Thorlabs offers a variety of nonlinear ß-BBO crystals for use with pulsed laser sources. These nonlinear crystals are mounted in Ø1" housings with engravings that indicate the rotation axis and polarization orientations for the input and output beams to aid in aligning the crystal for optimal phase matching. Type-I nonlinear crystals are available for Second-Harmonic Generation, and both Type-I and Type-II nonlinear crystals are available for Spontaneous Parametric Down Conversion.