Twinning and "natural quasi-phase matching" in Yb:YAB

Defects obtained during crystal growth of many laser and nonlinear crystals limit their commercial potential, as is the case of the yttrium aluminium borate family. Typically, in Yb:YAB, birefringent phase-matching is used to obtain self frequency doubled output from a fundamental laser operating near 1μm. Many crystals however show signs of twin inversion defects during crystal growth resulting in the reversal of the sign of the nonlinear coefficient and hence can result in quasi-phase-matched-like structures. In this report we examine the second harmonic conversion characteristics of a 3.8mm thick birefringently phase matched Yb:YAB crystal which is then examined in more detail by slicing into four thinner sections each approximately 400μm thick. Etching of crystal surfaces reveals clear boundaries between reversed domains, which under illumination with a laser at 1.064μm shows a cancellation of the nonlinear conversion along the interface of the two regions.