Achieving the full-wavelength phase-matching for efficient nonlinear optical frequency conversion in C(NH2)3BF4

Phase-matching of light waves is a critical condition for maximizing the efficiency of nonlinear frequency conversion processes in nonlinear optical crystals; however, phase-matching, commonly achieved by tuning birefringence, is often difficult to achieve over a wide wavelength range. Here, full-wavelength phase-matching crystals that can avoid phase-mismatching across the entire optical transparency range are proposed. The anisotropic strength of bonding in the dimension of energy is confirmed theoretically to be the key to the full-wavelength phase-matching ability. We demonstrate that a crystal of guanidinium tetrafluoroborate (C(NH 2 ) 3 BF 4 ) can be phase-matched throughout its entire optical transparency range and is able to generate harmonic light as short as ~193.2 nm, which is close to its deep-ultraviolet cut-off edge. Importantly, this crystal is stable, cheap and efficient compared with commercially available nonlinear optical crystals for generation of 266 nm light. This work lays the foundation for finding a new class of crystals in which the phase-matching wavelength fully covers its optical transparency range, and also provides a high-performance crystal for generating light at 266 nm—the fourth-harmonic of a commercial 1,064 nm laser.