J/cm2 in the subpicosecond range for 1053-nm pulses. In dielectrics, we find qualitative differences in the morphology of damage and a departure from the diffusion-dominated τ1/2 scaling that indicate that damage results from plasma formation and ablation for τ ≤ 10 ps and from conventional heating and melting for τ > 50 ps. A theoretical model based on electron production by multiphoton ionization, joule heating, and collisional (avalanche) ionization is in quantitative agreement with both the pulse-width and the wavelength scaling of experimental results.

Nanosecond-to-femtosecond laser-induced breakdown in dielectrics.

We report extensive laser-induced damage threshold measurements on dielectric materials at wavelengths of 1053 and 526 nm for pulse durations $\ensuremath{\tau}$ ranging from 140 fs to 1 ns. Qualitative differences in the morphology of damage and a departure from the diffusion-dominated ${\ensuremath{\tau}}^{\frac{1}{2}}$ scaling of the damage fluence indicate that damage occurs from ablation for $\ensuremath{\tau}<~10$ ps and from conventional melting, boiling, and fracture for $\ensuremath{\tau}>50$ ps. We find a decreasing threshold fluence associated with a gradual transition from the long-pulse, thermally dominated regime to an ablative regime dominated by collisional and multiphoton ionization, and plasma formation. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in quantitative agreement with the experimental results.

Measurement of the optical damage threshold in fused quartz

The damage thresholds of five different types of quartz glass used for the production of spectroscopic cuvettes for liquids were determined with single temporal and spatial mode nanosecond pulses at 532 nm. One of the glasses had a damage threshold of ≃420 J/cm(2), which was more than twice that of the other glasses.

Laser-induced damage in dielectrics with nanosecond to subpicosecond pulses.

We report extensive measurements of damage thresholds for fused silica and calcium fluoride at 1053 and 526 nm for pulse durations $\ensuremath{\tau}$ ranging from 270 fs to 1 ns. Qualitative differences in the morphology of damage and a departure from the diffusion-dominated ${\ensuremath{\tau}}^{1/2}$ scaling indicate that damage results from plasma formation and ablation for $\ensuremath{\tau}\ensuremath{\le}10$ ps and from conventional melting and boiling for $\ensuremath{\tau}>100$ ps. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in good agreement with experimental results.

Laser-excited nonlinear properties of BaF2 and its application in a single-shot spatially insensitive autocorrelator

A detailed investigation of laser-generated nonlinear processes of undoped barium fluoride is presented including two-photon-excited exciton fluorescence, color-center formation and damage-threshold measurements. A single-shot autocorrelator based on two-photon fluorescence of BaF2 was constructed to measure the phase and amplitude of pulses provided by large apeture excimer lasers. The capabilities of the autocorrelator are demonstrated at 248 nm and 268 nm with pulses in the regime of 100 fs to 10 ps.

Laser damage testing of CVD-grown diamond windows

Laser-induced damage threshold (LIDT) measurements have been made on bulk polycrystalline (BP) CVD diamond plates using 0.532, 1.06 and 10.6 μm radiation and compared with measurements on natural type IIa diamond samples. Using 10.6 μm radiation, damage thresholds for BP CVD diamond have been measured as 5.6–12.7 MW mm −2 (29–66 J cm −2) using a 50 ns laser spike. These values compare with measurements in type IIa diamond of 12.5–18.5 MW mm −2 made in this study, and values of 3.5–40 MW mm −2 reported elsewhere. The LIDTs measured show a strong correlation with bulk absorption, which is significant at 0.532 μm, and with sample surface finish, but poor correlation with the carbonaceous inclusions found in the samples. Extrapolation from the d.c. dielectric breakdown threshold to the time domain of interest would suggest that, in both current CVD diamond and type IIa single-crystal natural diamond, the damage threshold is being substantially reduced by material or sample imperfections.

Single shot bulk damage threshold and conversion efficiency measurements on flux grown KTiOPO 4 (KTP)

The single shot bulk laser damage threshold was measured on thirteen different flux grown KTP crystals. The measurements were made with the KTP oriented for type II phase matched second harmonic generation (SHG) at 1064 nm. The width of the O-switched Nd:YAG laser pulses was 8.5 ns. The crystals were grown under different experimental conditions. Some of the crystals were measured topographically in order to find possible differences in the damage thresholds between different growth sectors and to check the crystals for optical homogeneity. The measured average threshold for dielectric breakdown of KTP is 31.5±4GW/cm 2. Topographic measurements showed, that the samples have a good optical homogeneity with the exception of trivalent cation doped KTP. The presence of Sc 3+ and Cr 3+ lowers the damage threshold of KTP and areas with preferential chromium incorporation in KTP show a lowered damage threshold in comparison with areas containing less chromium. The conversion efficiency for type II phase matching of the 1064 nm beam of a Nd:YAG laser has a maximum value of 55%. The measured d eff was 3.1 pm/V.

Laser Induced Damage Threshold Measurements at the Microscopic Level

ABSTRACTWe have (tudied the laser damage threshold to silver films (500Å - 1000Å) grown on single crystal silicon <100>, in a newly developed laser damage UHV system at pressures of 10−1 torr. A 1.06μm Nd:Glass laser is used to damage the mirror surfaces in 1-on-1 pulse studies. In-situ damage characterization includes Auger, reflectivity of the primary beam, diffuse scattering of a helium-neon laser, and mass spectrometry detection of desorbed surface species. External characterization includes optical microscopy and SEM. All in-situ damage probes are well correlated and baseline damage occurs at fluences near 3.4 MW/cm2. Time-of-flight to the mass spectrometer shows ejected particles with energies in the 5 to 10 eV range indicating a plasma damage mechanism. Prior to typical thermally induced damage the external microscopy shows well defined precursor morphology changes which appear as feather-like microstructure at the submicron level.

Transmission and damage-threshold measurements in AgGaSe_2 at 21 μm

Measurements of transmission loss coefficients in AgGaSe(2) at 2.1 microm ranged from 0.012 to 0.072 cm(-1). Energy density, not peak-power density, proved to be the determining factor in AgGaSe(2) damage threshold. Coated-surface damage thresholds originally were found to be in the 1.1-2.5-J/cm(2) region by using 180-nsec laser pulses, but damage thresholds as high as 3.5 J/cm(2), with a 3 J/cm(2) average, were measured in coated samples with improved surface quality.

Laser-induced damage threshold measurements of optical dielectric coatings at λ=1.06 μm

The laser-induced damage thresholds for λ=1.06 μm of commercially available (produced at Institute of Optics) dielectric optical coatings, both antireflective and high reflectance, have been determined. The dependence of the optical coatings stability on design and selection of materials has been investigated. An improvement of the coatings durability by using nonquarterwave layers in addition to the basic design of the mirrors has been obtained. The choice of the coating materials is also discussed.

Photoemission studies on metals using picosecond ultraviolet laser pulses

We report the quantum efficiency of various metals irradiated by 266-nm, 4.66-eV laser pulses of 10-ps duration. The highest quantum efficiency obtained is 7.25×10−4 with samarium photocathodes. Current densities exceeding 66 and 21 kA/cm2 have been obtained from an area of 0.05 and 7 mm2, respectively. The maximum currents and current densities obtainable in these experiments are limited by the space charge. For surface fields exceeding 5×107 V/m on gold, the efficiency increases linearly with the field for the values investigated. Based on the quantum efficiency and optical damage threshold measurements, current densities exceeding 100 kA/cm2 seem feasible without damaging the photocathode.

Laser-induced damage threshold and absorption measurements in rare-gas-halide excimer laser components

We have developed a new system to measure the damage threshold with high reproducibility. We controlled the total energy on the sample by an optical system, without changing a spot-size on the sample surface. We made high-reflection coatings with and without over-coat, and anti-reflection coatings with and without under-coat for 248 nm KrF excimer laser light. In high-reflection coating, non-quarter-wavelength design was also tested. Damage threshold of coatings for 193 nm ArF excimer laser were also tested. We measured the absorption of coatings by the interferometric laser calorimetry. Revealing a cause of damage, we made a qualitative and quantitative analysis, so that the main cause of the damageof the optical coatings for 248 nm light was the residual absorption of the coatings.

Single layer Al_2O_3·H_2O and multilayer Al_2O_3·H_2O–SiO_2 optical coatings prepared from colloidal suspensions

Single layer coatings of Al(2)O(3).H(2)O have been prepared on fused silica and BK-7 substrates from aqueous colloidal suspensions of hydrated alumina at room temperature. These coatings were porous and had a refractive index of ~1.44. Multilayer, high reflectivity, dielectric coatings were also prepared by laying down quarterwave-thick, alternating coats of this alumina with silica, also prepared from colloidal suspension, and having a refractive index of 1.22. To achieve 99% reflectivity, 30-35 layers were required. Single shot, laser damage threshold measurements at 1064 nm wavelength with a pulse length of 10 ns were carried out on both the single layer and multilayer system. The thresholds averaged 21 and 12 J/cm(2), respectively.

Polarization dependence of absorption and laser induced surface damage thresholds in wire grid polarizers

We report a theoretical and experimental study of the absorption of wire grid polarizers vs orientation and wavelength. Measurement of absorption and surface damage thresholds are in good agreement with the theory.

Single Point Diamond Turning of an Organic Nonlinear Crystal, L-Arginine Phosphate

We have demonstrated that single point diamond turning can be used to generated high optical quality finished surfaces on a new organic nonlinear crystal, L-arginine phosphate (LAP). The proper choice of cutting conditions can produce surfaces with less than 5 angstroms rms surface roughness. Local softening or melting near the cutting tool tip may play a key role in the machining process by ensuring that material is removed by ductile cutting rather than brittle fracture. At the same time, the low melting temperature of LAP makes lubrication and cooling especially important to prevent extensive melting and tool fouling. In spite of the presence of a weak cleavage plane in LAP, the surface quality is relatively insensitive to crystallographic orientation. Tool wear is apparently negligible, so that surface flatness is governed by the stability of the diamond turning machine. Data on residual subsurface strain and preliminary measurements of the optical damage thresholds of diamond turned LAP surfaces will be presented.

Single-layer TiO_2 and multilayer TiO_2–SiO_2 optical coatings prepared from colloidal suspensions

Single-layer coatings on fused silica substrates have been prepared from aqueous colloidal suspensions of titania at room temperature. These coatings were porous and had a refractive index of 1.9. Multilayer high reflectivity dielectric coatings were also prepared by laying down quarterwave thick alternating coats at room temperature of this titania with silica, prepared from a colloidal suspension of silica. Single-shot laser damage threshold measurements at 1064-nm wavelength with a pulse length of 1 ns were carried out for both the titania and titania-silica systems. These averaged 7.5 and 6.2 J/cm(2), respectively.

Laser-induced (1064-μm) damage threshold measurement at the GEC Research Laboratories, Hirst Research Centre

The laser-induced damage threshold of a series of high-power laser coatings fabricated by Balzers has been measured with a 1.064-μm Nd:YAG Q-switched laser pulse at GEC Research Laboratories, Hirst Research Centre. The results are reported elsewhere [ K. H. Guenther , Appl. Opt.23, 3743 ( 1984)]. This paper outlines the design of the laser and the particular experimental procedures undertaken with this series of tests.

Measurement of optical losses and damage thresholds of multilayer coatings

A correlation between the absorption measured at λ = 514 and 1060 nm and the damage thresholds at λ = 1060 nm is established for evaporated dielectric multilayer stacks. In addition the diffused reflectance of the coatings was measured at λ = 633 nm by means of a Coblentz sphere.

Ripple structures associated with ordered surface defects in dielectrics

Laser-induced ripple structures on material surfaces have been observed by a number of workers for various materials including metals, semiconductors, and dielectrics. A model has been proposed [1] which correctly accounts for the spacing and polarization dependence of the ripples, and the association of the ripples with material defects. In this letter, experimental results which unambiguously show the association of these features with defects are presented. Quantitative measurements of laser-induced damage thresholds show a reduction of damage thresholds for surfaces with controlled linear defects aligned orthogonal to the incident laser field (as predicted in [1]).

Scandium oxide coatings for high-power UV laser applications

Scandium oxide has proved to be a damage-resistant high-index material in laser coatings for use at 248 nm. The results of damage threshold measurements on laser reflectors and antireflection coatings of various designs, material combinations, and deposition temperatures are presented. The most significant effects are observed for overcoat layers on high reflectors and undercoat layers on antireflection coatings. Thresholds >6 J/cm(2) for 20 nsec pulses were observed in both cases.