New non linear optical crystals for UV: beyond the limits for reliable Solid state laser systems. – UV-CHALLENGE

UV challenge experimental project is divided into 3 tasks
Task N°1 is related to crystal growth of YAB, CBF, derivative compounds and fabrication of frequency conversion elements. This task is done to set up a reliable elaboration process starting from the purification of the starting materials to the crystal growth process of the nonlinear materials. The main idea is to overcome the effect of impurities on the UV-transparency windows, particularly for YAB. Crystals like LBO, BBO, CLBO, or KABO will be grown or bought and used as references..
Task N°2 is devoted to Physical Properties of the nonlinear crystals. The aim of this task is to determine the physical parameters of the optical non-linear crystals. The understanding of the physical properties leads us to improve the crystals to increase the efficiency in the UV range. YAB and CBF crystals show currently altered performances as harmonic converters for UV partly due to a lack of understanding of the nature of impurities and their impact on physical and optical properties.
Task N°3 is mainly the characterization of nonlinear properties. This task will provide a complete analysis of the nonlinear optical performances of studied crystals. First of all, an original characterization bench will be developed to provide a full set of data of second harmonic generation parameters as a function of the fundamental wavelength. Secondly, the new nonlinear crystals produced in the project will be tested in operational conditions, namely with sources combining high peak power and high average power. A comparison with commercial crystals will be done. Thirdly, the effects limiting the nonlinear efficiency will extensively be studied. This includes damage threshold, photo-induced effects, effects of nonlinear index, effects of temperature increase induced by absorption and surface aging.

The obtained results nowadays can be summary as as follows:
1- Crystal Growth of several boules of YAB and CBF about 20 x 20 x 15 mm2 after optimization of the growth conditions.
- Extraction of several samples with 3 x 3 mm2 apertures cut in phase matching configuration for the frequency conversion in the UV.
2- Several chemical analysis measurements (ICP-AES) coupled with EXAFS studies of crystals YAB to show the presence of lanthanum in crystals grown with flux based lanthanum oxyde.
- GDMS analysis measurements associated with studies RPE, and optical absorption of YAB show the evidence of iron impurity (some ppm) in crystals.
- Measures of the thermal expansion coefficients of the YAB for show anisotropy responsible of cystal cracking during the growth
- Measurements of refractive indexes of the YAB by the method of the minimum deviation method and determination of Seillmeier equations. Phase matching angle calculations, for the GTH at 266 nm.
- SEM and Ombroscopy studies, to show up inclusions and spread defects in the crystals of YAB.
3-Measurement of frequency conversion efficiencies in the UV for YAB and CBF crystals:
3a- CBF crystals:
- Third harmonic generation at 343 nm - from a laser based on single-crystal fiber (YAG: Yb) delivering 20 W average power at 1030 nm (30 kHz, 15 ns) – allows to obtain with a CBF crystal cut in the plan XY a 300 mW average power corresponding to 1,5 % of 1030-343 nm conversion efficiency.
- Estimation of the actual not linear coefficients of CBF in
- Estimation of the effective nonlinear coefficients of CBF in both main plans YX and YZ:
3b-Cristaux cut by YAB:
- An energy of 113 µJ at 266 nm corresponding to 12,2 % conversion efficiency was obtained thanks to phase matched type I cut and polished YAB crystal with 15 x 15 x 2,94 mm3 dimensions.

The futur propects of the UV - CHALLENGE project are the following ones:
- do additional crystal growth of YAB and CBF crystals, as expected in the milestones, By an optimization of the growth conditions for developing crystals with very low(weak) iron impurity Fe3+ or reduce the iron to Fe2+in order to limit the UV residual absorption.
- decrease the number of twin area in the grown crystals of YAB, by an optimization of the conditions of growth
- Study of the effects limiting the conversion of frequency in the UV by:
1- measurements of the UV absorption of YAB crystal at 266 nm
2-measurement of UV frequency conversion using last grown YAB and CBF crystals. Measurements which should show better crystal transmission in the UV and thus of better performances.

Publications
1- S. Ilas, P. Loiseau, G. Aka, T. Taira, «240 kW peak power at 266 nm in nonlinear YAl3(BO3)4 single crystal,« Opt. Express 22, 30325 (2014)
2- L. Deyra, S. Ilas, X. Delen, P. Loiseau, F. Balembois, G. Aka, F. Salin, P. Georges, «Third harmonic generation at 343 nm in nonlinear Ca5(BO3)3F crystals,« Opt. Mater. Express 3, 1798 (2013).
3- L. Deyra, I. Martial, J. Didierjean, F. Balembois, and P. Georges, «3 W, 300 µJ, 25 ns pulsed 473 nm blue laser based on actively Q-switched Nd:YAG single-crystal fiber oscillator at 946 nm« Opt. Letters Vol. 38, No. 16, 3013 (2013)
4- L. Deyra, J. Didierjean, F. Balembois, and P. Georges «Deep-UV 236.5 nm laser by fourth-harmonic generation of a single-crystal fiber Nd:YAG oscillator« Opt. Letters Vol.39, No. 8, 2236 (2014).
International communications
1- D. Rytz, “Properties of YAB Crystals for Frequency Conversion to the UV”, Advanced Solid State Laser Conf. (ASSL), 27 Oct.-1 Nov., Paris, (France) Invited talk AM3A.1 (2013).
2- L. DEYRA, S. Ilas, X. Délen, P. Loiseau, F. Balembois, G. Aka, F. Salin, P. Georges, “Nonlinear Properties of Non-hygroscopic Ca5(BO3)3F Crystal at 343nm”, Advanced Solid State Laser Conf. (ASSL), 27 Oct.-1 Nov., Paris ,(France) Oral AM3A.3 (2013).
4- L. Deyra, S. Ilas, X. Délen, P. Loiseau, F. Balembois, G. Aka, F. Salin, P. Georges «Third harmonic generation at 343 nm in nonlinear Ca5(BO3)3F (CBF)« Oral : Non Linear Optics, Hawai, (21-26 July 2013)
5- A. Maillard, R. Maillard, P. Loiseau, G. Aka, P. Villeval, D. Rytz, “Defect similitude in LYSB and YAB crystals and ONL characterization”, Poster, Advanced Solid State Laser (ASSL) 16-21 Nov. 2014 Shanghai–China
7- J. Ren, L. Zheng, P. Loiseau, D. Rytz, G. Aka; “Growth and characterization of YAl3(BO3)4 single crystal for UV laser frequency conversion”; Oral : 4th French-German Workshop on Oxide, Dielectrics and Laser Crystals, Idar-Oberstein (Germany)10-11 Sept. 2015

The UV-CHALLENGE project (duration of 36 months) is proposed by an academic-industrial consortium. The latter includes three academic French laboratories [Laboratoire de Chimie de la Matière Condensée (LCMCP) at Ecole Nationale Supérieure de Chimie de Paris (ENSCP), Laboratoire Charles Fabry (LCF) at Palaiseau and Laboratoire de Matériaux Optique, Photonique et Système (LMOPS) at Metz (University de Lorraine)], two French industrial partners [EOLITE Systems in Bordeaux and Cristal Laser (CLASER) in Nancy] and one German industrial partner [Forschungsinstitut für Edelsteine/Edelmetalle GmbH (FEE)] which is member of the consortium without any requested financial support from ANR.
The main purpose of this project is to demonstrate the capabilities of new non linear optical (NLO) crystals for practical UV laser light generation. The generation of UV light by means of non linear optical (NLO) processes from an infrared fundamental beam is indeed a promising path to design all solid-state UV lasers. For this purpose, several non linear optical crystals have to be used, but there are only very few NLO crystals that can achieve the last stage of frequency conversion towards UV, especially when wavelengths below 270 nm are targeted. In this framework, the consortium involved in this project has already identified two families of crystals with real potentialities. They are based on YAl3(BO3)4 (YAB) and Ca5(BO3)3F (CBF). These crystals were identified in a previous project, called “UV-Borates” (2006-2010), and they have been developing to compete with the commercial crystals which are practically used today for UV conversion: LiB3O5 (LBO) for third harmonic generation (THG: 343 – 355 nm) and ß – BaB2O4 (ß – BBO) or CsLiB6O10 (CLBO) for fourth harmonic generation (FHG: 237 – 266 nm) by frequency doubling of green light. These commercial crystals have some severe drawbacks: they are hygroscopic (very critical for CLBO) and their lifetime in UV laser systems is unsatisfactory. YAB, CBF and some of their related compounds are proposed to solve these limitations and to increase the reliability and/or the efficiency of the THG and FHG processes. For deeper understanding of aging effect, this study will extend to the analysis of the behavior of commercial crystals like LBO and BBO.
UV-CHALLENGE is a multidisciplinary project involving materials science, crystal growth and machining, linear and non linear optics, conversion efficiency and material aging studies. The consortium of the present project consists of 6 partners having more than 4 years of collaboration in the field of nonlinear optical materials. The first objective is first to optimize the crystal growth conditions leading to enhanced optical quality crystals for fabrication of NLO crystals oriented and polished for specific configurations. Second, the precise determination of the physical properties as well as the relationships between optical and structural properties (impurities, twinning, absorption spectroscopy, EPR and Raman spectroscopy, surface characterization) of the new NLO crystals proposed in this project will be emphasized. Finally, the nonlinear optical properties and the aging behavior of the crystals will be tested in practice and compared to commercial NLO crystals. Aging in particular is a key issue to any practical application of UV sources. The achievement of these objectives will require a large panel of basic optical measurements, the accurate machining of many crystals, and the direct evaluation of UV conversion performances. All these studies will be analyzed and provided as feedback to the crystal growth process.