Angular dependence of the transverse Raman scattering in KDP and DKDP in geometries suitable for beam polarization control.

The angular dependance of the transverse Raman scattering in potassium dihydrogen phosphate (KDP) and its deuterated analogue (DKDP) for the entire range of crystal configurations suitable for laser beam polarization control has been investigated via experimental and modeling tools. This work was made possible by simultaneously rotating a spherical sample and the pump polarization to effectively measure the angular dependance of the transverse Raman signal in 360°. This novel method, which is applicable for the investigation of the Raman scattering in optically anisotropic materials, demonstrates that the spontaneous Raman scattering signal exhibits strong angular dependence that is modulated by depolarization and polarization rotation effects generated as the Raman signal traverses the material due to its birefringence. The results show that the total signal generated by the pump beam is the sum of the signals generated by the two components that have polarization parallel and orthogonal to the optic axis. The peak signal intensity, which is of importance for high-power laser applications, depends on the orientation of the optic axis and can vary by a factor of about 2. The excellent agreement between experimental data and modeling results validates the associated models and enables one to consider optimal crystal cut designs for specific applications.

Electric-field enhancement caused by subwavelength-sized particles located on the surface of multilayer dielectric mirrors.

A laser pulse impinging on the surface of an optical component can interact with particles, such as contamination debris, to produce a scattered electric field, which, either by itself or combined with the incident laser field, coherently can significantly increase the local field intensity. This effect can be of critical importance as it can reduce the laser-induced-damage threshold of the affected component. In this work, we use a field-propagation code to improve understanding regarding the factors that determine the magnitude and location of the electric-field enhancement for the case of subwavelength-sized particles located on the surface of multilayer dielectric mirrors.

Dynamics of electronic excitations involved in laser-induced damage in HfO2 and SiO2 films.

The dynamics of electron excitations associated with the initiation of laser-induced damage in hafnia and silica monolayer films are investigated using time-resolved damage testing involving a pair of 0.7 ps pulses with adjustable delay and laser pulse fluences. Results in hafnia indicate that the relaxation profile depends on the pump-pulse fluence (initial excitation), and as a result, it exhibits an effective lifetime that is variable. Analogous experiments in silica form two different types of damage morphologies that are observed on different ranges of delay times.

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.

Mechanisms of picosecond laser-induced damage in common multilayer dielectric gratings.

The modifications of multilayer dielectric (MLD) gratings arising from laser-induced damage using 0.6-ps and 10-ps laser pulses at 1053 nm are investigated to better understand the damage-initiation mechanisms. Upon damage initiation, sections of the affected grating pillars are removed, thereby erasing the signature of the underlying mechanisms of laser damage. To address this issue, we performed paired studies using macroscopic grating-like features that are 5 mm in width to reveal the laser-damage morphology of the different grating sections: pillar side wall, sole, and pillar top. The results suggest that, similarly to MLD coatings, there are two damage-initiation mechanisms corresponding to the different pulse durations.

Measurement of the angular dependence of the spontaneous Raman scattering in anisotropic crystalline materials using spherical samples: Potassium dihydrogen phosphate as a case example.

A specialized experimental configuration was developed to allow for more-accurate characterization of the spontaneous Raman scattering properties in anisotropic materials. This need stems from the challenges, arising from the complexity of light propagation, in obtaining accurate measurements of the angular dependence of the Raman scattering cross section in birefringent materials. The nonlinear optical material KH2PO4 (KDP) is used as a model medium. This study is motivated by the need to improve our understanding and management of transverse stimulated Raman scattering in KDP crystals and its deuterated analog, DKDP, typically used for frequency conversion and polarization control in large-aperture laser systems. Key to this experimental platform is the use of high-quality spherical samples that enable one to measure the Raman scattering cross section in a wide range of geometries using only a single sample. The effect of polarization rotation of both the pump light and the collected Raman signal must be carefully considered in data analysis and can give rise to artifacts, which can, in part, be mitigated by reducing the input and collection cone angles.

Investigation of parameters governing damage resistance of nematic liquid crystals for high-power or peak-intensity laser applications.

We investigate the damage resistance of saturated and unsaturated liquid crystals (LC's) under a wide range of laser excitation conditions, including 1053-nm pulse durations between 600 fs and 1.5 ns and nanosecond pulse excitation at 351 nm and 532 nm. This study explores the relationship between the LC's resistance to laser-induced breakdown (damage) and the electronic structure (π-electron delocalization) of the constituent molecules. The laser-induced damage threshold at all wavelengths and pulse durations was consistently higher in saturated materials than in their unsaturated counterparts. The wavelength's dependence in the results suggests that the energy coupling process that leads to laser-induced breakdown is governed by the energy separation between the ground state and the first and second excited states, while the pulse duration's dependence in the results reveals the important role of electron relaxation between the excited states. A qualitative description was developed to interpret the experimental observations.

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.

Influence of absorption-edge properties on subpicosecond intrinsic laser-damage threshold at 1053 nm in hafnia and silica monolayers.

Owing to their relatively high resistance to laser-induced damage, hafnia and silica are commonly used in multilayered optical coatings in high-power laser facilities as high- and low-refractive-index materials, respectively. Here, we quantify the laser-induced-damage threshold (LIDT) at 1053 nm in the short-pulse regime of hafnia and silica monolayers deposited by different fabrication methods, including electron-beam evaporation, plasma ion-assisted deposition and ion-assisted deposition. The results demonstrate that nominally identical coatings fabricated by different deposition techniques and/or vendors can exhibit significantly different damage thresholds. A correlation of the LIDT performance of each material with its corresponding absorption edge is investigated. Our analysis indicates a weak correlation between intrinsic LIDT and the optical gap of each material (Tauc gap) but a much better correlation when considering the spectral characteristics in the Urbach tail spectral range. Spectrophotometry and photothermal absorption were used to provide evidence of the correlation between the strength of the red-shifted absorption tail and reduced LIDT at 1053 nm.

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.

Interaction of short laser pulses with model contamination microparticles on a high reflector.

The response of model contamination particles located on the surface of a multilayer dielectric mirror when exposed to 1053 nm laser pulses of 10 ps or 0.6 ps duration is investigated. Four different particle types were studied: stainless steel, borosilicate glass, polyethylene, and polytetrafluoroethylene, all having an average diameter of about 40 µm. Irradiation with one laser pulse caused particles to eject from the surface with an onset fluence in the range 5× to 100×, depending on the particle type, below the particle-free, laser-induced damage threshold of the mirror. Morphological analysis showed, however, that the ejection process always generated ablation craters and/or secondary contamination, both of which can degrade the performance of the optic during subsequent pulses. Ejection and damage mechanisms are discussed for each particle type.

Optical properties of oxygen vacancies in HfO2 thin films studied by absorption and luminescence spectroscopy.

Hafnium oxide thin films with varying oxygen content were investigated with the goal of finding the optical signature of oxygen vacancies in the film structure. It was found that a reduction of oxygen content in the film leads to changes in both, structural and optical characteristics. Optical absorption spectroscopy, using nanoKelvin calorimetry, revealed an enhanced absorption in the near-ultraviolet (near-UV) and visible wavelength ranges for films with reduced oxygen content, which was attributed to mid-gap electronic states of oxygen vacancies. Absorption in the near-infrared was found to originate from structural defects other than oxygen vacancy. Luminescence generated by continuous-wave 355-nm laser excitation in e-beam films showed significant changes in the spectral profile with oxygen reduction and new band formation linked to oxygen vacancies. The luminescence from oxygen-vacancy states was found to have microsecond-scale lifetimes when compared with nanosecond-scale lifetimes of luminescence attributed to other structural film defects. Laser-damage testing using ultraviolet nanosecond and infrared femtosecond pulses showed a reduction of the damage threshold with increasing number of oxygen vacancies in hafnium oxide films.

Simulation of internal stress waves generated by laser-induced damage in multilayer dielectric gratings.

Multilayer dielectric (MLD) gratings used in ultrahigh-intensity laser systems often exhibit a laser-induced damage performance below that of their constituent materials. Reduced performance may arise from fabrication- and/or design-related issues. Finite element models were developed to simulate stress waves in MLD grating structures generated by laser-induced damage events. These models specifically investigate the influence of geometric and material parameters on how stress waves can lead to degradation of material structural integrity that can have adverse effects on its optical performance under subsequent laser irradiation: closer impedance matching of the layer materials reduces maximum interface stresses by ~20% to 30%; increasing sole thickness from 50 nm to 500 nm reduces maximum interface stresses by ~50%.

Enhanced laser conditioning using temporally shaped pulses.

Laser conditioning was investigated as a function of the temporal shape and duration of 351 nm nanosecond pulses for fused-silica substrates polished via magnetorheological finishing. The aim is to advance our understanding of the dynamics involved to enable improved control of the interaction of laser light with the material to optimize laser conditioning. Gaussian pulses that are temporally truncated at the intensity peak are observed to enhance laser conditioning, in comparison to a Gaussian pulse shape.

Mechanisms of surface contamination in fused silica by means of laser-induced electrostatic effects.

We demonstrate that fused-silica samples exposed to nanosecond laser pulses at 355 nm and 1064 nm develop long-lived electrostatic charges on their surfaces. These charges extend well beyond the area exposed to the laser beam. The results suggest this effect is dependent on laser fluence and wavelength. In addition, ejected particles generated during laser-induced breakdown are electrostatically charged. Experiments indicate that such electrostatic charges can produce forces that can support the transport of dielectric and metallic microspheres between surfaces. This in turn can promote increased contamination of optical components during operation at relevant excitation conditions.

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.

Factors influencing rat survival in a warm renal ischemia model: time to adapt the protocols.

INTRODUCTION: Survival in warm renal ischemia models is not only dependent on the treatment or surgical technique being evaluated, but also on factors inherent to the model itself. Use of rats of various strains in previous studies makes interstudy comparison difficult when trying to design an appropriate model control that would yield intermediate survival. In this study, impact of rat strain on survival after prolonged warm renal ischemia in the setting of delivery-controlled inhalational anesthesia was evaluated. MATERIALS AND METHODS: Under general delivery-controlled inhalation anesthesia with isoflurane, Dahl salt-sensitive, Wistar-Furth, Sprague-Dawley, and spontaneously hypertensive rats (n = 66 rats) were subjected to 150 minutes of unilateral renal warm ischemia time, subsequent reperfusion, and contralateral nephrectomy. Animals were followed up for 1 month, after which survivors were euthanized and morphologic changes in kidneys were scored. RESULTS: Thirty-day survival was: Dahl salt sensitive, 78%; Wistar-Furth, 67%; Sprague-Dawley, 55%; and spontaneously hypertensive rats, 0% (P < .0001). Histologic acute injury scores were higher for non-survivors versus 30-day survivors (P < .0001). CONCLUSION: Our data strongly suggest that rat strain is a major factor influencing survival and that strain and warm ischemia time selections must be considered together when designing a model control yielding intermediate survival. Further study is warranted in order to compare the effect of delivery-controlled inhalational versus historical anesthesia methods on animal survival.

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.