Determination of the Raman polarizability tensor in the optically anisotropic crystal potassium dihydrogen phosphate and its deuterated analog.

The Raman tensor of the dominant A1 modes of the nonlinear optical crystalline material potassium dihydrogen phosphate and its 70% deuterated analog have been ascertained. Challenges in determining the A1 mode tensor element values based on previous reports have been resolved using a specially designed experimental setup that makes use of spherical crystal samples. This novel experimental design enabled the determination of measurement artifacts, including polarization rotation of the pump and/or scattered light propagating through the sample and the contribution of additional overlapping phonon modes, which have hindered previous efforts. Results confirmed that the polarization tensor is diagonal, and matrix elements were determined with high accuracy.

Dynamics of secondary contamination from the interaction of high-power laser pulses with metal particles attached on the input surface of optical components.

We investigate the interaction of 355-nm and 1064-nm nanosecond laser pulses with nominally spherical metallic particles dispersed on the input surface of transparent substrates or high-reflectivity (HR) multilayer dielectric coatings, respectively. The objective is to elucidate the interaction mechanisms associated with contaminant-induced degradation and damage of transparent and reflective optical elements for high-power laser systems. The experiments involve time-resolved imaging capturing the dynamics of the interaction pathway, which includes plasma formation, particle ejection, and secondary contamination by droplets originating from the liquefied layer of the particle. The results suggest that HR coatings are more susceptible to secondary contamination by liquid droplets produced by the particles because of the different geometry of excitation and the location of plasma initiation. Modeling results focus on better understanding the melting of the particle surface, leading to ejections of liquid droplets and the pressure applied to the substrate, leading to mechanical damage.

Mechanisms of laser-induced damage in absorbing glasses with nanosecond pulses.

The propagation of 355-nm, nanosecond pulses in absorbing glasses is investigated for the specific case examples of the broadband absorbing glass SuperGrey and the Ce3+-doped silica glass. The study involves different laser irradiation conditions and material characterization methods to capture the transient material behaviors leading to laser-induced damage. Two damage-initiation mechanisms were identified: (1) melting of the surface as a result of increased temperature; and (2) self-focusing caused by a transient change in the index of refraction. Population of excited states greatly affects both mechanisms by increasing the transient absorption cross section via excited-state absorption and introducing a change of the refractive index to support the formation of graded-index lensing and self-focusing of the beam inside the material. The governing damage-initiation mechanism depends on the thermodynamic properties of the host glass, the electronic structure characteristics of the doped ion, and the laser-spot size.

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.

Role of defects in laser-induced modifications of silica coatings and fused silica using picosecond pulses at 1053 nm: II. Scaling laws and the density of precursors.

We investigate the role of defects in laser-induced damage of fused silica and of silica coatings produced by e-beam and PIAD processes which are used in damage resistant, multi-layer dielectric, reflective optics. We perform experiments using 1053 nm, 1-60 ps laser pulses with varying beam size, number of shots, and pulse widths in order to understand the characteristics of defects leading to laser-induced damage. This pulse width range spans a transition in mechanisms from intrinsic material ablation for short pulses to defect-dominated damage for longer pulses. We show that for pulse widths as short as 10 ps, laser-induced damage properties of fused silica and silica films are dominated by isolated absorbers. The density of these precursors and their fluence dependence of damage initiation suggest a single photon process for initial energy absorption in these precursors. Higher density precursors that initiate close to the ablation threshold at shorter pulse widths are also observed in fused silica, whose fluence and pulse width scaling suggest a multiphoton initiation process. We also show that these initiated damage sites grow with subsequent laser pulses. We show that scaling laws obtained in more conventional ways depend on the beam size and on the definition of damage for ps pulses. For this reason, coupling scaling laws with the density of precursors are critical to understanding the damage limitations of optics in the ps regime.

Dynamics of transient absorption in bulk DKDP crystals following laser energy deposition.

The transient changes in the optical properties of bulk DKDP material arising from its exposure to high temperatures and pressures associated with localized laser energy deposition are investigated. Two methods for initiation of laser-induced breakdown are used, intrinsic, involving relatively large energy deposition brought about by focusing of the laser beam to high intensities, and extrinsic, arising from more localized deposition due to the presence of pre-existing absorbing damage initiating defects. Each method leads to a very different volume of material being affected, which provides for different material thermal relaxation times to help better understand the processes involved.

The effect of laser pulse shape and duration on the size at which damage sites initiate and the implications to subsequent repair.

Growth of laser damage on SiO(2) optical components used in high power lasers can be reduced or eliminated by pre-exposure to pulses of a few hundred ps in duration. Such pre-exposure would cause weak locations on the optics surface to self-identify by initiating very small damage sites. The sites which initiate will be only a few microns in diameter and will have a very low probability of growing even without any further treatment. Repairing damage sites when small is important because both laser mitigation and acid etching are very successful in preventing such small sites from growing.

Dynamics of material modifications following laser-breakdown in bulk fused silica.

We report on the material response during the cooling phase in bulk fused silica following localized energy deposition via laser-induced breakdown.We use a time-resolved microscope system to acquire images of the region of energy deposition at delay times covering the entire timeline of events. In addition, this system is configured to perform pump-and-probe damage testing measurements to investigate the evolution of the transient absorption of the modified material. The main features of a damage site are established at approximately 30 ns after the pump pulse, i.e. cracks reach their final size within this time frame. The results reveal that the cracks and melted core exhibit a transient absorption up until about 300 ns and 200 micros delay times, respectively, and suggest that the melted region returns to solid phase at approximately 70 ms delay.

Laser damage performance of KD2-chiHchiPO4 crystals following X-ray irradiation.

We investigate the laser-induced damage performance of KD(2-chi)H(chi)PO(4) crystals following exposure to X-ray irradiation. Two important issues addressed by our study are i) the performance of the material when operational conditions lead to its exposure to ionizing irradiation and ii) the way the radiation-induced transient defects interact with the pre-existing precursor defects responsible for laser-induced damage. Our results indicate that the damage performance of the material is affected by exposure to X-rays. This behavior is attributed to a change in the physical properties of the precursors which, in turn, affect their ability to initiate damage following interaction with X-ray generated defects.

Expedited laser damage profiling of KDxH(2-x)PO4 with respect to crystal growth parameters.

We investigate the laser-induced damage resistance at 355 nm in deuterated potassium dihydrogen phosphate (DKDP) crystals grown with varying growth parameters, including speed of growth and temperature. The aim is to explore a new expedited method to study the growth parameters affecting the laser-induced damage resistance in DKDP material to obtain crystals with enhanced performance.

Investigation of laser annealing parameters for optimal laser-damage performance in deuterated potassium dihydrogen phosphate.

Laser annealing via preexposure to laser pulses at sub-damage-threshold fluences is known to improve the resistance of KDP crystals to laser-induced damage. Using a specific damage-testing method, we investigate the laser annealing process as a function of fluence and number of preexposure pulses (at 355 nm, 2.5 ns). Our aim is to reveal the key laser parameters in order to devise a practical and efficient protocol for optimizing performance of the material for operation in laser systems in the near UV. Results suggest that a near twofold improvement to the laser-damage performance can be achieved with a limited number of preexposure pulses.

Multiwavelength investigation of laser-damage performance in potassium dihydrogen phosphate after laser annealing.

The laser-induced damage performance of potassium dihydrogen phosphate and deuterated potassium dihydrogen phosphate nonlinear optical crystals after pre-exposure to lower-energy laser pulses (laser annealing, also known as laser conditioning) is investigated as a function of wavelength for both the damaging and conditioning pulses. We obtain a quantitative evaluation of the bulk damage performance of these materials by measuring the density of damage events as a function of laser parameters. This new method allows for a detailed assessment of the improvement of material performance from laser conditioning and reveals the key parameters for optimizing performance depending on the operational wavelength.