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We present the formation of periodic ripples in ZnO crystal irradiated by a wavelength-tunable femtosecond laser. The results indicate that in the surface thin layer, the periods change from 0.1 lambda to lambda with laser fluences and pulse numbers, and in the subsurface layer the periods are always lambda/2n, where n is the refractive index. The formation processes and mechanisms are also discussed.

We reported three types of complex micro/nanostructures on 6H-SiC crystal induced by the interferences of three femtosecond laser beams by arranging three types of laser polarization combinations. The micro/nanostructures are composed of two parts: two-dimensional long-periodic micropatterns determined by the interferential intensity pattern and short-periodic nanopatterns determined by the interferential polarization pattern. Theoretical calculation indicates that the different polarization combinations will lead to a distinct complex interferential polarization pattern and intensity pattern, and they accord well with the experimental results.

Two dimensional (2D) periodic microstructures composed of short periodic ripples, long periodic ripples and micro-holes are fabricated on ZnO crystals via the interference of two femtosecond laser beams. The relative reflectivity and transmissivity of visible light of these 2D microstructures decrease to the values of 30% and 20%, respectively. Theoretical and experimental studies indicate that besides the effects of increased surface area, the decrease of reflectivity is influenced greatly by the Mie scattering of surface microstructures, and the transmissivity, by the damage of crystalline structures.

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