Research and development of pure and Yb3+- activated nonlinear optical (NLO) borate type crystals for the realization of new compact and tunable high-power visible solid-state laser sources based on frequency conversion processes. – BORATESYB

This project can be divided into 5 parrallel tasks

Task 1: Crystal growth of pure and Yb-doped large size YCOB crystals.
The growth by Czochralski method will be optimized to avoid cracks, inclusions, heterogeneities and, in the case of undoped crystals, the growth will be scaled up from 1 inch in diameter to 2 inches. Very good optical quality, homogeneity and purity are aimed for YCOB and Yb:YCOB in order to cut, orientate and polish slabs for optical characterization.

Task 2: Compositional refinement and growth of pure and Yb-doped LGSB and BCBF crystals.
This task is related to the development of new crystals of LGSB type. Depending on the compositional parameters, LGSB single crystals of good optical quality could be grown by the Czochralski method. These compositional parameters also determine the transition from a centric to a acentric structure required for nonlinear properties. As a result, large crystals with dimensions of 25 mm in diameter will be grown and slabs will be prepared.
A similar study will be applied to BCBF in order to improve its optical quality and allows its doping by Yb3+ ions.

Task 3: Characterization of crystal properties.
The main objective is be to make a complete characterization of useful NLO parameters of crystals, particularly those related to SHG of NIR radiation by calculating the conditions for efficient frequency doubling.

Task 4: Spectroscopic characterization of Yb: YCOB and Yb: LGSB.
Spectroscopic measurements (absorption, emission) at different temperatures 10º - 300ºK will be carried out to determine the optical properties of Yb3+ in each host: crystal field effects and optical cross sections

Task 5: High-energy green laser sources in pure and Yb-doped YCOB and LGSB.
The main objective is to investigate the optical performances of the three borates for nonlinear optical and laser application. By combining these two properties, we also aim to obtain self frequency doubling crystals.

Incongruent melting LaxGdyScz(BO3)4 crystals were grown after optimization of the Czochralski process. Its favorable optical characteristics coupled with the opportunity to grow large dimension crystals make them very promising for NLO applications. As laser material, Yb:LGSB demonstrated 0.94 W at 1070 nm for 2.8 W of absorbed pump power, opening the way to SFD.
Spectroscopic properties and SFD laser performances were measured for Yb:YCa4O(BO3)3. For this crystal, a record of 330 mW of green light was obtained by laser diode pumping even if an instability of laser emission was observed.
Despite the complexity of Yb:BCBF chemical behavior, crystals were grown and first laser oscillation under laser diode pumping was demonstrated.

The two most promising crystals identified in this work are Yb:LGSB and Yb:YCOB.
For Yb:LGSB, further experiments concerning the polarization state of the near-infrared light and frequency conversion experiments are currently undertaken to determine the full potential of this crystals as new self-frequency doubling crystals.
Significant green laser output power was obtained with Yb:YCOB. Nevertheless, to equal Nd:GdCOB SFD performances, laser instability problem must be solved, notably by investigating new propagation directions, either in XY principal plane or out of principal plane to reach higher conversion efficiencies. It should be noted that Nd:GdCOB is the core product of a Chinese company, Lasence Inc. The development of Yb:YCOB could so be a serious competitor to the product developed by this company.
By this project, a new Czochralski furnace was bought and tested by the French part, which has considerably increased its potential for crystal growth for further development (growth of large scale crystals or in controlled atmosphere).

The results gathered in this project were largely communicated in international conferences (ASSL, ECCG5, IS-OM7) and French CNRS research network (CMDO and Cristech). As a result, they led to 5 posters, 2 orals and 1 invited communications. Moreover, 3 papers were accepted for publication in peer review journals and a last one is in preparation. It should be noted that the PhD student hired in this project was highlighted as first author of most of the communications (poster, oral, paper).

The aim of this project is to identify and develop new and innovative solutions for the realization of compact and tunable high-power DPSS (diode-pumped solid-state) laser systems with efficient emission in the green spectral range. The most efficient method currently available to achieve such laser sources is high-efficient frequency conversion of the solid-state infrared laser emissions into visible emissions by nonlinear optical (NLO) processes in suitable NLO crystals. Many efforts have been carried out to develop and study new NLO materials in order to improve their specific properties or workability. Experimental results show that borate type compounds constitute a veritable source of NLO crystals with good nonlinear properties. Among all the type of inorganic compounds now developed, 36% of structures based on borate are non-centrosymetric while only 15% of the all reported inorganic structures are non-centrosymetric. Therefore, all the recent NLO crystals discovered in the last 10 years are in majority borate based compounds such as: CsLiB6O10 (CLBO), BiB3O6 (BiBO), K2Al2B2O7 (KABO), KBe2BO3F2 (KBBF), ReCa4O(BO3)3 (Re = Gd, Y, La) - ReCOB, ReSc3(BO3)4 (Re = lanthanide). Nowadays, two families of borate crystals are known that melt congruently or comprise congruently melting compounds, which can be grown in large sizes by the Czochralski method: the family of rare-earth calcium oxyborates with chemical composition ReCa4O(BO3)4 (Re = Gd, Y or La), and the family of ReSc3(BO3)4
The proposed project is a joint research project between: Solid-State Quantum Electronics Laboratory – SSQEL of the National Institute for Laser, Plasma & Radiation Physics – INFLPR, Romania, and Laboratoire de Chimie de la Matière Condensée de Paris - LCMCP of the École Nationale Supérieure de Chimie de Paris (ENSCP) - Chimie ParisTech, France.
In order to obtain large dimension frequency converters for high-energy laser beams and also to realize new compact and tunable high-power SFD green lasers, we intend to study and develop two types of NLO borate crystals during this proposed project:
(i)- pure and Yb-doped YCa4O(BO3)3 (YCOB) crystal which is considered as the leader compound of the calcium rare earth oxyborate ReCOB (Re = Gd, Y, La) family compounds.
(ii)- pure and Yb-doped LaxGdyScz(BO3)4 (x + y + z = 4) - (LGSB) crystals which belong to the huntite family compounds ReSc3(BO3)4. The first growth experiment of trigonal LGSB crystal associated with its nonlinear properties makes this crystal a very promising crystal.
The entire work that we plan to do for this project should lead to the following expected results:- Grow for the first time high optical quality and large size, pure and Yb-doped YCOB and LGSB crystals suitable for high power frequency conversion in the green spectral range.
- Determine and define precise crystal growth conditions so that industrial growth could be done allowing a valorization of the grown crystals during this project. Two of our industrial collaborators, Cristal Laser in France and FEE in Germany, manufacture and sell nonlinear crystals worldwide. Depending on our results, their interest for YCOB and LGSB crystals is expected to rise.
- Make a complete characterization of NLO parameters and useful properties of the grown crystals
- Make a full spectroscopic characterization of Yb: YCOB and Yb: LGSB crystals and optimization of the Yb concentration in order to achieve efficient SFD processes.
- Offer new ideas and solutions for large current European and international projects related to the construction of the petawatt-class UHI lasers and ICF facilities