Walk-off effect in nonlinear crystals suitable for UVC generation

Lasers emitting in the ultraviolet C-band (UVC) have recently attracted considerable attention for germicidal purposes. Combining diode lasers with nonlinear crystals used for second-harmonic generation (SHG) is a promising approach thanks to their relatively low cost, small footprint and long lifetime. The output power in the UVC is limited by the output power of the diode lasers and by the conversion efficiency in the nonlinear crystal. This work compares the SHG conversion efficiency using a bulk approach to values expected using guided modes in waveguides. It discusses the phase-matching (PhM) condition for different input polarizations, the effective nonlinearity, and the Poynting vector walk-off. This last effect is particularly detrimental as it reduces the effective length for the nonlinear interaction in bulk, which ultimately limits the conversion efficiency towards the UVC. Values for the walk-off angle are computed for barium borate (BBO), and a comparison is provided with other nonlinear crystals.