Enhancements of Second Harmonic Generation and Birefringence in Traditional RbTiOPO_4-Type Crystals by Aliovalent Substitution of Ti Site

摘要

Strong second harmonic response and large birefringence of nonlinear optical (NLO) materials have important scientific and technical significance for tunable lasers. The second harmonic generation (SHG) response in NLO crystals has been generally enhanced by improving the polarization of bridged oxygen atoms or local structural distortions, while the synergy of the two contributions to enhance the SHG response is rarely explored. To this end, new crystals with traditional RbTiOPO_4 (RTP)-structure, Rb_(1-x)Ti_(1-x)Sb_xOPO_4 (x = 0.01, 0.03, and 0.06), containing novel functional SbO_6 octahedra and Rb vacancies are designed through a one-site aliovalent substitution and grown in this work. Rb_(0.94)Ti_(0.94)Sb_(0.06)OPO_4 exhibits an enhanced SHG response, nearly 1.48 times that of the RTP crystal, accompanied by the largest birefringence in phosphates (exp. 0.202 at 546 nm, cal. 0.156 at 1064 nm). Theoretical calculations and crystal structure analysis confirm that the higher polarizability of bridged oxygen and the local structural distortions in Rb_(1-x)Ti_(1-x)Sb_xOPO_4 strengthen the NLO activity of neighboring units, thus significantly prompt the SHG effect and optical anisotropy. This study provides a new paradigm to deeply improve the NLO performance of crystal materials and benefits for expanding their practical applications.