Multipulse Laser Damage Research Articles

AgScP2S6 van der Waals Layered Crystal: A Material with a Unique Combination of Extreme Nonlinear Optical Properties

Research in two-dimensional layered materials (2DLMs) has exploded over the past several years for a variety of applications in photonics and optoelectronics. The 2D nature of these materials allows for a very local electronic probe of material as well as flexible integration with other functional components. Herein, using the femtosecond Z-scan technique, we report a giant two photon absorption (TPA) process and its saturation in the van der Waals gapped silver scandium thiophosphate (AgScP2S6) crystal. We have found a TPA coefficient of the order of 104 cm/GW which is orders of magnitude larger compared to many existing semiconductors and nonlinear crystals. Furthermore, we found a TPA cross-section of 103 GM and characterized the optical limiting (OL) response (0.2 mJ/cm2) and the multipulse laser damage threshold (1.09 ± 0.19 J/cm2). The combination of giant TPA, extremely low OL, and very high damage threshold suggests that this material could be extremely useful in applications like optical limiters or switches.

A high-power long lifetime beam dump for the Thomson scattering diagnostic system in the XuanLong-50 experiment.

The Energy iNNovation's XuanLong-50 is a spherical torus experiment with up to 10s plasma operation duration. A 3 J/50Hz pulsed laser is used in the Thomson scattering diagnostic system that is developed to measure the time evolutions of plasma electron temperature and density profiles. The expected laser pulse number is about 7.5 × 106/year with a power load of 150W. To meet at least 1-year lifetime requirement, a Chevron type beam dump with polished molybdenum plates is designed and fabricated, which absorbs the laser beam energy in a 3D structure to reduce the laser fluence deposited on the material surface. To prevent the backscattered stray light from interfering with the Thomson scattering measurements, a 7.5m beam path with folding mirrors is set between the beam dump and the plasma scattering volumes. Details of the beam dump design procedure including the laser beam profile control, multi-pulse laser damage threshold, heat dissipation, Zemax modeling, folding mirror selection, and beam path enclosure are presented together with the testing results.

Thermally ruggedized ITO transparent electrode films for high power optoelectronics.

We present two strategies to minimize laser damage in transparent conductive films. The first consists of improving heat dissipation by selection of substrates with high thermal diffusivity or by addition of capping layer heatsinks. The second is reduction of bulk energy absorption by lowering free carrier density and increasing mobility, while maintaining film conductance with thicker films. Multi-pulse laser damage tests were performed on tin-doped indium oxide (ITO) films configured to improve optical lifetime damage performance. Conditions where improvements were not observed are also described. When bulk heating is not the dominant damage process, discrete defect-induced damage limits damage behavior.

Optical and electrical properties of indium tin oxide films near their laser damage threshold

In this study, we investigated whether the optical and electrical properties of indium tin oxide (ITO) films are degraded under laser irradiation below their laser ablation threshold. While performing multi-pulse laser damage experiments on a single ITO film (4.7 ns, 1064 nm, 10 Hz), we examined the optical and electrical properties in situ. A decrease in reflectance was observed prior to laser damage initiation. However, under sub-damage threshold irradiation, conductivity and reflectance of the film were maintained without measurable degradation. This indicates that ITO films in optoelectronic devices may be operated below their lifetime laser damage threshold without noticeable performance degradation.

Generic incubation law for laser damage and ablation thresholds

In multi-pulse laser damage and ablation experiments, the laser-induced damage threshold (LIDT) usually changes with the number of pulses in the train, a phenomenon known as incubation. We introduce a general incubation model based on two physical mechanisms—pulse induced change of (i) absorption and (ii) critical energy that must be deposited to cause ablation. The model is applicable to a broad class of materials and we apply it to fit data for dielectrics and metals. It also explains observed changes of the LIDT as a function of the laser repetition rate. We discuss under which conditions the crater-size method to determine LIDTs can be applied in multi-pulse experiments.

Multipulse laser damage in potassium titanyl phosphate: statistical interpretation of measurements and the damage initiation mechanism

Multipulse laser-induced damage is an important topic for many applications of nonlinear crystals. We studied multipulse damage in X-cut KTiOPO4. Using a 6-ns Nd:YAG laser with a weakly focused beam, a fatigue phenomenon was observed. We addressed whether this phenomenon necessarily implies material modifications. Two possible models were tested, both of them predicting increasing damage probability with increasing pulse number while all material properties are kept constant. The first model, pulse energy fluctuations and depointing, increases the probed volume during multiple pulse experiments. The probability to cause damage thus increases with increasing pulse number; however, this effect is too small to explain the observed fatigue. The second model assumes a constant single-shot damage probability p1, so a multipulse experiment can be described by statistically independent resampling of the material. Very good agreement was found between the 2000-on-1 volume damage data and this statistical multipulse model. Additionally, the spot size dependency of the damage probability is well described by a precursor presence model. Supposing that laser damage precursors are transient, the presented data explain the experimental results without supposing material modifications.

Laser damage investigations of Cu mirrors

Abstract Laser tests were performed on Cu mirrors as “first mirror” prototypes for laser diagnostics. Data on laser damage thresholds of surfaces under the influence of pulsed YAG laser radiation were obtained for single shot and after about 1.5 × 10 5 shots. The experiments were carried out using three types of copper mirrors: diamond-turned mirror, diamond-turned substrate with copper coating and reflection grating on a copper coated mirror. The lifetime of the copper mirrors was tested. Diffusion scattering was used as a monitor of mirror surface quality. The degradation of diamond turned copper mirrors in multiple pulse laser irradiation appears to be in satisfactory accordance with a model for multipulse laser damage of metal mirrors up to 1.5 × 10 5 laser pulses. The degradation of copper coated mirrors has a more complicated behaviour.

Frequency laser damage of Mo mirrors

Laser tests were performed on first mirror (FM) prototypes manufactured from single crystal and polycrystal Mo and with reflective Mo coatings on Mo substrate. Data on laser damage thresholds under the influence of the frequency-operated pulsed YAG laser (1064 nm wavelength, 12.5 Hz repetition rate, 10–30 mJ per pulse energy, 12 ns pulse duration) were obtained for single laser shot and after about 2.2×10 5 laser shots. The output beam of the laser operating in the TEM oo mode has a Gaussian profile. The single shot damage thresholds were measured and were equal to 2±0.4 J/cm 2 for polycrystal Mo and Mo/Mo mirrors, 3±0.5 J/cm 2 for single crystal Mo mirror. The long-time effect of frequency-operated YAG laser on mirror reflectivity was studied. It was shown that diffusion scattering is a very sensitive instrument to check the surface condition. Different types of Mo mirrors show various values of multipulse laser damage threshold. It may be connected as with initial quality of the surface and with different structures of the reflection layer for investigated samples of Mo. The degradation of a single crystal Mo mirror under multiple pulse laser irradiation is described with good accuracy by a predictive model for multipulse laser damage of metal mirrors [Appl. Optics 30–36 (1991) 5239] up to 1.5×10 5 laser pulses.

Materials selection for the in situ mirrors of laser diagnostics in fusion devices

When mirrors for the laser scattering diagnostic for large fusion devices need to be inside the vacuum chamber, they are subjected to irradiation by multiple high-energy laser pulses and bombardment by charge exchange atoms. Both of these assaults are known to degrade and eventually damage metal laser mirrors given sufficient time and flux. Our aim in this article is to use current data on these damage mechanisms to make design selections of metal mirror materials for application in fusion device diagnostics. We identify tradeoffs between low sputtering rates and multipulse laser damage resistance in candidate metals. The data for multipulse laser damage are incomplete and extend to a maximum of only 104 shots for a few metals. However, there is a clear trend of decreasing laser-damage threshold with increasing number of shots, and damage threshold fluences can fall to 10% of the single-pulse damaging laser fluence. Further experiments up to 106 or 108 laser shots need to be conducted on the likely mirror candidate metals for use in new plasma devices. We define a figure-of-merit based on current laser damage data and employ it in our analysis. Recent data on the sputtering yield and reflectance degradation of metal mirrors give a different priority ranking of candidate metals. Overall, the preferred material selection depends on the number of laser shots and the number of plasma pulses that the mirror must endure before replacement is allowed. For example, we find that for conditions typical of the LHD (10 s plasma pulses with a 10 Hz laser PRF), Au, Ag, and Cu are candidate materials if mirrors are replaced after 103 plasma pulses; Au and Rh are candidates if the replacement interval is 104 pulses; and if the replacement interval is further increased to 105 plasma pulses, then Mo is the candidate material. Other materials might also be candidates but the data on them are still insufficient.

Predicting multipulse laser-induced failure for molybdenum metal mirrors

In combination with known thermomechanical-fatigue data for Mo, we have conducted transient photothermal deflection (TPD) measurements to develop a model for the multipulse laser damage of Mo mirrors to predict their lifetimes. In laser-damage experiments to verify the model, Mo mirrors were irradiated with 10-ns Nd:YAG laser pulses at 1064 nm at a 10-Hz repetition rate. Digitized TPD waveforms indicated peak surface angular deflection that could then be converted into surface displacement. Numerical modeling of the vertical heat distribution enabled the peak surface-deflection signal to be converted into peak surface temperature. The thermomechanical model was verified by both the experimental and the numerical results. Conventional mechanical-fatigue data for Mo were used to derive a predictive equation for the laser-accumulation lifetime of Mo mirrors. Experiments were performed with 1-10(4) pulses per site, yielding laser-damage thresholds and accumulation curves. The accumulation behavior predicted from measurements of mechanical fatigue was in excellent agreement with the measured behavior. Thus a single-pulse TPD measurement of peak deformation at a subthreshold laser fluence, in conjunction with mechanical-fatigue data, may be used to estimate the safe operating fluence for a component in a multipulse laser environment.

Multipulse laser damage of GaAs surfaces

An in-depth study of the single and multiple-pulse laser (10 ns, 10 Hz, and 1064 nm) damage of (100) single-crystal gallium arsenide surfaces is presented. The surface morphologies resulting from both chemical etching and laser-induced damage are discussed. The damage behavior is determined from the damage probability and accumulation curves. The dependence of damage on accumulated incident energy in multipulse experiments is determined and compared with the behavior of other semiconductor and metal surfaces. Finally, Auger electron spectroscopy is employed to measure the elemental depth composition of GaAs wafers using Ar+ ion sputter etching. This technique is used to study the changes in elemental depth profiles caused by the laser-induced damage.

Laser-induced damage and ion emission of GaAs at 106 μm

This study focused on the multipulse laser damage and the subdamage threshold ion emission of GaAs. The initial goals were to determine the pulse-dependent damage threshold and to correlate ion emission with surface damage. A Q-switched Nd:YAG laser was used to irradiate the (100) GaAs samples. Using values of N from 1 to 100, we obtained accumulation curves based on 50% damage probability. Corresponding damage threshold fluences were 0.4-0.8 J/cm2 for N > 1 and 1.5 J/cm2 for N = 1. We observed large site-to-site fluctuations in ion emission and found the onset of emission at 0.2 J/cm2 for all cases. Once surface damage occurred, ion emission increased greatly. The observed behavior supports a surface cleaning model for the ion emission which precedes surface damage. Measurements of linear and nonlinear free carrier absorption were made, but no anomalous absorption was observed.