CAOL *2010 International Conference on Advanced Optoelectronics & Lasers, 10-14 September, 2010, Sevastopol, Ukraine

Nonlinear LBO and BBO crystals for extreme light sources

lInstitute of Geology and Mineralogy SB RAS, 3, Koptyug ave., 630090, Novosibirsk, Russia a.e.kokh@gmail.com

2CRISTAL LASER, Nancy, France

Abstract: Using the approach of heat filed symmetry change and

rotation LBO crystals of more than 1.5 kg of weight were grown

from MoOrcontaining melt-solution. High quality optical elements

up to 65 mm in diameter were produced. At the same time with this

approach we obtained high quality BBO crystals about 400 g using

NaBaB03 flux. Elements with apertures up to 30x30 mm2 were

produced. The progress in the dimensions of grown crystals

provides the opportunity to use them in laser systems of multi

petawatt and exawatt power level.

Due to good operating performance LiB30s crystals (LBO) are widely used for frequency conversion in the visible and near UV regions. Compared to KDP, the LBO have more preferable operating characteristics in particular higher nonlinearity, angular tolerance and damage threshold as well as smaller absorption and larger thermal conductivity. But its applying in broad-aperture high power laser systems was limited due to difficulties to grow crystals with big sizes. According to the Curie principle, rhombic LBO crystals were grown in the heat field of the same symmetry by Kyropoulos method.

A 1

Fig. l. Sketch of the growth zone.

1 - insulation

2 - heaters

3 - crystal

4 - melt-solutior

5 - radial temperature distribution

Fig. 1 presents the sketch of the growth zone. Eight separate heaters around the crucible are responsible for a specific temperature distribution in the horizontal section. Electrical connection of the heaters is realized through power commutator which in turn provides switched on/off state of any heater for any time period. In this work a sequential switching of four from eight heaters was used: 1234->2345->3456 . . . If the delay between switchings is high, an effective homogenization of the melt-solution may be achieved (Fig. 2a). Before the start of crystal growth the delay of switching is decreased to 3 sec, so the convection becomes similar to that if all heaters would be switched on at the same time (Fig. 2b). Crystal growth was performed from Li20-B20rMo03 melt­solution with molar ratio between oxides as 1: l.29: l. 71, correspondingly.

Fig. 2. Convective patterns on the surface of melt-solution; delay between switching of the heaters: (a) - 20 min, (b) -3 sec.

But since imperfections in heaters and insulation result in nonsymmetrical radial temperature distribution, this provides in fact a significant progress in the growth technology. By means of correction of switching delay for some group of heaters, it is possible to shift the cold point on the melt surface in the geometric center of the crucible which is highly important for large diameter crucibles. Grown crystals (Fig. 3) possesses high quality with optical absorption for most of the samples less than 5 ppm/cm at 1064 nm. Currently the maximum diameter of NLO element cut from the crystals is 65 mm [1].

978-1-4244-7046-411 0/$26.00 ©20 1 0 IEEE

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CAOL *2010 International Conference on Advanced Optoelectronics & Lasers, 10-14 September, 2010, Sevastopol, Ukraine

Fig. 3. As grown LBO crystal 1558g in weight.

In the optical experiments of Commissariat a l'Energie Atomique (Bordeaux, France), 217 J pulse of second harmonic wave (')..,=527 nm) was achieved with �92% efficiency [2].

Therefore, progress in LBO crystal growth and results of experiments on the transformation of laser radiation of high energy have risen the status and prospects of the crystal to a new level. Currently there are many debates of replacing the family of KDP crystals in high-power laser systems for more efficient LBO crystals, which will provide substantially higher levels of output power.

BBO crystals occupy a special niche in nonlinear optics. Despite the active search for new crystals for the near-UV range, BBO is still a widely-used commercial crystal. Its potential for powerful wide-aperture laser systems operating in the femtosecond regime remains very high. Due to the phase

transition at 925°C, P.BaB204 crystals are commonly grown from high temperature solution. In high-temperature solution methods solvent must provide extended primary crystallization

region of f3-BaB204 and reduce the viscosity and glass-forming ability of the melt.

Fig. 4. Surface of the melt-solution under L3 heat field.

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Also, it must be of low volatility, possess stable properties and provide very weak interaction with the crystal to exclude the

possibility of poisoning. Main solvents used for P.BaB204 crystal growth are compositions in BaO-B20rNa20 system; NaF; mixed oxide-fluoride compositions.

a)

. �

- �" " ' !� . .

. ". ,

Fig. 5. As grown BBO crystal 400 g in weight; side (a) and bottom (b) views.

Because of high melt viscosity, growth of inclusion free crystal is quite complicated. So reproducible production of crystals for large aperture elements is still the aim to achieve.

In our prior publications we have shown the efficiency of

f3-BaB204 (BBO) crystal growth in the heat field of three-fold axis symmetry by top-seed solution method (TSSG) [3, 4]. In such heat field liquid body splits into three convective cells, in which the thermal-gravitational flows moved up-and-down and formed three intensive flows at the surface merging in the centre (Fig. 4). This convective regime provides a permanent renewal of high temperature solution in the region near crystallization interface. In order to involve the whole body of

CAOL *2010 International Conference on Advanced Optoelectronics & Lasers, 10-14 September, 2010, Sevastopol, Ukraine

the high temperature solution in the convective motion it is very important to achieve vertical temperature distribution with the 'hot points' at the lower part of the growth crucible. At the same time temperature of the latter at the surface of high temperature solution must be always higher than crystallization temperature. In order to accomplish such temperature distribution two-zone heating furnace with three heating sectors in each zone has been developed. Load commutator included in thermoregulation system executes power distribution on each growth sector during the growth run.

The BaO-B20rNa20 system remains basic for growing BBO crystals. Using NaBaB03 compound as a solvent [5], we obtained high-quality crystals about 400 g of weight with the convex form of the crystallization front which is an evidence of the stability of the crystallization process (Fig. 5). Diameter of the crucible was 100 mm. Currently the maximum aperture of NLO element cut from the crystals is 30x30 mm

2 (Fig. 6).

Fig. 6. NLO element from BBO crystal, dimensions: 30x30xx6 mm

3

This work is supported by RFBR grant #09-02-12261-ofi_m in Russia and in France by OSEO Innovation, contract # A0610001L, by MIPI and by "Region de Lorraine", contract #A0610002L.

REFERENCES

[1] A. Kokh, N. Kononova, G. Mennerat et aI, " Growth of high quality large size LBO crystals for high energy second harmonic generation", J Crystal Growth, vol. 312, pp. 1774-1778, 2010.

[2] Dr. Gabrial Mennerat - private communications. [3] A.E. Kokh, N.G. Kononova, P.W. Mokruchnikov,

"An azimuthal pattern of heat field in �-BaB204 crystal growth" J Crystal Growth, vol. 216, pp. 359-362, 2000.

[4] A.E. Kokh, V.N. Popov, T.B. Bekker et aI, "Melt­solution BBO crystal growth under change of the heat field symmetry and its rotation" J Crystal Growth, vol. 275, pp. e669-e674, 2005.

[5] Fedorov, P.P., Kokh, A.E., Kononova, N.G., Bekker, T.B., "Investigation of phase equilibria and growth of BBO (�-BaB204) crystals in BaO-B203-Na20 ternary system" J Crystal Growth, vol. 310, pp. 1943-1949, 2008.

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