Crystals 2023, 13, 169. https://doi.org/10.3390/cryst13020169 www.mdpi.com/journal/crystals Review Pure and Yb-Doped LaxYySc4-x-y(BO3)4 Crystals: A Review of Recent Advances Alin Broasca 1,2 , Madalin Greculeasa 1,2 , Flavius Voicu 1 , Cristina Gheorghe 1 and Lucian Gheorghe 1, * 1 National Institute for Laser, Plasma and Radiation Physics, 077125 Magurele, Romania 2 Doctoral School of Physics, Faculty of Physics, University of Bucharest, 077125 Magurele, Romania * Correspondence: lucian.gheorghe@inflpr.ro Abstract: This paper reviews the progress in developing the LaxYySc4-x-y(BO3)4-LYSB and Yb-doped LaxYySc4-x-y(BO3)4-LYSB:Yb huntite-type crystals grown by the Czochralski method as new candi- dates for the next generation of nonlinear optical (NLO) and/or laser crystals. Considering the in- congruent melting of these crystals, the initial compositions of the melt and the pulling and rotation rates were optimized. Additionally, a special thermal setup was engineered to grow LYSB-type crystals by the Czochralski crystal growth method. The chemical compositions of the LYSB and LYSB:Yb grown crystals were found to be La0.78Y0.32Sc2.90(BO3)4 and La0.78Y0.32Yb0.04Sc2.86(BO3)4, respec- tively. Therefore, for the LYSB:Yb crystal, the doping concentration of Yb 3+ ions was considered to be 4 at.% with respect to the nonstoichiometric (La1-xYx)1.25Sc2.75(BO3)4 undoped compounds, i.e., LYSB:Yb (4 at.%). In terms of NLO properties, the obtained results demonstrate that LYSB and LYSB:Yb (4 at.%) crystals possess remarkable properties specific to huntite-type crystals. The main advantage of these crystals consists in the fact that they may be obtained with large dimensions and excellent optical quality by the Czochralski method, which recommends them as a new class of highly efficient crystals for different NLO applications, including second harmonic generation (SHG) of high-power or high-energy laser beams. The laser performances of the LYSB:Yb (4 at.%) crystal prove its favorable intrinsic properties to generate laser emissions in the 1 µm range with high efficiency. The efficient laser emission at ~1028 nm together with good NLO characteristics to convert its own emission into emission at ~514 nm via SHG make the LYSB:Yb (4 at.%) crystal a very promising active medium to be used in self-frequency doubling configuration. Keywords: single crystals; Czochralski; NLO crystals; Yb 3+ ; laser crystals 1. Introduction Nonlinear optical (NLO) crystals play a key role in the emergence of optoelectronic and photonic technologies, as they have the ability to change the characteristics of a laser beam in terms of frequency, amplitude, phase, and polarization. In the context of expand- ing laser applications over a wide variety of domains, the pursuit of new NLO crystals has attracted more interest in the scientific community. The specific criteria that a NLO crystal should meet are the following: (i) crystallizing in noncentrosymmetric structure (χ (2) ≠ 0); (ii) high transparency in the spectral range of interest; (iii) large nonlinear optical coefficients; (iv) moderate birefringence to meet the phase-matching conditions, especially in the UV spectral range; (v) large laser-induced damage threshold (LDT); (vi) chemical stability and good physical properties; and, last but not least, (vii) the possibility of ob- taining crystals with large dimensions and high quality. Since the 1980s, more than a hun- dred different NLO crystals have been developed and studied. Among these, NLO crys- tals from the borate family, such as β-BaB2O4 (β-BBO), LiB3O5 (LBO), CsLiB6O10 (CLBO), GdCa4O(BO3)3 (GdCOB), YCa4O(BO3)3 (YCOB), and YAl3(BO3)4 (YAB), stood out for their good NLO properties. However, the main disadvantage of BBO, LBO, CLBO, and YAB Citation: Broasca, A.; Greculeasa, M.; Voicu, F.; Gheorghe, C.; Gheorghe, L. Pure and Yb-Doped LaxYySc4-x-y(BO3)4 Crystals: A Review of Recent Advances. Crystals 2023, 13, 169. https://doi.org/10.3390/ cryst13020169 Academic Editor: Guanying Chen Received: 19 December 2022 Revised: 15 January 2023 Accepted: 16 January 2023 Published: 18 January 2023 Copyright: © 2023 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).