Laser damage performance of KD_2-χH_χPO_4 crystals following X-ray irradiation

Laser damage performance ofKD2−xHxPO4 crystals following X-ray

irradiation

R. A. Negres*, C. K. Saw, P. DeMange, and S. G. DemosLawrence Livermore National Laboratory,

7000 East Avenue, Livermore, California 94550*[email protected]

Abstract: We investigate the laser-induced damage performance ofKD2−xHxPO4 crystals following exposure to X-ray irradiation. Two impor-tant issues addressed by our study are i) the performance of the materialwhen operational conditions lead to its exposure to ionizing irradiationand ii) the way the radiation-induced transient defects interact with thepre-existing precursor defects responsible for laser-induced damage. Ourresults indicate that the damage performance of the material is affected byexposure to X-rays. This behavior is attributed to a change in the physicalproperties of the precursors which, in turn, affect their ability to initiatedamage following interaction with X-ray generated defects.

© 2008 Optical Society of AmericaOCIS codes: (140.0140) Lasers and laser optics; (140.3330) Laser damage.

References and links1. L. N. Rashkovich, KDP-family single crystals (Adam Hilger, Bristol, 1991).2. V. G. Dmitriev, G. G. Gurzadyan, and D. N. Nikogosyan, Handbook of Nonlinear Optical Crystals (Springer-

Verlag, Berlin, Germany, 1997), 2nd ed.3. D. H. Munro, S. N. Dixit, A. B. Langdon, and J. R. Murray, “Polarization smoothing in a convergent beam,”

Appl. Opt. 43, 6639–6647 (2004).4. J. J. De Yoreo, A. K. Burnham, and P. K. Whitman, “Developing KH2PO4 and KD2PO4 crystals for the world’s

most powerful laser,” Int. Mater. Rev. 47, 113–152 (2002).5. R. W. Hopper and D. R. Uhlmann, “Mechanism of inclusion damage in laser glass,” J. Appl. Phys. 41, 4023–4037

(1970).6. M. D. Feit and A. M. Rubenchik, “Implications of nanoabsorber initiators for damage probability curves, pulse-

length scaling, and laser conditioning,” Proc. SPIE 5273, 74–82 (2004).7. C. W. Carr, H. B. Radousky, and S. G. Demos, “The Wavelength Dependence of Laser Induced Damage: Deter-

mining the Damage Initiation Mechanisms,” Phys. Rev. Lett. 91, 127402 (2003).8. W. E. Hughes and W. G. Moulton, “Electron Spin Resonance of Irradiated KH2PO4 and KD2PO4,” J. Chem.

Phys. 39, 1359–1360 (1963).9. G. Volkel, W. Windsch, and W. Urbanowitschius, “Dynamics of irradiation defect centers in potassium dihydro-

gen phosphate observed by EPR,” J. Mag. Reson. 18, 57–63 (1975).10. J. A. McMillan and J. M. Clemens, “Paramagnetic and optical studies of radiation-damage centers in K(H1-

xDx)2PO4,” J. Chem. Phys. 68, 3627–3631 (1978).11. E. Dieguez, J. M. Cabrera, and F. Agullo Lopez, “Optical absorption and luminescence induced by X-rays in

KDP, DKDP, and ADP,” J. Chem. Phys. 81, 3369–3374 (1984).12. J. W. Wells, E. Budzinski, and H. C. Box, “Electron-spin-resonance and ENDOR studies of irradiated potassium

dihydrogen phosphate,” J. Chem. Phys. 85, 6340–6346 (1986).13. S. D. Setzler, K. T. Stevens, L. E. Halliburton, M. Yan, N. P. Zaitseva, and J. J. De Yoreo, “Hydrogen atoms in

KH2PO4 crystals,” Phys. Rev. B 57, 2643–2646 (1998).14. K. T. Stevens, N. Y. Garces, L. E. Halliburton, M. Yan, N. P. Zaitseva, J. J. De Yoreo, G. C. Catella, and J. R.

Luken, “Identification of the intrinsic self-trapped hole center in KD2PO4,” Appl. Phys. Lett. 75, 1503–1505(1999).

(C) 2008 OSA 13 October 2008 / Vol. 16, No. 21 / OPTICS EXPRESS 16326#97370 - $15.00 USDReceived 16 Jun 2008; revised 26 Aug 2008; accepted 6 Sep 2008; published 29 Sep 2008

15. N. Y. Garces, K. T. Stevens, L. E. Halliburton, S. G. Demos, H. B. Radousky, and N. P. Zaitseva, “Identificationof electron and hole traps in KH2PO4 crystals,” J. Appl. Phys. 89, 47–52 (2001).

16. M. M. Chirila, N. Y. Garces, L. E. Halliburton, S. G. Demos, T. A. Land, and H. B. Radousky, “Production andthermal decay of radiation-induced point defects in KD2PO4 crystals,” J. Appl. Phys. 94, 6456–6462 (2003).

17. P. DeMange, C. W. Carr, H. B. Radousky, and S. G. Demos, “System for evaluation of laser-induced damageperformance of optical materials for large aperture lasers,” Rev. Sci. Instrum. 75, 3298–3301 (2004).

18. H. A. Kramers, “On the theory of X-ray absorption and the continuous X-ray spectrum,” Philos. Mag. 46, 836–871 (1923).

19. H. Endert and M. L. Martin, “The effect of chromium impurities on the laser damage thresholds of KDP crystals,”Cryst. Res. Technol. 16, K65-K66 (1981).

20. H. Endert and W. Melle, “Laser-Induced damage in KDP crystals,” Phys. Status Solidi 74, 141–148 (1982).21. N. P. Zaitseva, J. J. De Yoreo, M. R. Dehaven, R. L. Vital, K. E. Montgomery, M. Richardson, and L. J. Atherton,

“Rapid growth of large-scale (40-55 cm) KH2PO4 crystals,” J. Cryst. Growth 180, 255–262 (1997).22. A. K. Burnham, M. Runkel, M. D. Feit, A. M. Rubenchik, R. L. Floyd, T. A. Land, W. J. Siekhaus, and R. A.

Hawley-Fedder, “Laser-Induced Damage in Deuterated Potassium Dihydrogen Phosphate,” Appl. Opt. 42, 5483–5495 (2003).

23. R. A. Negres, N. P. Zaitseva, P. Demange, and S. G. Demos, “Expedited laser damage profiling of KDxH2−xPO4with respect to crystal growth parameters,” Opt. Lett. 31, 3110–3112 (2006).

24. M. L. Spaeth, Lawrence Livermore National Laboratory (internal communication, 2006).

1. Introduction

Potassium dihydrogen phosphate, KH2PO4 (KDP) and its deuterated analog, DKDP, are tech-nologically important and unique materials for use in high-power, large-aperture laser systemsfor various applications (e.g., electro-optics switching, polarization smoothing and nonlinearoptical frequency conversion) [1, 2, 3, 4]. The use of nonlinear optical materials in such lasersystems is continuously expanding and the laser output and frequency conversion efficiencyrely on increased laser intensities. As a result, laser-induced damage in these materials rep-resents a key limiting factor. Localized damage initiation in KDP/DKDP crystals has beenattributed to either impurity nanoparticles incorporated during growth or clusters of intrinsicdefects that form during growth[5, 6, 7]. However, the exact nature of these defects has not yetbeen identified. Moreover, large-aperture laser systems under development, designed to expandthe frontiers in high-energy density physics, will create an operational environment where op-tical components, such as the frequency conversion crystals (KDP/DKDP), may be exposed toX-rays and other ionizing radiation. This in turn may lead to a change in the damage perfor-mance 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 havebeen identified using either optical techniques or electron paramagnetic resonance (EPR) spec-troscopy [8, 9, 10, 11, 12, 13, 14, 15, 16]. Hydrogen atoms, oxygen vacancies, self-trappedholes, and holes trapped adjacent to hydrogen vacancies are among these defects. These earlierresults confirmed that most of the radiation-induced point defects in the pure material are shortlived above 200 K and revealed the complex relaxation pathways as one defect species decaysand 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 ofdefects formed during X-ray radiation may affect the laser induced damage performance ofKDP/DKDP materials.

In this Letter, we evaluate the laser-induced bulk damage performance at 355-nm of DKDPcrystal samples subjected to X-ray irradiation at room temperature with different exposuretimes. Specifically, we map the damage density at fixed testing fluence versus location alongthe 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 laserdamage characteristics in DKDP crystals by either forming new damage initiation centers orinteracting 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 toand after X-ray irradiation along x). (Bottom) Typical intensity distribution in the continu-ous X-ray spectrum from Rh target (100 keV, 3 mA) and the linear attenuation coefficientof X-rays in DKDP (left and right inset graphs, respectively).

It should be noted that atomically dispersed point defects (at low concentration) are not effi-cient light-absorbers as needed to initiate localized breakdown in these materials [6]. However,aggregations (clusters) of intrinsic point defects (prompted by stress, lattice imperfections, im-purities, or other factors) leading to localized high concentration of atomic defects can act inpart as absorbing particles by providing additional electronic states in the band gap and thusefficient coupling of laser light into the material. This in turn leads to the formation of macro-scopic 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 profileat focus and dimensions of ∼50 µm in width and ∼3 mm in height. Scattered light images ofeach tested volume (obtained using a counter-propagating cw He-Ne laser diagnostic beam) arecaptured orthogonally to the laser propagation direction z, and the number of damage events perunit volume, or damage pinpoint density (PPD, in mm−3), is measured over the region of thecrystal exposed to only peak fluence (∼0.12 mm3). The damage performance of the materialis 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 descriptionof the damage performance over a wide range of laser fluences. The second method involvesmeasuring the PPD at a fixed damage testing fluence as a function of location and/or post-processing 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


Fig. 2. Damage density profiles at 355-nm measured in high-PPD, pristine and 8-hr irradi-ated material (open and closed squares, respectively), and low-PPD pristine material (opencircles). 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.

changes in their optical properties. The typical energy distribution of continuous X-ray emis-sion spectra[18] and linear attenuation coefficient of X-rays in DKDP (from NIST PhysicalReference Data) are illustrated at the bottom of Fig. 1, where λ (A)=12.366/E(keV). Theseplots suggests that ∼50 keV or higher energy photons from the X-ray source will experiencesmall attenuation in the material and thus produce point defects nearly uniformly throughoutthe bulk of the samples.

The DKDP samples investigated here were 70% deuterated, harvested from a tripler-cut plateof conventionally grown material and polished to optical quality on all sides [4]. We preparedseveral 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 indicatedthat growth conditions, raw material, and other variations can greatly affect the damage per-formance of these materials [19, 20, 21, 22]. Large variations in damage densities have alsobeen observed after testing within the same crystal boule, e.g., across growth and sector bound-aries [21, 23]. Therefore, damage testing in pristine material is a necessary step to provide themeasurement baseline and factor out any growth-related inhomogeneity in the damage perfor-mance of the samples. However, there is always a statistical spread in the damage density valuesrecorded 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-PPDDKDP material. We have also limited our analysis to homogenous samples only, as indicated bydamage tests performed prior to irradiation at several bulk locations (∼250 µm apart) along thex axis. The distinct damage density profiles from each type of pristine material are illustratedin Fig. 2 (by open squares and open circles) and suggest that the damage thresholds in the twotypes of material are approximately the same despite the very different rising slope of PPD withincreasing fluence (note the log-log scale for better visualization of both low and high damagedensities). The representative results shown in Figs. 2-5 were obtained from two cubic samplesfrom 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


Fig. 3. Local damage pinpoint density versus X-ray beam penetration depth after testingat fixed fluence (at 355-nm) in high-PPD, pristine and 2-hr irradiated material (open andclosed squares, respectively). Solid lines through the data illustrate the average PPD values.

of location along the direction of X-ray beam propagation (i.e., at different positions along aline at fixed height) due to the decreasing dose of irradiation with penetration depth (assumingexponential attenuation). For comparison, the measurements were performed prior to and af-ter irradiation of the samples up to 2, 4, and 8 hours. Following each exposure, we measuredthe PPD at a fixed laser damage testing fluence at several positions located in between thosetested in pristine (prior to X-ray irradiation) material. These tests have revealed uniform PPDvalues (within experimental errors) throughout the thickness of the irradiated samples (i.e., in-dependent of the penetration depth), for all exposure times and both types of material. As anexample, Fig. 3 illustrates the local damage densities (the average of measurements at four loca-tions within 1 mm) versus X-ray penetration depth after testing at ∼11.2 J/cm2 in both pristineand 2-hr irradiated high-PPD material. These measurements suggest that the attenuation of theX-ray photons responsible for the observed effects was not significant enough to lead to mea-surable changes in the damage performance. Therefore, for the purpose of this discussion, wereport the average values of the damage densities measured at all locations along the directionof the X-ray beam as a function of only exposure time to X-ray radiation.

The results of damage testing at 11.2±1 J/cm2 in high-PPD material as a function of exposuretime are presented in detail in Fig. 4 in terms of frequency count of measured PPDs before andafter X-ray irradiation (at neighboring locations). These histograms demonstrate an increase inthe overall damage density distributions at fixed fluence from irradiated vs pristine material. Inaddition, this effect is reduced with increasing exposure time. The same results are summarizedin 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 isalso evident from the shift of the entire damage density profile to lower fluences after exposureas 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, respec-tively. The average PPD values after damage testing at several fixed fluences along the directionof X-ray beam propagation are summarized in Figs. 2 and 5. The results suggest that the dam-age resistance of low-PPD DKDP material is improved upon irradiation with X-rays, but this


Fig. 4. Frequency count of local damage pinpoint densities versus exposure time after test-ing at fixed fluence (at 355-nm) in pristine and irradiated high-PPD material (clear andhashed boxes, respectively).

improvement is partially reversed with prolonged exposure. The former is in contrast to the be-havior illustrated above from high-PPD material, where a decline in the damage performance(i.e., increase in PPD) has been observed.

3. Discussion

The results discussed above indicate that the damage performance of DKDP material is affectedby 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, ascompared to the values recorded from each type of pristine material. The latter effect is similarto laser annealing (also referred to as laser conditioning), the process by which preexposure tosub-damage-threshold laser irradiation leads to a measurable increase in the damage resistanceof KDP/DKDP materials. Although we do not consider in this work X-ray irradiation as analternative method to laser annealing, the combination of the two methods may offer additionalbenefits (further improve the damage performance of the material) and also contribute to thefundamental understanding of the nature of the damage initiating defects.

The present results do not provide evidence that new damage precursors are formed fromexposure to X-ray irradiation since, for the low-PPD material, an overall improvement in the


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.

damage characteristic is observed. Instead, the results suggest that there are interaction effectsbetween X-ray induced short-lived (transient) point defects [8, 9, 10, 11, 12, 13, 14, 15, 16] andthe pre-existing damage precursors. This interaction is macroscopically manifested as eitherincrease or decrease in the individual threshold for damage initiation of the precursors. This inturn leads to a shift of the PPD vs fluence profile (as shown in Fig. 2) or a change in the resultingPPD when testing at a fixed fluence (as shown in Fig. 5). More recent experimental resultssupport the model that the damage precursors are clusters of intrinsic material defects which actas multi-level absorbing particles [7]. The absorbing nanoparticle model of damage initiationpredicts 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. haveproposed 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 intheir properties that govern their ability to initiate damage, namely, a) the electronic structure ofthe constituent atomic defects, b) the size of the precursor and, c) the density of the constituentdefects within the precursor. The experimental results suggest that the outcome of the aboveinteractions depends on the initial properties of the precursors which are different betweenthe low- and the high-PPD materials. Arguably, the electronic structure of the precursors is thesame in different DKDP materials. Furthermore, there should be a wide range of precursor sizesin both types of materials leading to the damage profiles shown in Fig. 2. Therefore, it may bethe density of the precursor defect clusters that differs the most between the two materials andultimately determines the final outcome of their interactions with the X-ray produced transientdefects. Moreover, such interactions can involve multiple steps, as highlighted in the work byChirila et al. [16].

We hypothesize that the lower damage performance material (the high-PPD type) containsmore dense defects as compared to those from low-PPD material. Upon X-ray irradiation, trap-ping 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 profileshifting to lower fluences as shown in Fig. 2 or higher PPD when testing at a fixed fluenceas 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 tran-sient defects by its less dense precursor defect clusters leads to conformational changes withthe formation of clusters of increased density but smaller size. These modified clusters will ex-hibit increased damage thresholds (due to their smaller size) compared to those prior to X-rayirradiation. Although the exact interaction mechanisms between transient X-ray induced pointdefects and pre-existing clusters of defects leading to damage initiation in DKDP crystals maybe undetermined, this work provides a first account of such interactions not only in DKDPcrystals but in any optical material.

Acknowledgments

We thank Dr. S. O. Kucheyev and Dr. L. E. Halliburton for useful discussions. This workwas performed under the auspices of the U.S. Department of Energy by Lawrence LivermoreNational Laboratory under Contract DE-AC52-07NA27344.


Documents

Laser damage performance of KD_2-χH_χPO_4 crystals following X-ray irradiation