  High-power infrared laser systems with broadband tunability are of great
importance due to their wide range of applications in spectroscopy and
free-space communications. These systems require nonlinear optical (NLO)
crystals for wavelength up/down conversion using sum/difference frequency
generation, respectively. NLO crystals need to satisfy many competing criteria,
including large nonlinear optical susceptibility, large laser induced damage
threshold (LIDT), wide transparency range and phase-matchability. Here, we
report bulk single crystals of SnP_2S_6 with a large non-resonant SHG
coefficient of d33= 53 pm/V at 1550nm and a large LIDT of 350 GW/cm^2 for
femtosecond laser pulses. It also exhibits a broad transparency range from 0.54
{\mu}m to 8.5{\mu}m (bandgap of ~2.3 eV) and can be both Type I and Type II
phase-matched. The complete linear and SHG tensors are measured as well as
predicted by first principles calculations, and they are in excellent
agreement. A proximate double-resonance condition in the electronic band
structure for both the fundamental and the SHG light is shown to enhance the
non-resonant SHG response. Therefore, SnP2S6 is an outstanding candidate for
infrared laser applications.
