Laser-induced breakdown in the bulk of transparent dielectric materials is associated with the generation of extreme localized conditions of temperatures and pressures. In this work, we perform pump and probe damage testing experiments to investigate the evolution of transient absorption by the host material arising from modifications following confined laser energy deposition in fused silica and DKDP materials. Specifically, we measure the size of the damage sites observed in the region of spatial overlap between the pump and probe pulses versus probe time delay and energy. Results of this proof-of-principle experimental work confirm that material modifications under extreme conditions created during a damage event include transient optical absorption. In addition, we found that the relaxation times of the induced absorption are very distinct for DKDP and SiO{sub 2} even under identical excitation conditions, on the order of 100 ns and 100 {micro}s, respectively

Bude, J D

DeMange, P

Demos, S G

Feit, M D

Negres, R A

UNT Digital Library

UCRL-PROC-235690Pump and probe damage testing forinvestigation of transient materialmodifications associated with laserdamage in optical materialsR. A. Negres, M. D. Feit, P. DeMange, J. D. Bude,S. G. DemosOctober 19, 2007Boulder Damage Symposium XXXIXBoulder, CO, United StatesSeptember 24, 2007 through September 26, 2007Disclaimer  This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.  Pump and probe damage testing for investigation oftransient material modifications associated withlaser damage in optical materialsR. A. Negres, M. D. Feit, P. DeMange, J. D. Bude, and S. G. DemosLawrence Livermore National Laboratory7000 East Avenue, Livermore, CA 94550, USAABSTRACTLaser-induced breakdown in the bulk of transparent dielectric materials is associated with the generation ofextreme localized conditions of temperatures and pressures. In this work, we perform pump and probe damagetesting experiments to investigate the evolution of transient absorption by the host material arising from modi-fications following confined laser energy deposition in fused silica and DKDP materials. Specifically, we measurethe size of the damage sites observed in the region of spatial overlap between the pump and probe pulses ver-sus probe time delay and energy. Results of this proof-of-principle experimental work confirm that materialmodifications under extreme conditions created during a damage event include transient optical absorption. Inaddition, we found that the relaxation times of the induced absorption are very distinct for DKDP and SiO2even under identical excitation conditions, on the order of 100 ns and 100 µs, respectively.Keywords: Laser-induced material modifications, optical materials, localized energy deposition1. INTRODUCTIONIn large bandgap materials, laser intensities in excess of 1011 W/cm2 are required to achieve intrinsic breakdown.1However, defects in the material can initiate localized breakdown at more than two orders of magnitude lowerintensities. Laser damage caused by nanosecond scale pulses in practical optical materials is nearly always dueto precursors, i.e. very small absorbers not present in the ideal material. Even though the amount of energyabsorbed by such precursors can be as small as tens of nanojoules, the energy density is high enough that theprecursor is vaporized and forms a plasma with temperature in the eV range (104 K). The hot high pressure(∼ 30 GPa) plasma generation launches a shockwave into the surrounding material.2, 3 This localized energy isfast dissipating and is accompanied by a sequence of transient material modifications that lead to the formationof a void.3, 4 Material modifications associated with laser damage arise from key fundamental processes thatare not well understood, such as solid state material response to localized extreme conditions, energy transportmechanisms through complex material phases, and material displacement and lattice transformation. Moreover,the sequence of events remains largely unknown.In this work, we present initial results using pump and probe damage testing to investigate the localizeddynamics of transient optical absorption of the modified material following laser-induced breakdown. We explorethe dependence of the transient absorption on size of initial energy deposition volume (from pump pulse) andprobe laser fluence. We also estimate the absorption relaxation times for bulk DKDP and bulk fused silica,chosen for their very different structural, mechanical and thermal properties.2. EXPERIMENTPump-probe measurements can be used to obtain information on complex transient phenomena. The generalprinciple is the following. A sample is exposed to a first (pump) pulse, which generates some kind of excitation(or other modification) in the sample. After an adjustable time interval, a second (probe) pulse is used toirradiate the sample, and through its interaction with the material (as manifested using a variety of techniques)obtain information regarding transient changes introduced by the pump pulse. By monitoring the probe signalSend correspondence to R. A. Negres: negres2@llnl.govas a function of the time delay, one obtains information on the decay of the generated excitation, or on otherprocesses initiated by the pump. The temporal resolution is fundamentally limited only by the pulse duration. Inparticular for the pump-probe laser-induced studies proposed in this work, the probe beam can also be absorbedand thus change the final (time equal infinity) state.In this work, the pump-probe technique is integrated within a novel damage testing system that allowsdetection of damage events in the bulk of optical materials with very high spatial resolution.5 Moreover, thedelay between pump and probe pulses can be varied over a wide range, from nanoseconds to microseconds, andeven milliseconds, in contrast to those achieved by optical delay lines. This approach can be realized by usingtwo independent, nanosecond Q-switched Nd:YAG laser systems that are synchronized within five nanosecondsusing an electronic pulse delay generator. A sketch of the experimental setup is shown in Fig. 1. The pumppulse is obtained from a first Nd:YAG, Q-switched laser with output at the fundamental (1064 nm) and 2nd and3rd harmonics [532- and 355-nm with ∼3-ns Full-Width-Half-Maximum (FWHM)]. The pump beam is used toinduce damage sites in the bulk of the material and is kept at a fixed fluence just above the damage threshold ofthe material (with 10% shot-to-shot variation). The probe beam is obtained from a second Nd:YAG, Q-switchedlaser with output at the fundamental (1064 nm) and 2nd and 3rd harmonics (with ∼7-ns FWHM at 355-nm)which is aligned to spatially overlap the pump beam inside the sample, with comparable or larger spot size thanthat of the pump. A mechanical, computer controlled fast shutter (FS) enables single pulse operation. The timedelay, pulse widths and energies of pump and probe pulses are monitored in real time using fast photodiodes(PhD). A He-Ne laser beam is focused by a 250-mm focal length cylindrical lens (CL) through the back of thesamples to illuminate the tested volume and detect bulk damage events (see Fig. 2). A CCD camera coupled to amicroscope equipped with a long-working-distance objective lens is used to record high-resolution light-scatteringimages (2 µm per pixel) in order to monitor changes in the morphology of bulk damage sites induced by thepump pulse in the presence of the probe background fluence. The materials investigated here were conventionalgrown DKDP and UV grade fused silica. Both samples were 5×5×1 cm3 plates with optical quality finish onfour sides. Figures 2(a) and 2(b) illustrate in more detail the pump-probe damage testing configurations andbeam paths within the sample for two different excitation methods used in this study, spontaneous and intrinsicbulk damage initiation, respectively.PROBE LASER355-nm, 7 ns, 10 HzPUMP LASER355 or 532-nm, 3 ns, 10 HzPULSE DELAYGENERATORLampTriggerQ-SwitchTriggerBSSampleDamage testingTrigger InBDFSDELAYPhD BSPhDBSFigure 1. Schematic view of pump and probe damage testing using two independent, nanosecond Q-switched Nd:YAGlaser systems (BS-beam splitter, PhD-photodiode, BD-beam dump, FS-fast shutter).For the case of spontaneous damage in DKDP [see Fig. 2(a)], the pump and probe beams at 355-nm areco-propagating and focused within the bulk of the material using a 200-mm cylindrical lens (i.e., slit beams atthe sample location). We note that no spontaneous bulk damage is observed in fused silica. The pump peakfluence was fixed at 9 J/cm2 (gaussian beam profile with 2.2 mm by 60 µm) and resulted in a damage pinpointdensity of ∼400 mm−3 (see Ref. 5). The probe fluence was varied and kept below 17 J/cm2 to minimize theamount of damage due to probe pulse alone (gaussian beam profile with 2.4 mm by 110 µm). The individualdamage sites created separately by the pump and probe pulses were 5 µm and 10 µm in size, respectively. In(a)(b)Collinear Pump andProbe at 355-nmCL DKDPHe-NelaserCLX20ObjectiveCCD     xyz stagePump at 532-nmX10ObjectiveSampleHe-NelaserCLX20ObjectiveCCD     BSProbeat 355-nmxyz stageSLMFigure 2. Closeup view of beam paths to the sample(dotted region in Fig. 1) in (a) collinear, one-color and(b) non-collinear, two-color pump and probe geometriesfor spontaneous and intrinsic damage, respectively (CL-cylindrical lens, SL-spherical lens, M-mirror).6.1x 2.2 mm2DKDP(b)(a)(c)Figure 3. Typical scattered light images of bulk damagesites in the pump and probe interaction region for (a)spontaneous damage in DKDP and (b) intrinsic damagein DKDP and SiO2 under identical excitation conditions.(c) Size assessment of the damage region is performed interms of cross-sectional area (i.e., x·y) of damage sites (ex-ample shows damage sites in SiO2).this experimental configuration, the spatial distribution of pump and probe fluences along the slit beams leads toa distribution of sizes of damage sites within the interaction region, as illustrated by the light-scattering imageshown in Fig. 3(a) (with 5 µm per pixel resolution). We have assumed that the largest observed damage sitescorrespond to the spatial overlap of pump and probe pulses at peak fluences and thus their sizes will be reportedas a function of probe peak fluence.In order to achieve intrinsic bulk damage in DKDP and SiO2, intensities in excess of 200 GW/cm2 at 532-nm(for 3-ns pulses) need to be employed. For this purpose and to prevent surface damage on the substrates, wehave used a 10× objective in the pump beam path, as shown schematically in Fig. 2(b). This two-color, non-collinear pump-probe geometry allows for independent control of the probe beam size at the interaction region(i.e., focused using a separate spherical lens, SL). Shot-to-shot deterministic damage within the bulk can beachieved above the breakdown threshold. Therefore, the pump intensity at 532-nm was fixed at ∼440 GW/cm2for both DKDP and SiO2. The probe fluence at 355-nm (7-ns) was varied up to 80 J/cm2 in silica and up to28 J/cm2 in DKDP. We note that, under identical excitation conditions, the damage sites in DKDP are ∼4×larger than those observed in SiO2, as illustrated in Fig. 3(b). In addition, typical damage sites in fused silicaconsist of two pits with diameters of ∼40 µm and ∼20 µm, respectively, with the latter always upstream fromthe focus.63. RESULTS AND DISCUSSIONWith the above described experimental configurations, we monitored the size of the damage sites observed inthe pump and probe interaction region as a function of probe time delay at fixed probe fluences. Specifically,the overall size of damage sites is evaluated from light-scattering images in terms of cross-sectional area of thedamage region, i.e. width × height. Figure 3(c) illustrates the procedure for measuring the size of the damagearea and comparison of damage sites from pump only to those observed after pump and probe interaction inSiO2.The results of pump and probe damage testing in DKDP are presented in Figs. 4(a) and 4(b) for spontaneousand intrinsic bulk damage, respectively. The size of the damage area in the pump and probe interaction regionwas measured as a function of probe time delay from 1 ns up to 500 ns for fixed probe fluences (not all shown).Similarly, measurements of the damage area for the case of intrinsic bulk damage in SiO2 are summarizedFigure 4. Cross-sectional area of bulk damage sites in the pump and probe interaction region for the case of (a) spontaneousand (b) intrinsic damage in DKDP, and (c) intrinsic damage in SiO2 as a function of probe time delay for fixed probefluences. Size of damage from pump only is also shown for comparison (solid lines).in Fig. 4(c) for two different probe fluences and time delays up to 100 µs. In addition, the damage size inSiO2 corresponding to few seconds (i.e., infinite) probe delay is also shown at ∼0.5 ms on the same graph (forillustration purposes), as it is relevant to damage growth. For comparison, the size of damage sites from pumponly is plotted as a constant baseline in each graph (solid lines). Throughout this work, individual data pointsrepresent the average over measurements performed at five testing locations for each probe delay and fluencecombination. The error bars are then derived from the standard deviation of the measurement.The increase in the final size of damage sites following pump and probe interaction as compared to thosefrom pump only confirms the presence of enhanced absorption by the host material surrounding the locationof energy deposition. Figures 4(a)-(c) illustrate several interesting features of the localized dynamics of thetransient absorption. Namely:i) The size of the damaged region, and thus the amount of energy absorbed from the probe pulse and depositedinto the material, first increases with delay time and then decreases. In DKDP, the peak in the absorption profileis located at ∼20 ns and ∼50 ns, for spontaneous and intrinsic damage, respectively, while the maximum sizeddamage region in SiO2 is observed at ∼100 ns [indicated by the vertical arrows in Figs. 4(a)-(c)]. Moreover, inthe former case, the peak location shifts to longer delays with increasing focal volume (i.e., increasing energydeposition volume).ii) The transient absorption of the laser-modified material is longer lived in fused silica, decaying over ∼100 µs,compared to only ∼100 ns observed in DKDP. Even under under identical excitation conditions (as is the casefor intrinsic damage initiation), the relaxation times in the two materials differ by three orders of magnitude.This behavior, in conjunction with the different peak locations in the absorption profiles of DKDP and silica, asdepicted in Figs. 4(b) and 4(c), suggest a strong dependence on the host material properties. This assumption isfurther supported by the fact that the relaxation time for DKDP is the same for both spontaneous and intrinsicdamage [see Figs. 4(a)-(b), under very different excitation conditions].iii) For the case of fused silica, the size of damage sites after pump and probe damage testing with seconds delay(i.e., commonly referred to as damage growth with subsequent pulses) appears to be larger than those observedat 100 µs time delay. This observation suggests that the mechanisms for additional energy deposition by theprobe pulse in the material may be different in the various stages of transient material modifications leading toa damage site (i.e. crack growth may be a contributing factor in the later stages while induced absorption dueto localized high temperatures and pressures are playing a role at early times).The results presented in this study clearly show that the initial energy deposition and resulting plasma modifythe surrounding material to become absorbing. This process may occur due to either thermal or mechanicalenergy transport from the initial damage event associated with extreme localized conditions of temperatures andpressures and generation of a shockwave. The maximum damaged area in the case of intrinsic SiO2 damage isabout 5 ×10−2 mm2 (diameter of ∼250 µm) for 100 ns delay. The corresponding velocity is comparable to thespeed of sound in silica (2.5 µm/ns vs 5.8 µm/ns) while the thermal conduction length is only about 1 µm for 100ns. Thus we assume that the intense pressure pulse following energy deposition (by the pump pulse) modifies thesurrounding material as it propagates. As a consequence, the volume of the induced absorbing region increasesat early times. At the same time, the shockwave weakens as it spreads and the material modification decays intime. The superposition of the above effects leads to the behaviors depicted in Figs. 4(a)-(c), namely, there is amaximum size damaged region as a function of probe time delay in both materials.4. SUMMARYThe results of pump and probe damage testing presented in this work have revealed the presence of inducedtransient absorption within the modified host material well outside the energy deposition volume. We haveshowed that the response of the material is largely independent of the excitation process and is characteristic ofthe material properties. The induced absorption can be understood in terms of PdV work done by a shockwavelaunched from the initial energy deposition by the pump pulse creating defects of different lifetimes in differentmaterials.ACKNOWLEDGMENTSThis work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore NationalLaboratory under Contract DE-AC52-07NA27344.REFERENCES1. B. C. Stuart, M. D. Feit, S. Herman, A. M. Rubenchik, B. W. Shore, and M. D. Perry, “Nanosecond-to-femtosecond laser-induced breakdown in dielectrics,” Phys. Rev. B 53, pp. 1749–1761, 1996.2. C. W. Carr, H. B. Radousky, A. M. Rubenchik, M. D. Feit, and S. G. Demos, “Localized dynamics duringlaser-induced damage in optical materials,” Phys. Rev. Lett. 92, p. 087401, 2004.3. S. Juodkazis, K. Nishimura, S. Tanaka, H. Misawa, E. G. Gamaly, B. Luther-Davies, L. Hallo, P. Nicolai, ,and V. T. Tikhonchuk, “Laser-induced microexplosion confined in the bulk of a sapphire crystal: Evidenceof multimegabar pressures,” Phys. Rev. Lett. 96, p. 166101, 2006.4. E. G. Gamaly, S. Juodkazis, K. Nishimura, H. Misawa, B. Luther-Davies, L. Hallo, P. Nicolai, and V. T.Tikhonchuk, “Laser-matter interaction in the bulk of a transparent solid: Confined microexplosion and voidformation,” Phys. Rev. B 73, p. 214101, 2006.5. 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, p. 3298, 2004.6. A. V. Smith, B. Do, and M. Soderlund, “Nanosecond laser-induced breakdown in pure and yb+3 doped fusedsilica,” in Laser-Induced Damage in Optical Materials:2006, G. J. Exarhos, A. H. Guenther, K. L. Lewis,D. Ristau, M. J. Soileau, and C. J. Stolz, eds., p. 640321, Proc. SPIE 6403, 2007.

Pump and probe damage testing for investigation of transient material modifications associated with laser damage in optical materials

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Pump and probe damage testing for investigation of transient material modifications associated with laser damage in optical materials

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