Growth and characterization of 3.5 at.% Nd:LGSB bifunctional crystal
Introduction
Since its discovery in 1960, the laser has become an indispensable tool in many applications, including ultrafast material processing [1], automotive industry [2,3], medicine [4,5], military field [6,7], photolithography [8], or research. In order to meet the requirements for all these applications, the laser emission must cover a wide spectral range from ultraviolet (UV) to infrared (IR), including the visible (VIS) range. Currently, tunable lasers [[9], [10], [11]] and the lasers based on trivalent Pr or Tb ions [[12], [13], [14], [15]] are the only well-known solid-state lasers with direct emission in the VIS range. An alternative method to generate VIS laser radiation is the use of nonlinear optical (NLO) crystals that can convert the near-infrared (NIR) laser emission of the known solid-state lasers by second-order nonlinear processes. On the other hand, a more compact laser system can be built by using bifunctional laser and nonlinear optical crystals, in which the laser effect and the NLO phenomena take place simultaneously. This type of crystals must have suitable sites for laser-active ions so that when doped with appropriate rare-earth ions, they can combine the laser emission in the NIR domain with the second harmonic generation (SHG) properties of the host to generate green laser radiation by self-frequency doubling (SFD) processes [16].
In general, a bifunctional crystal should meet several requirements, such as high quantum efficiency, good thermal conductivity, high absorption efficiency at the pump wavelength, efficient laser emission in the NIR range, and phase-matching properties (with very low losses). Another important aspect is given by the possibility of obtaining crystals with high optical quality and large dimensions, being a well-known fact that the incorporation of large quantities of trivalent rare-earth ions in NLO crystals can cause crystal growth issues and degrade the optical quality of the grown crystals. For instance, the 4 at.% Nd3+-doped yttrium aluminum borate (Nd:YAB) crystal proved to be a promising bifunctional crystal, delivering a green output power of 225 mW under the pump power of 1.6 W at 807 nm with a laser diode [17]. However, because of its incongruent melting, the Nd:YAB crystal can be grown only by flux method, thus being very difficult to obtain sufficiently large and high-quality single crystals. To overcome the problems regarding the growth of the crystal, the research has focused on scandium derivatives with huntite-type structure, having the general formula LnSc3(BO3)4 (Ln = lanthanide) [18], which can exhibit congruent or “nearly” congruent melting.
Very recently, we developed the Nd-doped LaxGdySc4-x-y(BO3)4 (Nd:LGSB) crystal as a new bifunctional crystal with nearly congruent melting, which can be grown with large size and high-quality by the Czochralski method. Based on the laser results recorded on 2.3 at.% Nd:LGSB and 4.6 at.% Nd:LGSB bifunctional crystals [19,20], it was concluded that the concentration of Nd3+ ions must be further optimized to improve both NIR and green laser emission efficiency. Herein, we report on the growth, characterization of structural and optical properties, and NIR laser emission along the phase-matching direction for SHG of 1062 nm radiation of a 3.5 at.% Nd:LGSB bifunctional crystal. Preliminary results on the generation of green 531 nm laser emission by SFD in 3.5 at.% Nd:LGSB are also discussed. Efficient continuous-wave (CW) laser emission at 1062 nm and improved efficiency for the generation of green radiation at 531 nm by SFD were obtained in comparison with our previous reports.
Section snippets
Material synthesis
High-purity oxide powders of La2O3, Nd2O3, Gd2O3, Sc2O3 (99.999%), and B2O3 (99.98%) were used for the synthesis of LaxNdyGdzSc4-x-y-z(BO3)4 (Nd:LGSB) polycrystalline compound by the solid-state reaction method, according to the following chemical equation:0.32La2O3 + 0.019Nd2O3 + 0.286Gd2O3 + 1.375Sc2O3 + 2B2O3 → La0.640Nd0.038Gd0.572Sc2.75(BO3)4
X-ray powder diffraction
The X-ray powder diffraction (XRPD) spectra of Nd:LGSB starting compound and the as-grown crystal were measured at room temperature using a
Crystal growth, X-ray and compositional characterization
The X-ray powder diffraction (XRPD) peaks of La0.64Nd0.038Gd0.572Sc2.75(BO3)4 polycrystalline compound at room temperature are well indexed by the trigonal structure with space group R32 (classical huntite structure), as can be seen in Fig. 1 (in black color). For the growth of Nd:LGSB crystal by the Czochralski method, the use of a specific thermal assembly that ensures a balance between the thermal gradients and the evaporation of certain compounds from the melt is required in order to avoid
Conclusions
High quality 3.5 at.% Nd:LGSB crystal with dimensions of 12 mm in diameter and 35 mm in length was grown by the Czochralski technique. Structural, optical and laser emission properties along the phase-matching direction for SHG of 1062 nm radiation were reported. The Nd:LGSB crystal has a trigonal structure with a rhombohedral lattice corresponding to a classical huntite-type structure. The activator Nd3+ ions substitute only the La3+ ions in the trigonal prismatic sites with D3(32) symmetry.
CRediT authorship contribution statement
Alin Broasca: Conceptualization, Investigation, Data curation, Writing – original draft. Madalin Greculeasa: Investigation, Data curation. Flavius Voicu: Investigation, Data curation. George Stanciu: Investigation, Data curation. Stefania Hau: Investigation, Data curation. Cristina Gheorghe: Investigation, Data curation, Formal analysis, Writing – original draft. Catalina-Alice Brandus: Investigation, Data curation, Writing – original draft. Nicolaie Pavel: Data curation, Formal analysis,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financed by the Romanian Ministry of Research, Innovation and Digitization, through grant agreement PCE 49/2021 within PNCDI III, project number PN-III-P4-ID-PCE-2020-2203, and through project 16N/2019 within program NUCLEU-LAPLAS VI.
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