NaPN2: Deep-ultraviolet nonlinear optical material with unprecedented strong second-harmonic generation coefficient

The electronic band structures and second harmonic generations (SHGs) of phosphorus nitrides LiPN2, NaPN2, and CuPN2 are studied by first-principles calculations. NaPN2 has a band gap of ∼6.2 eV (200 nm) and the strongest SHG coefficient among phosphorus-based deep-ultraviolet nonlinear optical (NLO) materials, i.e., d36=2.35 pm/V at static limit and d36=3.77 pm/V under external optic field with wavelength 1064 nm. Compared with the SHG coefficients of borate NLO materials working in the region of deep ultraviolet, NaPN2 presents an unprecedented high SHG coefficient, which is the manifestation of its enhanced orbital hybridization owing to the shorter bond lengths from high pressure facilitated synthesis.