基于第一性原理的新型非线性光学晶体探索

First principle study of nonlinear optical crystals

非线性光学晶体能对常见波段的激光进行频率转换,从而获得宽波段、可调谐的激光光源.此类光电功能材料在军事和民用领域具有重要的战略价值和应用价值.经过30多年的发展,应用于可见光及邻近波段的非线性光学晶体技术已经基本成熟,但深紫外和中远红外波段的非线性光学晶体技术的发展仍存在诸多不足,还需要在这些波段进行新型优质的非线性光学晶体探索.近年来,为了改变传统低效的"炒菜式"实验探索,加快新材料研发速度,基于密度泛函理论的第一性原理计算方法在新型非线性光学晶体探索中得到了广泛的应用.本文总结了近几年深紫外和中红外波段非线性光学晶体的新进展,通过介绍几种新型非线性光学晶体材料的研发过程,突出了第一性原理计算在新材料探索过程中起到的关键作用;探讨了非线性光学晶体研发的研究难点与趋势,以及第一性原理方法在未来新材料探索中的重点攻关方向.

Nonlinear optical （NLO） crystal is one of the important opt-electrical functional materials that can convert laser frequency and obtain wide band tunable coherent sources, thus it possesses crucial strategic and application value in military and civil fields. On the basis of more than 30 years＇ efforts, the NLO crystals in visible and near infrared region, including β-BaB204 LiB305 and KTiOPO4, have been basically mature. However, there are still many shortcomings for those NLO crystals used in deep ultraviolet （DUV） and mid/far-infrared （IR） regions, thus putting forward more requirements for high performance crystals. For DUV KBe2BO3F2 （KBBF） crystals, the main shortcomings are the use of toxic BeO raw materials and strong layer growth tendency. Wide transparent region and high second harmonic generation （SHG） effect are also expected in new developed DUV NLO crystals. More importantly, a large enough birefringence is highlighted to satisfy the phase-matchable condition and DUV harmonic generation capacity below 200 am. On the other hand, the main requirement for mid/far-infrared NLO crystals is to maintain the balance between high laser damage threshold and strong SHG response. Indeed, it is a very difficult task to search for good NLO crystals through the traditional ＂trial and error＂ experimental methods. Theoretical studies, especially first principles calculations, can provide an efficient way to investigate and design new NLO materials with superior properties. In this paper, the recent progress of deep-UV and mid-IR NLO crystals is summarized. In addition, the crucial role of first principles calculations in new material exploration and design is highlighted by introducing several typical new NLO crystals, including defect diamond-like compound AgZnPS4, trigonal alkaline metal fluorooxoborate KB406F and alkaline earth fiuorooxoborate SrB5O2F3. Moreover, some advanced analysis tools are introduced, such as real space atomic cutting method, SHG-weighted mapping, flexible dipole moment model, and non-bonding atomic orbitals analysis, and used to investigate the structure-property relationship in langasite LasSnGa5O14, metal cyanurate Ca3（C3N3O3）2, vanadium-carbonate K3[V（O2）2O]C3, etc. Further, the flow chart of high-throughput first principles calculations of NLO crystal is proposed. According to the known or predicted crystal structure, we can obtain the chemical stability, band gap, NLO coefficient, birefringence and phase-matchable capacity quickly, thus easily judging the research potential of a new NLO material. On the basis of these ideas, a great blueprint for NLO crystal ＂material genome engineering＂ is highly put forward. Finally, the difficulties in research and challenges in NLO material investigations are discussed, and the direction of future research priorities based on first principles calculations are pointed out.