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We investigate the laser-induced damage performance of KD2–xHxPO4 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.
©2008 Optical Society of America
1. Introduction
Potassium dihydrogen phosphate, KH2PO4 (KDP) and its deuterated analog, DKDP, are technologically important and unique materials for use in high-power, large-aperture laser systems for various applications (e.g., electro-optics switching, polarization smoothing and nonlinear optical frequency conversion) [1, 2, 3, 4]. The use of nonlinear optical materials in such laser systems is continuously expanding and the laser output and frequency conversion efficiency rely on increased laser intensities. As a result, laser-induced damage in these materials represents a key limiting factor. Localized damage initiation in KDP/DKDP crystals has been attributed to either impurity nanoparticles incorporated during growth or clusters of intrinsic defects that form during growth[5, 6, 7]. However, the exact nature of these defects has not yet been identified. Moreover, large-aperture laser systems under development, designed to expand the frontiers in high-energy density physics, will create an operational environment where optical components, such as the frequency conversion crystals (KDP/DKDP), may be exposed to X-rays and other ionizing radiation. This in turn may lead to a change in the damage performance of these materials as they may be affected by radiation-induced effects.
A number of point defects produced in KDP/DKDP crystals during exposure to X-rays have been identified using either optical techniques or electron paramagnetic resonance (EPR) spectroscopy [8, 9, 10, 11, 12, 13, 14, 15, 16]. Hydrogen atoms, oxygen vacancies, self-trapped holes, and holes trapped adjacent to hydrogen vacancies are among these defects. These earlier results confirmed that most of the radiation-induced point defects in the pure material are short lived above 200 K and revealed the complex relaxation pathways as one defect species decays and the electrons or holes are trapped at a different lattice site to form a new defect species[16]. However, there is no experimental evidence in the literature addressing how this plurality of defects formed during X-ray radiation may affect the laser induced damage performance of KDP/DKDP materials.
In this Letter, we evaluate the laser-induced bulk damage performance at 355-nm of DKDP crystal samples subjected to X-ray irradiation at room temperature with different exposure times. Specifically, we map the damage density at fixed testing fluence versus location along the direction of the X-ray beam in both pristine (prior to irradiation) and irradiated material. The objective is to understand how radiation-induced transient point defects can affect the laser damage characteristics in DKDP crystals by either forming new damage initiation centers or interacting with the pre-existing damage initiating defects (referred to as damage precursors).
Fig. 1. (Top) Schematic view of laser-induced damage testing in DKDP crystals (prior to and after X-ray irradiation along x). (Bottom) Typical intensity distribution in the continuous X-ray spectrum from Rh target (100 keV, 3 mA) and the linear attenuation coefficient of X-rays in DKDP (left and right inset graphs, respectively).
It should be noted that atomically dispersed point defects (at low concentration) are not efficient light-absorbers as needed to initiate localized breakdown in these materials [6]. However, aggregations (clusters) of intrinsic point defects (prompted by stress, lattice imperfections, impurities, or other factors) leading to localized high concentration of atomic defects can act in part as absorbing particles by providing additional electronic states in the band gap and thus efficient coupling of laser light into the material. This in turn leads to the formation of macroscopic damage on the order of 1-10 µm, as observed in bulk KDP/DKDP.
2. Experiments and results
The damage testing method used in this study has been described in detail elsewhere [17]. A schematic view of the apparatus is presented in Fig. 1. The third harmonic (at 355-nm) of a ~3 ns pulsed Nd:YAG laser is focused by a 200-mm focal-length cylindrical lens (CL) into the bulk of the crystal samples. We thus obtain a slit beam with a near-Gaussian profile at focus and dimensions of ~50 µm in width and ~3 mm in height. Scattered light images of each tested volume (obtained using a counter-propagating cw He-Ne laser diagnostic beam) are captured orthogonally to the laser propagation direction 𝑧, and the number of damage events per unit volume, or damage pinpoint density (PPD, in mm-3), is measured over the region of the crystal exposed to only peak fluence (~0.12 mm3). The damage performance of the material is quantified using two methods. First, we experimentally obtain the damage density profiles (PPD versus damage testing fluence at 355-nm) which provide a more detailed description of the damage performance over a wide range of laser fluences. The second method involves measuring the PPD at a fixed damage testing fluence as a function of location and/or postprocessing parameters.
An X-ray source with a Rh target (100 keV, 3 mA) was used to irradiate the samples (placed ~2 inches away) at ambient conditions along the x axis for 2 hrs up to 8 hrs exposure time. The integrated photon flux of X-rays at the front surface of the sample was determined to be ~1.9×109 photons/sec·cm2. Upon visual inspection, irradiated samples revealed no apparent changes in their optical properties. The typical energy distribution of continuous X-ray emission spectra[18] and linear attenuation coefficient of X-rays in DKDP (from NIST Physical Reference Data) are illustrated at the bottom of Fig. 1, where λ(Å)=12.366/E(keV). These plots suggests that ~50 keV or higher energy photons from the X-ray source will experience small attenuation in the material and thus produce point defects nearly uniformly throughout the bulk of the samples.
Fig. 2. Damage density profiles at 355-nm measured in high-PPD, pristine and 8-hr irradiated material (open and closed squares, respectively), and low-PPD pristine material (open circles). Average PPDs at fixed fluences from irradiated low-PPD material are also shown. Dashed lines through the data points are drawn as a guide to the eye.
The DKDP samples investigated here were 70%deuterated, harvested from a tripler-cut plate of conventionally grown material and polished to optical quality on all sides [4]. We prepared several cubic and rectangular samples with dimensions of ~1×1×1 cm3 and ~1.5×1×5 cm3, respectively. Previous studies of laser-induced damage in KDP/DKDP crystals have indicated that growth conditions, raw material, and other variations can greatly affect the damage performance of these materials [19, 20, 21, 22]. Large variations in damage densities have also been observed after testing within the same crystal boule, e.g., across growth and sector boundaries [21, 23]. Therefore, damage testing in pristine material is a necessary step to provide the measurement baseline and factor out any growth-related inhomogeneity in the damage performance of the samples. However, there is always a statistical spread in the damage density values recorded at neighboring locations, for any given fluence (illustrated later in Fig. 4).
Two sets of samples were selected for this study corresponding to a low-PPD and a high-PPD DKDP material. We have also limited our analysis to homogenous samples only, as indicated by damage tests performed prior to irradiation at several bulk locations (~250 µm apart) along the x axis. The distinct damage density profiles from each type of pristine material are illustrated in Fig. 2 (by open squares and open circles) and suggest that the damage thresholds in the two types of material are approximately the same despite the very different rising slope of PPD with increasing fluence (note the log-log scale for better visualization of both low and high damage densities). The representative results shown in Figs. 2-5 were obtained from two cubic samples from low-PPD material and one rectangular sample from high-PPD material.
We first investigated for a possible effect on the bulk damage characteristics as a function of location along the direction of X-ray beam propagation (i.e., at different positions along a line at fixed height) due to the decreasing dose of irradiation with penetration depth (assuming exponential attenuation). For comparison, the measurements were performed prior to and after irradiation of the samples up to 2, 4, and 8 hours. Following each exposure, we measured the PPD at a fixed laser damage testing fluence at several positions located in between those tested in pristine (prior to X-ray irradiation) material. These tests have revealed uniform PPD values (within experimental errors) throughout the thickness of the irradiated samples (i.e., independent of the penetration depth), for all exposure times and both types of material. As an example, Fig. 3 illustrates the local damage densities (the average of measurements at four locations within 1 mm) versus X-ray penetration depth after testing at ~11.2 J/cm2 in both pristine and 2-hr irradiated high-PPD material. These measurements suggest that the attenuation of the X-ray photons responsible for the observed effects was not significant enough to lead to measurable changes in the damage performance. Therefore, for the purpose of this discussion, we report the average values of the damage densities measured at all locations along the direction of the X-ray beam as a function of only exposure time to X-ray radiation.
Fig. 3. Local damage pinpoint density versus X-ray beam penetration depth after testing at fixed fluence (at 355-nm) in high-PPD, pristine and 2-hr irradiated material (open and closed squares, respectively). Solid lines through the data illustrate the average PPD values.
The results of damage testing at 11.2±1 J/cm2 in high-PPD material as a function of exposure time are presented in detail in Fig. 4 in terms of frequency count of measured PPDs before and after X-ray irradiation (at neighboring locations). These histograms demonstrate an increase in the overall damage density distributions at fixed fluence from irradiated vs pristine material. In addition, this effect is reduced with increasing exposure time. The same results are summarized in Fig. 5 by plotting only the average of the PPD distributions as a function of exposure time. Furthermore, the decrease in the overall damage performance of irradiated high-PPD material is also evident from the shift of the entire damage density profile to lower fluences after exposure as compared to that from pristine material (illustrated in Fig. 2 for 8-hr exposure).
The samples representing the low-PPD material were irradiated for 2 and 8 hours, respectively. The average PPD values after damage testing at several fixed fluences along the direction of X-ray beam propagation are summarized in Figs. 2 and 5. The results suggest that the damage resistance of low-PPD DKDP material is improved upon irradiation with X-rays, but this improvement is partially reversed with prolonged exposure. The former is in contrast to the behavior illustrated above from high-PPD material, where a decline in the damage performance (i.e., increase in PPD) has been observed.
Fig. 4. Frequency count of local damage pinpoint densities versus exposure time after testing at fixed fluence (at 355-nm) in pristine and irradiated high-PPD material (clear and hashed boxes, respectively).
3. Discussion
The results discussed above indicate that the damage performance of DKDP material is affected by exposure to X-rays but the effect is reduced with increasing exposure time. Most noticeably, in the case of 2-hr exposure, the damage threshold (as measured by our system) is reduced by ~30%in high-PPD material while the threshold increases by ~70%in the low-PPD material, as compared to the values recorded from each type of pristine material. The latter effect is similar to laser annealing (also referred to as laser conditioning), the process by which preexposure to sub-damage-threshold laser irradiation leads to a measurable increase in the damage resistance of KDP/DKDP materials. Although we do not consider in this work X-ray irradiation as an alternative method to laser annealing, the combination of the two methods may offer additional benefits (further improve the damage performance of the material) and also contribute to the fundamental understanding of the nature of the damage initiating defects.
The present results do not provide evidence that new damage precursors are formed from exposure to X-ray irradiation since, for the low-PPD material, an overall improvement in the damage characteristic is observed. Instead, the results suggest that there are interaction effects between X-ray induced short-lived (transient) point defects [8, 9, 10, 11, 12, 13, 14, 15, 16] and the pre-existing damage precursors. This interaction is macroscopically manifested as either increase or decrease in the individual threshold for damage initiation of the precursors. This in turn leads to a shift of the PPD vs fluence profile (as shown in Fig. 2) or a change in the resulting PPD when testing at a fixed fluence (as shown in Fig. 5). More recent experimental results support the model that the damage precursors are clusters of intrinsic material defects which act as multi-level absorbing particles [7]. The absorbing nanoparticle model of damage initiation predicts that the size of the precursors is determining their individual damage threshold, i.e., smaller precursors initiate damage at a higher fluence [5, 6]. More recently, Spaeth et al. have proposed that the defect density of the cluster precursors may play a key role [24]. Therefore, the change in the individual damage threshold of the precursors can be attributed to changes in their properties that govern their ability to initiate damage, namely, a) the electronic structure of the constituent atomic defects, b) the size of the precursor and, c) the density of the constituent defects within the precursor. The experimental results suggest that the outcome of the above interactions depends on the initial properties of the precursors which are different between the low- and the high-PPD materials. Arguably, the electronic structure of the precursors is the same in different DKDP materials. Furthermore, there should be a wide range of precursor sizes in both types of materials leading to the damage profiles shown in Fig. 2. Therefore, it may be the density of the precursor defect clusters that differs the most between the two materials and ultimately determines the final outcome of their interactions with the X-ray produced transient defects. Moreover, such interactions can involve multiple steps, as highlighted in the work by Chirila et al. [16].
Fig. 5. Average damage pinpoint density versus exposure time after testing at fixed fluences (at 355-nm) in both pristine (0 hrs) and irradiated high-PPD and low-PPD material.
We hypothesize that the lower damage performance material (the high-PPD type) contains more dense defects as compared to those from low-PPD material. Upon X-ray irradiation, trapping of transient defects at the damage precursor location may lead to an increase in its size (and density, to a smaller degree) and thus lowering of the damage threshold of the precursors [6], which is manifested as a decline in the damage characteristics of the material (PPD profile shifting to lower fluences as shown in Fig. 2 or higher PPD when testing at a fixed fluence as shown in Fig. 5). In order to explain the improvement in the damage performance of the low-PPD material following X-ray irradiation, we may need to assume that trapping of transient defects by its less dense precursor defect clusters leads to conformational changes with the formation of clusters of increased density but smaller size. These modified clusters will exhibit increased damage thresholds (due to their smaller size) compared to those prior to X-ray irradiation. Although the exact interaction mechanisms between transient X-ray induced point defects and pre-existing clusters of defects leading to damage initiation in DKDP crystals may be undetermined, this work provides a first account of such interactions not only in DKDP crystals but in any optical material.
Acknowledgments
We thank Dr. S. O. Kucheyev and Dr. L. E. Halliburton for useful discussions. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
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