Self-assembled periodic sub-wavelength structures by femtosecond laser direct writing.

Self-assembled, sub-wavelength periodic structures are induced in fused silica by a tightly focused, linearly polarized, femtosecond laser beam. Two different types of periodic structures, the main one with period (Lambda(E)) in the direction of the laser beam polarization and the second with period (Lambda(k)) in the direction of the light propagation, are identified from the cross-sectional images of the modified regions using scanning electron microscopy. We demonstrate the spatial coherence of these nanogratings in the plane perpendicular to the beam propagation direction. The range of effective pulse energy which could produce nanogratings narrows when the pulse repetition rate of writing laser increases. The period Lambda(E) is proportional to the wavelength of the writing laser and period Lambda(k) in the head of the modified region remains approximately the wavelength of light in fused silica.