Laser-induced Damage Experiments Research Articles

Optimizing sub-nanosecond laser conditioning of DKDP crystals by varying the temporal shape of the pulse.

We propose a strategy to optimize the laser conditioning of DKDP crystals by varying the temporal shape of sub-nanosecond pulses. Four sub-ns temporally shaped pulses with nearly the same full width at half maxima of ∼600 ps but different rising-falling statuses were designed to conduct laser-induced damage (LID) and laser conditioning experiments on DKDP crystals. The shape of the pulse substantially influences the damage pinpoints size and LID threshold (LIDT) of the crystals in the sub-nanosecond range. After sub-nanosecond laser conditioning, the ns R-on-1 LIDT showed that slow-rising fast-falling pulse (R400-F200 and High-foot pulses) conditioning achieved a 14%-20% LIDT enhancement than the traditional Gaussian pulse (R300-F300 pulse). The 8-ns laser damage morphologies after slow-rising fast-falling pulse conditioning showed cracks, whereas those after fast-rising slow-falling pulse (R200-F400 pulse) conditioning were pinpoint core, as usual. These results suggest that the rising front plays an important role in the LID and laser conditioning of the DKDP crystals. A pulse with a slower rising front is beneficial for thermal modification, thereby leading to better LID properties. This strategy greatly expands and enriches the manipulation methods to improve the LIDT of DKDP crystals, and sheds light on understanding the laser damage mechanisms.

Beam-size effects on the measurement of sub-picosecond intrinsic laser induced damage threshold of dielectric oxide coatings.

Laser-induced damage experiments on HfO2 and Nb2O5 thin films were performed with 500 fs pulse duration at 1030 nm wavelength. Threshold fluences as a function of beam size have been determined for effective beam diameters ranging from 40 to 220 µm, in a single shot regime. The results suggest no beam-size effect related to material properties in the investigated range, but size effects related to the metrology. The results indicate the importance of appropriate focusing conditions and beam measurement to qualify the optics for use in lasers with large beam sizes.

Laser cleaning of the contaminations on the surface of tire mould

During the manufacturing of tires, surface pollutants on tire mould will lead to the production of unqualified tires. Tire moulds need to be regularly cleaned. Laser cleaning is recognized as a non-destructive, effective, precise and environmental friendly method. In this paper, laser cleaning was used to remove contaminants on tire mould surface. First, laser induced damage experiments were performed. The results showed that the roughness and hardness of the cast steel sample surface seldom changed under the energy range of 140.1–580.2 mJ laser irradiation 1 pulse and the energy range of 44.7–168.9 mJ laser irradiation 100 pulses. In the laser cleaning experiments, the cleaning thresholds and the optimal cleaning parameters were obtained. Results indicated that laser cleaning was safe and effective for tire mould contamination removal.

The role of defects in laser-induced modifications of silica coatings and fused silica using picosecond pulses at 1053 nm: I. Damage morphology.

Laser-induced damage with ps pulse widths straddles the transition from intrinsic, multi-photon ionization and avalanche ionization-based ablation with fs pulses to defect-dominated, thermal-based damage with ns pulses. We investigated the morphology of damage for fused silica and silica coatings between 1 ps and 60 ps at 1053 nm. Using calibrated laser-induced damage experiments, in situ imaging, and high-resolution optical microscopy, atomic force microscopy, and scanning electron microscopy, we show that defects play an important role in laser-induced damage down to 1 ps. Three types of damage are observed: ablation craters, ultra-high density pits, and smooth, circular depressions with central pits. For 10 ps and longer, the smooth, circular depressions limit the damage performance of fused silica and silica coatings. The observed high-density pits and material removal down to 3 ps indicate that variations in surface properties limit the laser-induced damage onset to a greater extent than expected below 60 ps. Below 3 ps, damage craters are smoother although there is still evidence as seen by AFM of inhomogeneous laser-induced damage response very near the damage onset. These results show that modeling the damage onset only as a function of pulse width does not capture the convoluted processes leading to laser induced damage with ps pulses. It is necessary to account for the effects of defects on the processes leading to laser-induced damage. The effects of isolated defects or inhomogeneities are most pronounced above 3 ps but are still discernible and possibly important down to the shortest pulse width investigated here.

Fluorescence Lifetime Mapping of NADH Reveals DNA Repair Activity in Live Cells

Reduced nicotinamide adenine dinucleotide (NADH) is an important coenzyme that catalyzes many cellular functions including energy production and DNA damage repair. The ratio of free and bound NADH has recently emerged as a regulator of transcriptional activity and age-associated diseases. The DNA damage response, which requires NADH to gain full functionality, senses and repairs any breaks and errors in the genome. Uncovering the role of this ubiquitous molecule in response to DNA damage will further our understanding of how DNA protects itself and provide insight on how cancer cells evade these processes. In laser-induced damage experiments, HeLa cells were damaged with a femto-second pulsed two-photon laser at a wavelength of 780nm. Fluorescence lifetime imaging microscopy (FLIM) data was acquired and processed using the FLIM-phasor method. Induction of DNA damage and initiation of DNA repair were confirmed via immunostaining for γH2AX and PAR. The lifetime of NADH species in each pixel of FLIM images were transformed into polar coordinates on the phasor plot. The relative fraction of bound NADH was then calculated from the phasor plot and plotted over time. Low laser power damage resulted in a transient change toward a high fraction of bound NADH that was restored in approximately 8 hours. High laser power damage resulted in an immediate (∼5 minutes) and persistent (after 12 hours) shift toward a high fraction of bound NADH. The data show that the extent of DNA damage correlates to increases in the fraction of bound NADH in the nucleus owing to increased binding to DNA repair-associated substrates. Thus, monitoring NADH activity can be an intrinsic biosensor for DNA repair activity. Future studies will explore the role of NADH in chromatin remodeling to facilitate transcriptional regulation of DNA repair.

Mechanism for defect dependence of damage morphology in HfO2/SiO2 high reflectivity coating under nanosecond ultraviolet laser irradiation

High reflectivity coating is designed and fabricated with HfO2/SiO2 stacks. The laser-induced damage experiment is prepared by a third-order harmonic generation of Nd:YAG laser (355nm, 8ns). Typical damage micrographs are obtained using atomic force microscope (AFM) and scanning electron microscope (SEM). A theoretical model based on the thermal transfer is tried to describe the defect dependence of damage morphology. Three situations are considered in this model: single defect, couple defects and high absorption film. The calculated results show that the second situation agrees well with the experiment.

Laser-induced damage of rugate and quarter-wave stacks high reflectors deposited by ion-beam sputtering

Rugate and high/low quarter-wave stacks high reflector coatings for 1064 nm have been prepared with Ta 2 O 5 and SiO 2 by an ion-beam sputtering technique. A laser-induced damage experiment of the samples has been conducted at 1064 nm with pulse duration of 5 ns [full width at half maximum (FWHM)]. These two samples’ damages both initiate at defects and show almost the same damage threshold within the experimental error. Nevertheless, the damage morphology on rugate is less severe at higher fluences. The thermal shock wave induced by a nanosecond pulsed laser is considered to be the main cause of catastrophic damage.

Measuring the effective pulse duration of nanosecond and femtosecond laser pulses for laser-induced damage experiments

We report on our approach to measure the quantity named effective pulse duration as defined in the ISO 21254-1:2011 standard, which deals with laser-induced damage (LID) threshold measurements. The approach is applied to measure pulses from two laser sources: an injection-seeded electro-optically Q-switched Nd:YAG nanosecond system with 10-Hz pulse repetition frequency, and a fully integrated Ti:sapphire laser with 150–400 fs and 2-kHz pulse repetition frequency. For comparison, the full-width-half-maximum (FWHM) of the same pulses is also measured. The analysis and description of the measurement process, the experimental results, and the corresponding uncertainties are presented. A smaller combined uncertainty is obtained for the effective pulse duration than for the FWHM-defined pulse duration for each time scale involved in experiments. This suggests that the effective pulse duration is the appropriate parameter to characterize the pulse duration in LID experiments.

Residual stress near cracks of K9 glass under 1 064-nm nanosecond laser irradiation

We present the birefringence measurements induced in K9 specimen by cracks produced by 1 064-nm Nd:YAG laser. The birefringence data are converted into units of stress, permitting the estimation of residual stress near cracks. The laser parameters and characterization of the optical material influence the value of residual stress. Residual stress in optical materials can affect fracture; thus, this factor should be considered in any formulation that involves enhanced damage resistance of optical components used in laser-induced damage experiments. The probability of the initial damage and the direction of the energy dissipation in cracks determine the residual stress distribution. Moreover, thermal-stress coupling enlarges the asymmetry of residual stress distribution. Therefore, the physical mechanism of asymmetric damage is useful for understanding the nature of optical materials under high-power laser irradiation.

Microstructure and 355 nm Laser-Induced Damage Characteristics of Al<sub>2</sub>O<sub>3</sub> Films Irradiated with Oxygen Plasma under Different Energy

Al2O3 films were prepared using electron beam evaporation at room temperature. The samples were irradiated with oxygen plasma under different energy. The variations in average surface defect density and root mean square (RMS) surface roughness were characterized using an optical microscope and an atomic force microscope. Surface average defect density increased after plasma treatment. The RMS surface roughness of the samples decreased from 1.92 nm to 1.26 nm because of surface atom restructuring after oxygen plasma conditioning. A 355 nm laser-induced damage experiment indicated that the as-grown sample with the lowest defect density exhibited a higher laser-induced damage threshold (1.12 J/cm2) than the other treated samples. Laser-induced damage images revealed that defect is one of the key factors that affect laser-induced damage on Al2O3 films.

Residual stress near cracks of K9 and fused silica under 1064 nm nanosecond laser irradiation

Measurements of birefringence induced in K 9 and fused silica specimens by cracks produced by 1064 nm Nd : YAG laser have been presented. The Birefringence data is converted into the units of stress, thus permitting the estimation of residual stress near crack. The intensity of residual stress in K 9 glass is larger than that in fused silica under the same condition. The similarity of residual stress distribution along the y -axis reveals that the nature of shock wave transmission in optical materials under 1064 nm laser irradiation is the same with each other. The value of residual stress can be influenced by laser parameters and characterization of optical material. Simulation based on a theoretical model giving the residual stress field around a crack is developed for comparison with experiment results. The probability of initial damage and the direction of the energy dissipation in cracks determine the residual stress distribution. The thermal stress coupling enlarges the asymmetry of residual stress distribution. Residual stress in optical material has a strong effect on fracture and should be taken into account in any formulation that involves the enhanced damage resistance of optical components used in laser induced damage experiments.

Stimulated Brillouin Scattering Damage of Large-Aperture Fused Silica Grating

Laser induced damage experiment is carried out on a large aperture laser facility. Severe damage is observed on a large-aperture fused silica grating which presents dense craters on the front surface and six cracks alternatively located at the front and the rear surface. The bizarre fact about the damage on the grating is that, unlike other optics, the damage craters are almost on the front surface. According to observation, damage phenomenon is due to the stimulated Brillouin scattering (SBS) effect occurring in the grating, which includes the transverse SBS, the back SBS and the zigzag SBS.

Pump-pump experiment in KH2PO4 crystals: Coupling two different wavelengths to identify the laser-induced damage mechanisms in the nanosecond regime

Laser-induced damage experiments based on an original pump-pump set-up have been carried out in the nanosecond regime for KH2PO4 (KDP) crystal. The impact of a simultaneous mixing of 351 nm and 1064 nm pulses has been experimentally studied and compared to a model based on heat transfer, the Mie theory and a Drude model. This study sheds light on the physical processes implied in the KDP laser damage. In particular, a three-photon ionization mechanism is shown to be responsible for laser damage in KDP.

氧化铪/氧化硅薄膜在真空中的污染与激光损伤

The performances of HfO2/SiO2 single- and multi-layer coatings in vacuum influenced by contamination are studied. The surface morphology, the transmittance spectrum, and the laser-induced damage threshold are investigated. The results show that the contamination in vacuum mainly comes from the vacuum system and the contamination process is different for the HfO2 and SiO2 films. The laser-induced damage experiments at 1064 nm in vacuum show that the damage resistance of the coatings will decrease largely due to the organic contamination.

Coupling statistics and heat transfer to study laser-induced crystal damage by nanosecond pulses

By coupling statistics and heat transfer, we investigate numerically laser-induced crystal damage by multi-gigawatt nanosecond pulses. Our model is based on the heating of nanometric absorbing defects that may cooperate when sufficiently aggregated. In that configuration, they induce locally a strong increase of temperature that may lead to a subsequent damage. This approach allows to predict cluster size distribution and damage probabilities as a function of the laser fluence. By studying the influence of the pulse duration onto the laser-induced damage threshold, we have established scaling laws that link the critical laser fluence to its pulse duration tau. In particular, this approach provides an explanation to the deviation from the standard tau(1/2) scaling law that has been recently observed in laser-induced damage experiments with KH(2)PO(4) (KDP) crystals [J.J. Adams et al., Proc. of SPIE 5991, 5991R-1 (2005)]. In the present paper, despite the 3D problem is tackled, we focus our attention on a 1D modeling of thermal diffusion that is shown to provide more reliable predictions than the 3D one. These results indicate that absorbers involved in KDP damage may be associated with a collection of planar defects. First general comparisons with some experimental facts have been performed.

Study on LIDT of MDGs for different fabrication processes

Progress of ultra-short high peak power laser depends significantly on laser-induced damage threshold (LIDT) of diffraction grating used in chirped-pulse amplification system. Laser-induced damage experiment was conducted by the standard one-on-one modes and the damage morphology was detected under scanning electron microscope. LIDT of the samples, which are masked on hafnia and on Chrome, are 3.15 and 4.14 J/cm 2, respectively. Damage process corresponding to the two kinds of samples is described in detail by analyzing the relative damage morphology. The explanation of the difference between the two kinds of samples is given with the analysis of the pollution, which was introduced during the manufacturing process.

Surface damage of (−)2-(α-methylbenzylamino)-5-nitropyridine single crystals induced by pulsed laser radiation

Pulsed laser-induced surface damage experiments were made on the cleaved (001) face of (-)2-(α-methylbenzylamino)-5-nitropyridine crystals at a pulse length of 25 ns FWHM. The highest single-pulse damage threshold value was 24.2 J/cm(2) at 532 nm for light polarized along the crystallographic b axis.