Near- and far-field second-harmonic imaging of quasi-phase-matching crystals

Second-harmonic scanning near- and far-field optical microscopy of an electric-field poled KTiOPO₄ quasi-phase-matching crystal has been accomplished. This has been done in order to reveal the walls that form the intersections between inverted and non-inverted crystal domains. The domain walls are seen clearly only in images recorded by means of second-harmonic generation because of a large nonlinear contrast, and they appear as bright stripes when studied in a reflection geometry but they are dark when studied in transmission. The images show that the duty cycle of the quasi-phase-matching crystal differs from the ideal and that the walls are not completely smooth. These effects, in combination with the observed scattering from the domain walls, are expected to lower the output of the crystal when used for frequency doubling. We conclude that the wall thickness is no more than approximately 100 nm, which makes it a suitable test object for the resolution capabilities of scanning near-field optical microscopes that are used for nonlinear imaging.