Photoluminescence characterization of KH2PO4 crystal: application to three-dimensional growth-sector identification Céline Gouldieff,1,* Frank R. Wagner,1 Bertrand Bertussi,2 François Guillet,2 and Jean-Yves Natoli1 1
Aix Marseille Université, CNRS, Centrale Marseille, Institut Fresnel UMR 7249, 13397 Marseille, France 2
CEA, DAM, LE RIPAULT, F-37260 Monts, France
*Corresponding author: [email protected] Received 2 January 2014; revised 9 March 2014; accepted 23 March 2014; posted 9 April 2014 (Doc. ID 203111); published 8 May 2014
In this work, rapidly grown KH2 PO4 (KDP) crystals extracted from the prismatic and the pyramidal growth sectors of crystal boules were analyzed using photoluminescence measurements. From the spectra, we deduced a robust criterion to discriminate between both growth sectors in an unknown KDP plate. Moreover, spatially resolved photoluminescence was shown to enable a local probing of different planes in the bulk of the material leading to accurate and nondestructive three-dimensional mapping of the sector boundary, which is often the weakest point in terms of laser-damage resistance in rapidly grown KDP crystals. © 2014 Optical Society of America OCIS codes: (160.4330) Nonlinear optical materials; (160.3380) Laser materials; (260.3800) Luminescence; (300.2530) Fluorescence, laser-induced. http://dx.doi.org/10.1364/AO.53.003063
1. Introduction
Rapid growth of KH2 PO4 (KDP) crystals was developed during the last 30 years [1] in order to provide large-sized high-quality frequency conversion crystals for inertial confinement fusion class lasers such as the NIF in the USA [2], the Laser MegaJoule in France [3,4], or Gekko XII in Japan [5]. However, contrary to conventional growth, rapid growth generates in the same boule two different sectors, named “pyramidal” and “prismatic” sectors. As illustrated schematically in Fig. 1, these two sectors are generated by the growth of crystal seed faces according to two crystalline directions: [100] growth direction for the prismatic sector, and [101] for the pyramidal sector [1].
1559-128X/14/143063-06$15.00/0 © 2014 Optical Society of America
In a KDP boule, the pyramidal sector is the purest zone of the crystal (as shown by ICP-AES measurements; see Table 2 in [6]), whereas the prismatic sector contains a higher density of impurities (see Table 1 in [1] and Table 2 in [7]) in the majority of metal ions. Indeed, impurities are incorporated much more easily in the prismatic sector than in the pyramidal sector because the attachment energy is lower on the f100g faces compared to the f101g faces [8]. Consequently, both growth sectors were shown to have different optical properties in terms of absorption [6,7], photoluminescence signals (see Fig. 3 in [9]), and laser-damage resistance [10]. In particular, the growth-sector boundary was evidenced to be the weak point of the crystal in terms of resistance to laser damage [11,12]. When comparing different tests on several samples, it is mandatory to know the origin of each sample in order to take into account the potential influences of the growth sector 10 May 2014 / Vol. 53, No. 14 / APPLIED OPTICS
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Fig. 1. Schematic cross section of a rapidly grown KDP crystal showing the growth sectors.
on the laser-damage resistance. Moreover, the large dimensions of the plates needed in inertial confinement fusion facilities require plates in which the growth-sector boundary may be present on part of the crystal. Even though an adapted laser conditioning procedure can allow both growth sectors to have very similar LIDT values, better knowledge of the position and shape of the boundary should allow reinforcing the laser conditioning in the growthsector boundary area. Considering the impact that the growth sector may have on the different physical properties, we present in this paper a robust criterion to nondestructively and locally determine from which growth sector a sample originates, and, in case the sample was cut across a sector boundary, to precisely determine the shape and the position of the boundary.
translation stage, which allows choosing the crystal site to be tested. The photoluminescence measurement module is made up of a collection lens that focuses the emitted light on the entry side of an optical fiber assembly and a notch filter (532 and 266 nm) that blocks diffused pump light. The optical fiber assembly then guides the photoluminescence signal to the spectrometer, which comprises a monochromator and an intensified CCD camera. All presented spectra are corrected using the wavelength-dependent sensitivity of the photoluminescence measurement setup (lens Lc to ICCD), which was obtained using a calibrated halogen/ deuterium lamp. Even if in high-power lasers, KDP crystals are used to convert light from 1053 nm to only 351 nm, we chose to pump at 266 nm for several reasons. First of all, the photoluminescence yield is higher at 266 nm than at 355 nm, even though the pump photon energy of 4.66 eV is still much less than the band gap of KDP (7.7 eV [14]). We thus detect defectinduced photoluminescence, and it has been demonstrated in [15] that the photoluminescence spectra reveal the same defects whatever the chosen pump wavelength (266 or 355 nm). Nevertheless, even when pumping at 266 nm, the signal is very low. In order to optimize the signal-tonoise ratio, we use an intensified CCD camera, and we pump a relatively large volume of tested KDP crystal.
2. Experimental Aspects
B. Testing Modes
A.
The measurements were realized in two different configurations:
Experimental Setup
The setup is schematically depicted in Fig. 2 [13]. The sample is pumped by a wavelength of 266 nm, provided by forth harmonic generation of a pulsed Nd:YAG laser. The pulse duration of the longitudinal multimode laser is 8 ns, and the pulse repetition rate is 100 Hz. The sample is mounted in a motorized X–Y
– The “large-beam configuration” using a nonfocused laser beam (6 mm beam diameter). – The “small-beam configuration” using a slightly focused laser beam by adding a lens in front of the sample (L2 in Fig. 2) in order to limit the pumped volume to a cylinder of approximately 2 mm in diameter. In this second configuration, the part of the crystal that produces the collected photoluminescence signal is not the whole thickness of the material but a smaller volume, close to the entrance side of the laser beam (see inset in Fig. 2). C.
Fig. 2. Sketch of the experimental setup including the photoluminescence measurement system. The pump source is a frequency quadrupled Nd:YAG laser (100 Hz, 8 ns). The inset presents the details of the light collection system and geometry. 3064
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Tested Samples
All measurements shown here were realized on three different KDP samples extracted from the same crystal boule. Two of them can be considered as “pure” samples, whose dimensions are 50 mm × 50 mm × 10 mm. The first crystal was extracted from the pyramidal sector of the crystal boule, and the second was extracted from the prismatic sector of the same crystal boule. The third sample, whose dimensions are 100 mm × 100 mm × 10 mm, is “mixed”; i.e., it includes material from both growth sectors. The x-ray topograph
Fig. 3. X-ray images of the mixed crystal, highlighting different lines (left) that can be interpreted as growth bands and a possible growth-sector boundary (right). The dashed outlines represent the crystal sides (real dimensions 100 mm × 100 mm).
[16] of this third sample is depicted in Fig. 3. The white line in the image to the right indicates where one may assume the location of the growth-sector boundary as deduced from this x-ray topography measurement. 3. Main Results and Discussion A. Discussion of Previous Methods to Determine the Growth Sector of an Unknown KDP Crystal
There exist several methods that are traditionally used to determine the growth sectors of KDP crystals. Depending on the concentration and on the nature of impurities that fix in both growth sectors, the boundary between them can be clearly visible, which makes growth-sector identification very easy [17]. But in general, the growth boundary cannot be identified by eye. The simplest and most direct idea to determine the growth sector, from which a KDP plate originates is a geometrical method. This method consists in considering that the growth boundary is plane-shaped as schematically described in Fig. 1. But, as illustrated in Fig. 3 in [17], where chromium impurities were added to make the growth sector visible, the shape of the growth-sector boundary clearly depends on the crystal growth conditions, and is in general not plane-shaped, which makes the geometrical method very unreliable. X-ray topography [16] is very useful to image the structural defects in crystals, but the resulting topography highlights at the same time dislocations, the growth-sector boundary, and growth bands. Moreover, even if the growth-sector boundary can be identified, x-ray topography does not determine the nature of the growth sectors on both sides of the boundary without any further information on the KDP plate to analyze. It also requires special equipment and KDP plates that are polished on both sides. UV absorption measurements may, in some cases, be a simple and rapid means of discriminating the growth sectors (see Fig. 2 in [6] or Figs. 2 and 3 in [7]). It was, however, shown that pyramidal and prismatic sectors could lead to very similar UV absorption levels if the crystal was grown with pure enough starting materials (see Fig. 43 and text on
Fig. 4. Transmittance spectra of both pure crystals cut in the pyramidal and the prismatic sectors (full lines) compared to data reported by Fujioka et al. [6] from which some data points were extracted (dotted lines). Our transmission spectra for the pyramidal and the prismatic sectors are very similar to the transmission spectrum obtained by Fujioka et al. for a conventionally grown (thus pyramidal) KDP. Transmission spectra are thus not sufficient to state on the growth sector of a KDP crystal.
p. 72 in [1]). In our case, the transmission spectra of both “pure” crystals, extracted well inside one growth sector of the boule, were recorded and are compared to the literature in Fig. 4. It is evident that even in the UV, pyramidal and prismatic crystals have transmission levels close to each other. In particular, the transmission of the prismatic growth sector in our crystals is very similar to the transmission obtained by Fujioka et al. [6] for their conventionally grown pyramidal crystal. Indeed, absorption values depend a lot on the quality of the crystal, which has been largely improved in recent years. Further, the absorption values were shown to vary significantly within one growth sector (see Figs. 3 and 5(a) in [10]) or according to the quantity of laser radiation seen by the sample (laser conditioning) (see Fig. 5(b) in [10]). These examples provide evidence that an absorptionbased method is not sufficient to state on the growth sector, from which an unknown crystal has been extracted. Moreover, the method requires KDP plates that are polished on both sides. Very much like absorption measurements, the intensity of defect-induced photoluminescence (PL) may sometimes be used to identify the growth sector of a KDP crystal. We, however, observed strong changes in PL intensity depending on differences in the irradiation history and/or the thermal history of the samples (Fig. 5). Thus, PL intensity variations can only provide relevant information if all samples were grown, irradiated, and thermally treated in the same conditions. Finally, extensive laser-damage studies have been carried out over the past few decades, highlighting that the prismatic growth sector is not systematically the weakest one, although impurities fix preferentially in this sector [6]. Indeed, a possible explanation is that the laser-induced damage threshold depends 10 May 2014 / Vol. 53, No. 14 / APPLIED OPTICS
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Fig. 5. Photoluminescence spectra obtained in prismatic KDP (left) and in pyramidal KDP (right) sectors. In both graphs the two curves represent the most intense and the least intense PL spectra obtained, depending on the UV irradiation history and the thermal history of the samples. All curves show, however, a systematic difference around 3.8 eV, i.e., the presence of a large peak for prismatic KDP and the absence of the peak for pyramidal plates.
much more on the growth rate than on the growth sector considered [11,12]. Moreover, this technique is destructive, which prevents any further use of the KDP plate tested. In conclusion, none of the above-mentioned techniques is suitable to rapidly identify the growth sector of a KDP crystal with unknown thermal and irradiation history. In the following (Section 3.B), we present a new criterion deduced from our photoluminescence measurements and based on the shape of the spectra. We show that this criterion is robust with respect to different thermal and irradiation histories. Moreover, applying this criterion to a KDP plate containing both growth sectors, we demonstrate that it is possible to determine the shape and the position of the growth-sector boundary (Section 3.C).
the photoluminescence signal in the case of the prismatic sector systematically shows a large peak or shoulder centered at approximately 3.8 eV. Moreover, in the case of the pyramidal sector this large peak is absent in all spectra. We have observed this characteristic difference on many spectra corresponding to many different thermal and radiative crystal histories, but only the lowest and the highest intensity spectra are shown in the graphs for clarity. This criterion (presence or absence of the 3.8 eV peak) remained valid in crystals cut from two different boules and no matter the previous UV irradiation dose or thermal annealing received by the crystal, which indicates that the presence or absence of the 3.8 eV peak is characteristic to the growth sector rather than dependent on sample history. Nevertheless, the study of photoluminescence spectra should be extended to other KDP boules from different facilities. C. Localization of the Sector Boundary in the “Mixed” Crystal
In this section we show how the photoluminescence criterion can be applied to the three-dimensional (3D) reconstruction of the growth-sector boundary. According to the x-ray image (Fig. 3), the sector boundary may be positioned in the upper region of the crystal. Thus, two resolved photoluminescence measurements have been performed, one at the top right corner (X 95, Y 95), above the probable boundary position, and one at the lower right corner of the crystal (X 95, Y 05), far below the
B. Study of Photoluminescence Spectra: Signature of the Prismatic and Pyramidal Sectors
Both monosectorial crystals were tested at 266 nm in the large-beam configuration, i.e., with a 6 mm laser beam diameter, to obtain photoluminescence spectra with good signal-to-noise ratio. Figure 5 shows different typical photoluminescence spectra of the prismatic and pyramidal samples, respectively. Both spectra presented in each graph were measured on the same sample using constant pumping and acquisition parameters. In each graph, the strong differences observed in the signal levels and shapes are due to different histories of the samples in terms of laser pre-irradiation and thermal annealing, and to the recording of photoluminescence spectra at different positions of the sample. As both graphs are drawn with the same x and y scales, one can easily see that the observed photoluminescence intensity at any wavelength is a priori much more dependent on the history of the sample than on the growth sector. Thus, the photoluminescence intensity cannot be a sufficient criterion to discriminate the growth sector of any tested sample. However, considering the shapes of the photoluminescence spectra, one can notice that 3066
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Fig. 6. Photoluminescence spectra obtained in two extreme locations of the “mixed” KDP plate. All coordinates are in millimeters. According to the criterion presented in Section 3.B., the tested sites correspond to two different pure growth sectors, assuring that the plate contains a growth-sector boundary. The evolution of the spectra in each graph is caused by the pumping at 266 nm (2 mJ∕cm2 , 8 ns).
Fig. 7. Growth-sector boundary localization (a) based on the shape criterion of the photoluminescence spectra presented in Section 3.B and applied to the spectra recorded at each site depicted by a solid marker. From this 2D representation and from the visible differences observed on the front and back sides of the plate, we propose a possible 3D representation of the sector boundary (b).
probable boundary position. The results are presented in Fig. 6. The spectra obtained for the (95 mm, 95 mm) site are very similar to those obtained in the case of a pyramidal crystal (Fig. 5 right). Moreover, the spectra obtained at the lower corner of the mixed KDP crystal are very similar to the spectra obtained for the pure prismatic KDP crystal (Fig. 5 left). This result confirms that the crystal contains both growth sectors (pyramidal and prismatic), and, supposing that the boundary follows the line highlighted in the x-ray image, it additionally gives the information that the crystal mostly contains material from the prismatic sector. However, if a tested site is not purely pyramidal or prismatic, the resulting spectrum is a linear combination of both spectra. The relative composition of the material in the detection volume can then be extracted if spectra of pure pyramidal and pure prismatic volumes are known. For more detailed information, a precise scan around the assumed growth-sector boundary position was made in the focused configuration, and on both sides of the crystal. The spectra have been examined following the 3.8 eV peak criterion, and the results are shown in Fig. 7(a). In the figure, all red solid markers show the pure prismatic sector, the green solid markers are for the pyramidal growth sector, and the yellow ones represent the sites where the PL spectra correspond to a mix of pyramidal PL signal and prismatic PL signal. One can notice that the results obtained for the front side and for the back side are not identical. This means that the probed volume responsible for the analyzed photoluminescence spectra is located close to the laser entrance side. This local nature of the photoluminescence signal may be enhanced in the future by using a confocal setup if a sufficiently sensitive detector is used. In principle, the proposed photoluminescence analysis is thus suited to give detailed 3D information on the sector boundary. In our case, the front- and backside measurements can be combined to give already a rough 3D picture of the sector boundary in this crystal [Fig. 7(b)]. Example of the progressive change of photoluminescence spectra from the pyramidal spectrum to the prismatic one is given in Fig. 8 for several sites
Fig. 8. Photoluminescence spectra obtained at the position X 91 mm for different Y positions, highlighting three groups of spectra: one for pure pyramidal material (Y from 90 to 94 mm), one for pure prismatic material (Y from 72 to 82 mm), and one group where the tested volume is composed at 40% of the prismatic material and at 60% of the pyramidal one (Y ranging from 84 to 88 mm).
located at X 91 mm and measured on the front side. Comparing the photoluminescence signals for different Y positions to the spectra of the pure growth sectors and their linear combinations, one can identify three groups: for 72 mm ≤ Y ≤ 82 mm, a large shoulder around 3.8 eV is clearly visible. For these positions the detection volume thus contains 100% of prismatic material. For Y positions ranging from 90 to 94 mm the photoluminescence signal corresponds to purely pyramidal material. For the intermediate positions (84 mm ≤ Y ≤ 88 mm) the signal looks like the spectrum obtained by mixing 40% of the prismatic signal and 60% of the pyramidal signal. 4. Conclusion
We recorded photoluminescence spectra of rapidly grown KDP samples extracted from pyramidal and prismatic growth sectors from two boules. For a pump wavelength of 266 nm, we observed strong differences in the signal levels and shapes that are due to different histories of the samples in terms of laser preirradiation and thermal annealing, and to the recording of photoluminescence spectra at different positions of the samples. Nevertheless a 10 May 2014 / Vol. 53, No. 14 / APPLIED OPTICS
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robust criterion for the identification of the growth sectors could be deduced from these measurements: a broad photoluminescence peak at 3.8 eV is present on all spectra of prismatic crystals and absent on all spectra of pyramidal crystals. This criterion allows us to discriminate between pyramidal and prismatic growth sectors, in situations in which simple absorption measurements, for example, are not sufficient. Moreover, in small-beam configurations, spatially resolved photoluminescence spectra enabled us to precisely determine the position of the growth-sector boundary within an unknown KDP crystal. Confocal photoluminescence detection would enable 3D reconstruction of the growth-sector boundary. References 1. N. Zaitseva and L. Carman, “Rapid growth of KDP-type crystals,” Prog. Cryst. Growth Charact. Mater. 43, 1–118 (2001). 2. R. Hawley-Fedder, H. Robey, T. Biesiada, M. DeHaven, R. Floyd, and A. Burnham, “Rapid growth of very large KDP and KD*P crystals in support of the National Ignition Facility,” Proc. SPIE 4102, 152–161 (2000). 3. M. L. Andre, “The French megajoule laser project (LMJ),” Fusion Eng. Des. 44, 43–49 (1999). 4. N. Fleurot, C. Cavailler, and J. L. Bourgade, “The laser megajoule (LMJ) project dedicated to inertial confinement fusion: development and construction status,” Fusion Eng. Des. 74, 147–154 (2005). 5. T. Sasaki and A. Yokotani, “Growth Of large KDP crystals for laser fusion experiments,” J. Cryst. Growth 99, 820–826 (1990). 6. K. Fujioka, S. Matsuo, T. Kanabe, H. Fujita, and M. Nakatsuka, “Optical properties of rapidly grown KDP crystal improved by thermal conditioning,” J. Cryst. Growth. 181, 265–271 (1997).
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7. L. Ye, Z. Li, G. Su, X. Zhuang, and G. Zheng, “Study on the optical properties of rapidly grown KDP crystals,” Opt. Commun. 275, 399–403 (2007). 8. P. Hartman, “The morphology of zircon and potassium dihydrogen phosphate in relation to the crystal structure,” Acta Crystallogr. 9, 721–727 (1956). 9. S. G. Demos, M. C. Staggs, M. Yan, H. B. Radousky, and J. J. De Yoreo, “Observation of photoexcited emission clusters in the bulk of KDP and laser conditioning under 355-nm irradiation,” Proc. SPIE 3578, 509–515 (1999). 10. M. Pommiès, D. Damiani, B. Bertussi, J. Capoulade, H. Piombini, J. Y. Natoli, and H. Mathis, “Detection and characterization of absorption heterogeneities in KH2PO4 crystals,” Opt. Commun. 267, 154–161 (2006). 11. R. A. Negres, N. P. Zaitseva, P. DeMange, and S. G. Demos, “Expedited laser damage profiling of KDxH2-xPO4 with respect to crystal growth parameters,” Opt. Lett. 31, 3110–3112 (2006). 12. M. Yan, R. A. Torres, M. J. Runkel, B. W. Woods, I. D. Hutcheon, N. P. Zaitseva, and J. J. De Yoreo, “Investigation of impurities and laser-induced damage in the growth sectors of rapidly grown KDP crystals,” Proc. SPIE 2966, 11–16 (1997). 13. A. Ciapponi, S. Palmier, F. Wagner, J. Y. Natoli, H. Piombini, D. Damiani, and B. Bertussi, “Laser induced fluorescence as a tool for the study of laser damage precursors in transparent materials,” Proc. SPIE 6998, 69981E (2008). 14. S. G. Demos, M. Staggs, M. Yan, H. B. Radousky, and J. J. De Yoreo, “Investigation of optically active defect clusters in KH2PO4 under laser photoexcitation,” J. Appl. Phys. 85, 3988–3992 (1999). 15. A. Ciapponi, F. Wagner, J. Y. Natoli, B. Bertussi, and F. Guillet, “Photoluminescence and photothermal deflection measurements in KDP crystals for high power applications,” Proc. SPIE 7504, 750417 (2009). 16. A. Surmin, F. Guillet, S. Lambert, D. Damiani, and M. Pommies, “Structural study of large scale KDP crystals using high energy X-ray diffraction,” Proc. SPIE 5991, 59911W (2005). 17. N. Zaitseva, L. Carman, I. Smolsky, R. Torres, and M. Yan, “The effect of impurities and supersaturation on the rapid growth of KDP crystals,” J. Cryst. Growth 204, 512–524 (1999).
Aix Marseille Université, CNRS, Centrale Marseille, Institut Fresnel UMR 7249, 13397 Marseille, France 2
CEA, DAM, LE RIPAULT, F-37260 Monts, France
*Corresponding author: [email protected] Received 2 January 2014; revised 9 March 2014; accepted 23 March 2014; posted 9 April 2014 (Doc. ID 203111); published 8 May 2014
In this work, rapidly grown KH2 PO4 (KDP) crystals extracted from the prismatic and the pyramidal growth sectors of crystal boules were analyzed using photoluminescence measurements. From the spectra, we deduced a robust criterion to discriminate between both growth sectors in an unknown KDP plate. Moreover, spatially resolved photoluminescence was shown to enable a local probing of different planes in the bulk of the material leading to accurate and nondestructive three-dimensional mapping of the sector boundary, which is often the weakest point in terms of laser-damage resistance in rapidly grown KDP crystals. © 2014 Optical Society of America OCIS codes: (160.4330) Nonlinear optical materials; (160.3380) Laser materials; (260.3800) Luminescence; (300.2530) Fluorescence, laser-induced. http://dx.doi.org/10.1364/AO.53.003063
1. Introduction
Rapid growth of KH2 PO4 (KDP) crystals was developed during the last 30 years [1] in order to provide large-sized high-quality frequency conversion crystals for inertial confinement fusion class lasers such as the NIF in the USA [2], the Laser MegaJoule in France [3,4], or Gekko XII in Japan [5]. However, contrary to conventional growth, rapid growth generates in the same boule two different sectors, named “pyramidal” and “prismatic” sectors. As illustrated schematically in Fig. 1, these two sectors are generated by the growth of crystal seed faces according to two crystalline directions: [100] growth direction for the prismatic sector, and [101] for the pyramidal sector [1].
1559-128X/14/143063-06$15.00/0 © 2014 Optical Society of America
In a KDP boule, the pyramidal sector is the purest zone of the crystal (as shown by ICP-AES measurements; see Table 2 in [6]), whereas the prismatic sector contains a higher density of impurities (see Table 1 in [1] and Table 2 in [7]) in the majority of metal ions. Indeed, impurities are incorporated much more easily in the prismatic sector than in the pyramidal sector because the attachment energy is lower on the f100g faces compared to the f101g faces [8]. Consequently, both growth sectors were shown to have different optical properties in terms of absorption [6,7], photoluminescence signals (see Fig. 3 in [9]), and laser-damage resistance [10]. In particular, the growth-sector boundary was evidenced to be the weak point of the crystal in terms of resistance to laser damage [11,12]. When comparing different tests on several samples, it is mandatory to know the origin of each sample in order to take into account the potential influences of the growth sector 10 May 2014 / Vol. 53, No. 14 / APPLIED OPTICS
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Fig. 1. Schematic cross section of a rapidly grown KDP crystal showing the growth sectors.
on the laser-damage resistance. Moreover, the large dimensions of the plates needed in inertial confinement fusion facilities require plates in which the growth-sector boundary may be present on part of the crystal. Even though an adapted laser conditioning procedure can allow both growth sectors to have very similar LIDT values, better knowledge of the position and shape of the boundary should allow reinforcing the laser conditioning in the growthsector boundary area. Considering the impact that the growth sector may have on the different physical properties, we present in this paper a robust criterion to nondestructively and locally determine from which growth sector a sample originates, and, in case the sample was cut across a sector boundary, to precisely determine the shape and the position of the boundary.
translation stage, which allows choosing the crystal site to be tested. The photoluminescence measurement module is made up of a collection lens that focuses the emitted light on the entry side of an optical fiber assembly and a notch filter (532 and 266 nm) that blocks diffused pump light. The optical fiber assembly then guides the photoluminescence signal to the spectrometer, which comprises a monochromator and an intensified CCD camera. All presented spectra are corrected using the wavelength-dependent sensitivity of the photoluminescence measurement setup (lens Lc to ICCD), which was obtained using a calibrated halogen/ deuterium lamp. Even if in high-power lasers, KDP crystals are used to convert light from 1053 nm to only 351 nm, we chose to pump at 266 nm for several reasons. First of all, the photoluminescence yield is higher at 266 nm than at 355 nm, even though the pump photon energy of 4.66 eV is still much less than the band gap of KDP (7.7 eV [14]). We thus detect defectinduced photoluminescence, and it has been demonstrated in [15] that the photoluminescence spectra reveal the same defects whatever the chosen pump wavelength (266 or 355 nm). Nevertheless, even when pumping at 266 nm, the signal is very low. In order to optimize the signal-tonoise ratio, we use an intensified CCD camera, and we pump a relatively large volume of tested KDP crystal.
2. Experimental Aspects
B. Testing Modes
A.
The measurements were realized in two different configurations:
Experimental Setup
The setup is schematically depicted in Fig. 2 [13]. The sample is pumped by a wavelength of 266 nm, provided by forth harmonic generation of a pulsed Nd:YAG laser. The pulse duration of the longitudinal multimode laser is 8 ns, and the pulse repetition rate is 100 Hz. The sample is mounted in a motorized X–Y
– The “large-beam configuration” using a nonfocused laser beam (6 mm beam diameter). – The “small-beam configuration” using a slightly focused laser beam by adding a lens in front of the sample (L2 in Fig. 2) in order to limit the pumped volume to a cylinder of approximately 2 mm in diameter. In this second configuration, the part of the crystal that produces the collected photoluminescence signal is not the whole thickness of the material but a smaller volume, close to the entrance side of the laser beam (see inset in Fig. 2). C.
Fig. 2. Sketch of the experimental setup including the photoluminescence measurement system. The pump source is a frequency quadrupled Nd:YAG laser (100 Hz, 8 ns). The inset presents the details of the light collection system and geometry. 3064
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Tested Samples
All measurements shown here were realized on three different KDP samples extracted from the same crystal boule. Two of them can be considered as “pure” samples, whose dimensions are 50 mm × 50 mm × 10 mm. The first crystal was extracted from the pyramidal sector of the crystal boule, and the second was extracted from the prismatic sector of the same crystal boule. The third sample, whose dimensions are 100 mm × 100 mm × 10 mm, is “mixed”; i.e., it includes material from both growth sectors. The x-ray topograph
Fig. 3. X-ray images of the mixed crystal, highlighting different lines (left) that can be interpreted as growth bands and a possible growth-sector boundary (right). The dashed outlines represent the crystal sides (real dimensions 100 mm × 100 mm).
[16] of this third sample is depicted in Fig. 3. The white line in the image to the right indicates where one may assume the location of the growth-sector boundary as deduced from this x-ray topography measurement. 3. Main Results and Discussion A. Discussion of Previous Methods to Determine the Growth Sector of an Unknown KDP Crystal
There exist several methods that are traditionally used to determine the growth sectors of KDP crystals. Depending on the concentration and on the nature of impurities that fix in both growth sectors, the boundary between them can be clearly visible, which makes growth-sector identification very easy [17]. But in general, the growth boundary cannot be identified by eye. The simplest and most direct idea to determine the growth sector, from which a KDP plate originates is a geometrical method. This method consists in considering that the growth boundary is plane-shaped as schematically described in Fig. 1. But, as illustrated in Fig. 3 in [17], where chromium impurities were added to make the growth sector visible, the shape of the growth-sector boundary clearly depends on the crystal growth conditions, and is in general not plane-shaped, which makes the geometrical method very unreliable. X-ray topography [16] is very useful to image the structural defects in crystals, but the resulting topography highlights at the same time dislocations, the growth-sector boundary, and growth bands. Moreover, even if the growth-sector boundary can be identified, x-ray topography does not determine the nature of the growth sectors on both sides of the boundary without any further information on the KDP plate to analyze. It also requires special equipment and KDP plates that are polished on both sides. UV absorption measurements may, in some cases, be a simple and rapid means of discriminating the growth sectors (see Fig. 2 in [6] or Figs. 2 and 3 in [7]). It was, however, shown that pyramidal and prismatic sectors could lead to very similar UV absorption levels if the crystal was grown with pure enough starting materials (see Fig. 43 and text on
Fig. 4. Transmittance spectra of both pure crystals cut in the pyramidal and the prismatic sectors (full lines) compared to data reported by Fujioka et al. [6] from which some data points were extracted (dotted lines). Our transmission spectra for the pyramidal and the prismatic sectors are very similar to the transmission spectrum obtained by Fujioka et al. for a conventionally grown (thus pyramidal) KDP. Transmission spectra are thus not sufficient to state on the growth sector of a KDP crystal.
p. 72 in [1]). In our case, the transmission spectra of both “pure” crystals, extracted well inside one growth sector of the boule, were recorded and are compared to the literature in Fig. 4. It is evident that even in the UV, pyramidal and prismatic crystals have transmission levels close to each other. In particular, the transmission of the prismatic growth sector in our crystals is very similar to the transmission obtained by Fujioka et al. [6] for their conventionally grown pyramidal crystal. Indeed, absorption values depend a lot on the quality of the crystal, which has been largely improved in recent years. Further, the absorption values were shown to vary significantly within one growth sector (see Figs. 3 and 5(a) in [10]) or according to the quantity of laser radiation seen by the sample (laser conditioning) (see Fig. 5(b) in [10]). These examples provide evidence that an absorptionbased method is not sufficient to state on the growth sector, from which an unknown crystal has been extracted. Moreover, the method requires KDP plates that are polished on both sides. Very much like absorption measurements, the intensity of defect-induced photoluminescence (PL) may sometimes be used to identify the growth sector of a KDP crystal. We, however, observed strong changes in PL intensity depending on differences in the irradiation history and/or the thermal history of the samples (Fig. 5). Thus, PL intensity variations can only provide relevant information if all samples were grown, irradiated, and thermally treated in the same conditions. Finally, extensive laser-damage studies have been carried out over the past few decades, highlighting that the prismatic growth sector is not systematically the weakest one, although impurities fix preferentially in this sector [6]. Indeed, a possible explanation is that the laser-induced damage threshold depends 10 May 2014 / Vol. 53, No. 14 / APPLIED OPTICS
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Fig. 5. Photoluminescence spectra obtained in prismatic KDP (left) and in pyramidal KDP (right) sectors. In both graphs the two curves represent the most intense and the least intense PL spectra obtained, depending on the UV irradiation history and the thermal history of the samples. All curves show, however, a systematic difference around 3.8 eV, i.e., the presence of a large peak for prismatic KDP and the absence of the peak for pyramidal plates.
much more on the growth rate than on the growth sector considered [11,12]. Moreover, this technique is destructive, which prevents any further use of the KDP plate tested. In conclusion, none of the above-mentioned techniques is suitable to rapidly identify the growth sector of a KDP crystal with unknown thermal and irradiation history. In the following (Section 3.B), we present a new criterion deduced from our photoluminescence measurements and based on the shape of the spectra. We show that this criterion is robust with respect to different thermal and irradiation histories. Moreover, applying this criterion to a KDP plate containing both growth sectors, we demonstrate that it is possible to determine the shape and the position of the growth-sector boundary (Section 3.C).
the photoluminescence signal in the case of the prismatic sector systematically shows a large peak or shoulder centered at approximately 3.8 eV. Moreover, in the case of the pyramidal sector this large peak is absent in all spectra. We have observed this characteristic difference on many spectra corresponding to many different thermal and radiative crystal histories, but only the lowest and the highest intensity spectra are shown in the graphs for clarity. This criterion (presence or absence of the 3.8 eV peak) remained valid in crystals cut from two different boules and no matter the previous UV irradiation dose or thermal annealing received by the crystal, which indicates that the presence or absence of the 3.8 eV peak is characteristic to the growth sector rather than dependent on sample history. Nevertheless, the study of photoluminescence spectra should be extended to other KDP boules from different facilities. C. Localization of the Sector Boundary in the “Mixed” Crystal
In this section we show how the photoluminescence criterion can be applied to the three-dimensional (3D) reconstruction of the growth-sector boundary. According to the x-ray image (Fig. 3), the sector boundary may be positioned in the upper region of the crystal. Thus, two resolved photoluminescence measurements have been performed, one at the top right corner (X 95, Y 95), above the probable boundary position, and one at the lower right corner of the crystal (X 95, Y 05), far below the
B. Study of Photoluminescence Spectra: Signature of the Prismatic and Pyramidal Sectors
Both monosectorial crystals were tested at 266 nm in the large-beam configuration, i.e., with a 6 mm laser beam diameter, to obtain photoluminescence spectra with good signal-to-noise ratio. Figure 5 shows different typical photoluminescence spectra of the prismatic and pyramidal samples, respectively. Both spectra presented in each graph were measured on the same sample using constant pumping and acquisition parameters. In each graph, the strong differences observed in the signal levels and shapes are due to different histories of the samples in terms of laser pre-irradiation and thermal annealing, and to the recording of photoluminescence spectra at different positions of the sample. As both graphs are drawn with the same x and y scales, one can easily see that the observed photoluminescence intensity at any wavelength is a priori much more dependent on the history of the sample than on the growth sector. Thus, the photoluminescence intensity cannot be a sufficient criterion to discriminate the growth sector of any tested sample. However, considering the shapes of the photoluminescence spectra, one can notice that 3066
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Fig. 6. Photoluminescence spectra obtained in two extreme locations of the “mixed” KDP plate. All coordinates are in millimeters. According to the criterion presented in Section 3.B., the tested sites correspond to two different pure growth sectors, assuring that the plate contains a growth-sector boundary. The evolution of the spectra in each graph is caused by the pumping at 266 nm (2 mJ∕cm2 , 8 ns).
Fig. 7. Growth-sector boundary localization (a) based on the shape criterion of the photoluminescence spectra presented in Section 3.B and applied to the spectra recorded at each site depicted by a solid marker. From this 2D representation and from the visible differences observed on the front and back sides of the plate, we propose a possible 3D representation of the sector boundary (b).
probable boundary position. The results are presented in Fig. 6. The spectra obtained for the (95 mm, 95 mm) site are very similar to those obtained in the case of a pyramidal crystal (Fig. 5 right). Moreover, the spectra obtained at the lower corner of the mixed KDP crystal are very similar to the spectra obtained for the pure prismatic KDP crystal (Fig. 5 left). This result confirms that the crystal contains both growth sectors (pyramidal and prismatic), and, supposing that the boundary follows the line highlighted in the x-ray image, it additionally gives the information that the crystal mostly contains material from the prismatic sector. However, if a tested site is not purely pyramidal or prismatic, the resulting spectrum is a linear combination of both spectra. The relative composition of the material in the detection volume can then be extracted if spectra of pure pyramidal and pure prismatic volumes are known. For more detailed information, a precise scan around the assumed growth-sector boundary position was made in the focused configuration, and on both sides of the crystal. The spectra have been examined following the 3.8 eV peak criterion, and the results are shown in Fig. 7(a). In the figure, all red solid markers show the pure prismatic sector, the green solid markers are for the pyramidal growth sector, and the yellow ones represent the sites where the PL spectra correspond to a mix of pyramidal PL signal and prismatic PL signal. One can notice that the results obtained for the front side and for the back side are not identical. This means that the probed volume responsible for the analyzed photoluminescence spectra is located close to the laser entrance side. This local nature of the photoluminescence signal may be enhanced in the future by using a confocal setup if a sufficiently sensitive detector is used. In principle, the proposed photoluminescence analysis is thus suited to give detailed 3D information on the sector boundary. In our case, the front- and backside measurements can be combined to give already a rough 3D picture of the sector boundary in this crystal [Fig. 7(b)]. Example of the progressive change of photoluminescence spectra from the pyramidal spectrum to the prismatic one is given in Fig. 8 for several sites
Fig. 8. Photoluminescence spectra obtained at the position X 91 mm for different Y positions, highlighting three groups of spectra: one for pure pyramidal material (Y from 90 to 94 mm), one for pure prismatic material (Y from 72 to 82 mm), and one group where the tested volume is composed at 40% of the prismatic material and at 60% of the pyramidal one (Y ranging from 84 to 88 mm).
located at X 91 mm and measured on the front side. Comparing the photoluminescence signals for different Y positions to the spectra of the pure growth sectors and their linear combinations, one can identify three groups: for 72 mm ≤ Y ≤ 82 mm, a large shoulder around 3.8 eV is clearly visible. For these positions the detection volume thus contains 100% of prismatic material. For Y positions ranging from 90 to 94 mm the photoluminescence signal corresponds to purely pyramidal material. For the intermediate positions (84 mm ≤ Y ≤ 88 mm) the signal looks like the spectrum obtained by mixing 40% of the prismatic signal and 60% of the pyramidal signal. 4. Conclusion
We recorded photoluminescence spectra of rapidly grown KDP samples extracted from pyramidal and prismatic growth sectors from two boules. For a pump wavelength of 266 nm, we observed strong differences in the signal levels and shapes that are due to different histories of the samples in terms of laser preirradiation and thermal annealing, and to the recording of photoluminescence spectra at different positions of the samples. Nevertheless a 10 May 2014 / Vol. 53, No. 14 / APPLIED OPTICS
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robust criterion for the identification of the growth sectors could be deduced from these measurements: a broad photoluminescence peak at 3.8 eV is present on all spectra of prismatic crystals and absent on all spectra of pyramidal crystals. This criterion allows us to discriminate between pyramidal and prismatic growth sectors, in situations in which simple absorption measurements, for example, are not sufficient. Moreover, in small-beam configurations, spatially resolved photoluminescence spectra enabled us to precisely determine the position of the growth-sector boundary within an unknown KDP crystal. Confocal photoluminescence detection would enable 3D reconstruction of the growth-sector boundary. References 1. N. Zaitseva and L. Carman, “Rapid growth of KDP-type crystals,” Prog. Cryst. Growth Charact. Mater. 43, 1–118 (2001). 2. R. Hawley-Fedder, H. Robey, T. Biesiada, M. DeHaven, R. Floyd, and A. Burnham, “Rapid growth of very large KDP and KD*P crystals in support of the National Ignition Facility,” Proc. SPIE 4102, 152–161 (2000). 3. M. L. Andre, “The French megajoule laser project (LMJ),” Fusion Eng. Des. 44, 43–49 (1999). 4. N. Fleurot, C. Cavailler, and J. L. Bourgade, “The laser megajoule (LMJ) project dedicated to inertial confinement fusion: development and construction status,” Fusion Eng. Des. 74, 147–154 (2005). 5. T. Sasaki and A. Yokotani, “Growth Of large KDP crystals for laser fusion experiments,” J. Cryst. Growth 99, 820–826 (1990). 6. K. Fujioka, S. Matsuo, T. Kanabe, H. Fujita, and M. Nakatsuka, “Optical properties of rapidly grown KDP crystal improved by thermal conditioning,” J. Cryst. Growth. 181, 265–271 (1997).
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7. L. Ye, Z. Li, G. Su, X. Zhuang, and G. Zheng, “Study on the optical properties of rapidly grown KDP crystals,” Opt. Commun. 275, 399–403 (2007). 8. P. Hartman, “The morphology of zircon and potassium dihydrogen phosphate in relation to the crystal structure,” Acta Crystallogr. 9, 721–727 (1956). 9. S. G. Demos, M. C. Staggs, M. Yan, H. B. Radousky, and J. J. De Yoreo, “Observation of photoexcited emission clusters in the bulk of KDP and laser conditioning under 355-nm irradiation,” Proc. SPIE 3578, 509–515 (1999). 10. M. Pommiès, D. Damiani, B. Bertussi, J. Capoulade, H. Piombini, J. Y. Natoli, and H. Mathis, “Detection and characterization of absorption heterogeneities in KH2PO4 crystals,” Opt. Commun. 267, 154–161 (2006). 11. R. A. Negres, N. P. Zaitseva, P. DeMange, and S. G. Demos, “Expedited laser damage profiling of KDxH2-xPO4 with respect to crystal growth parameters,” Opt. Lett. 31, 3110–3112 (2006). 12. M. Yan, R. A. Torres, M. J. Runkel, B. W. Woods, I. D. Hutcheon, N. P. Zaitseva, and J. J. De Yoreo, “Investigation of impurities and laser-induced damage in the growth sectors of rapidly grown KDP crystals,” Proc. SPIE 2966, 11–16 (1997). 13. A. Ciapponi, S. Palmier, F. Wagner, J. Y. Natoli, H. Piombini, D. Damiani, and B. Bertussi, “Laser induced fluorescence as a tool for the study of laser damage precursors in transparent materials,” Proc. SPIE 6998, 69981E (2008). 14. S. G. Demos, M. Staggs, M. Yan, H. B. Radousky, and J. J. De Yoreo, “Investigation of optically active defect clusters in KH2PO4 under laser photoexcitation,” J. Appl. Phys. 85, 3988–3992 (1999). 15. A. Ciapponi, F. Wagner, J. Y. Natoli, B. Bertussi, and F. Guillet, “Photoluminescence and photothermal deflection measurements in KDP crystals for high power applications,” Proc. SPIE 7504, 750417 (2009). 16. A. Surmin, F. Guillet, S. Lambert, D. Damiani, and M. Pommies, “Structural study of large scale KDP crystals using high energy X-ray diffraction,” Proc. SPIE 5991, 59911W (2005). 17. N. Zaitseva, L. Carman, I. Smolsky, R. Torres, and M. Yan, “The effect of impurities and supersaturation on the rapid growth of KDP crystals,” J. Cryst. Growth 204, 512–524 (1999).
