Quasi-periodic structure can be introduced into nonlinear optical materials such as LiTaO3 crystals. Such structures were used for quasi-phase-matching second-harmonic generation. These materials are now shown to be able to couple second-harmonic generation and sum-frequency generation through quasi-phase-matching. The approach led to a direct third-harmonic generation with high efficiency through a coupled parametric process. The result verifies that high-order harmonics may be generated in a quadric nonlinear medium by a number of quasi-phase-matching processes, and therefore, exhibits a possible important application of quasi-periodic structure materials in nonlinear optics.

Direct observation of ferroelectric domain structures in LiTaO3 crystal, without etching or surface coating, has been realized by using environmental scanning electron microscopy in secondary electron emission mode. The new method can nondestructively provide domain contrast image at submicron resolution, and the domain contrast image is very stable. Conditions for best domain contrast of LiTaO3 crystals have been established.

We have designed and fabricated a novel nonlinear optical superlattice of LiTaO3 in which two antiparallel 180

A new type of conical second-harmonic generation was discovered in a 2D X(2) photonic crystal—a hexagonally poled LiTaO3 crystal. It reveals the presence of another type of nonlinear interaction—a scattering involved optical parametric generation in a nonlinear medium. Such a nonlinear interaction can be significantly enlarged in a modulated X(2) structure by a quasi-phase-matching process. The conical beam records the spatial distribution of the scattering signal and discloses the structure information and symmetry of the 2D X(2) photonic crystal.

Quasi-phase matching facilitates the phase matching of any parametric interaction by setting an appropriately modulated period. We propose a dual-periodic structure that is used in the construction of a frequencyconversion device. For example, we have designed and fabricated such a dual-periodic structure in a LiTaO3 crystal. Using a fundamental source at 1.064mm, we obtained ultraviolet radiation at 355nm and green radiation at 532nm simultaneously by frequency tripling and frequency doubling the fundamental source. Theoretically, this idea can be further extended to the design of a multiperiodic structure for achievement of more quasi-phase-matched processes in a single optical superlattice.