Nonlinear optical fluoroborate crystals for UV lasers – FLUOLUV

FLUOLUV is a 48-months PRC involving three national academic research groups [IRCP, SIMAP, NEEL] and one Japanese team [IMS] that will participate to the project from own resources. All will put together their complementary skills in the field of materials and optics to develop nonlinear optical (NLO) crystals for an efficient UV conversion.
The generation of UV light by means of NLO processes from an infrared fundamental beam is indeed the desired path to design all solid-state, compact and reliable UV lasers. However, there are only very few commercial NLO crystals that can achieve the last stage of frequency conversion towards UV despite the big market of UV lasers. For this purpose, the consortium will develop new alkaline-earth fluoroborate NLO crystals aimed to solve some limitations of current NLO crystals. IRCP group has already grown first single crystals from flux method and demonstrated their capability to generate 355 nm, despite a crystal quality to improve. Chemical substitution should extend and tune this range of capabilities deeper in UV to achieve for the first time laser generation at 266 nm by frequency doubling of 532 nm in an angle non-critical phase matching (NCPM) configuration. However, the demonstration of the full potentialities of these fluoroborate crystals requires a complete characterization and optimization of their properties. To boost the development of these nonlinear materials, the consortium set up a methodology consisting of a complementary multidisciplinary approach as follows:
1. Theoretical approach: Thermodynamic and ab-initio simulations. The thermodynamic properties of the solid/liquid phases and compounds as well as the gaseous species of the reciprocal fluoroborate family system will be modelled using the Calphad approach. In addition, DFT calculations will be used to calculate the energy of formation and the heat capacity of this family of materials as well as relevant physical properties. Theoretical data will be compared to experimental values. All generated results will be used to develop a coherent Gibbs energy dataset and calculate the relevant phase diagrams to define optimum crystal growth conditions: melt composition and temperature interval for growth by taking into account the gas atmosphere and volatilization effects.
2. Material approach: Preparation of fluoroborates single crystals. New flux growth compositions and conditions based on an original top-down approach fed by theoretical calculations will be tested to improve the fluoroborate family crystal quality. The target is to provide high-quality crystals and oriented samples of 10 to 20 mm length for laser frequency conversion tests. The composition tuning of the materials will be investigated to generate 266 nm laser wavelength by frequency doubling in NCPM conditions for the first time.
3. Nonlinear optical properties investigations: The dielectric frame orientation and the optical properties will be determined with special care about their temperature dependance. Direct measurements of phase-matching directions and the associated conversion efficiencies, angular and spectral acceptances will be performed using the sphere method. We will focus on UV generation at 355 and 266 nm by quadratic processes from 1064 nm fundamental laser wavelength to provide reliable dispersion equations of the principal refractive indices as well as the magnitudes and relative signs of all the nonlinear coefficients. This corpus of data will enable the calculation of the best orientations of the fluoroborate crystals for optimized configurations of different UV generators.

4. UV frequency conversion efficiencies. The optical damage threshold and effects limiting the UV nonlinear conversion will be estimated. The practical applications of these NLO fluoroborates will be demonstrated by developing long-term UV emitting prototypes using sub-ns high peak power microchip laser based on 1064 nm-Nd:YAG.