Formation and evolution of ultrashort pulse-induced nanogratings in Borosilicate glass

Abstract

The versatility of ultrashort laser pulses as a tool for laser materials processing has augmented particular interest in the past decade. Especially birefringent modifications, so-called nanogratings, have found to exhibit tremendous potential for manifold photonic functionalities. These self-assembling structures, orienting always perpendicular to the laser polarization, have been up to now extensively studied in bulk fused silica. Commonly it is assumed that the formation of nanogratings is actually limited to anomalous glasses like silica or slightly doped silica. However, we recently found that even in glasses like borosilicate or BK7 nanogratings can be observed within certain parameter regimes. Here we present an extensive study of the fundamental constituents of nanogratings in bulk borosilicate glass using small angle X-ray scattering (SAXS) in combination with focused ion beam milling (FIB) and scanning electron microscopy (SEM). The irradiation produces void-like sheets (10-20 nm wide) as well as elongated cracks of up to 400 nm. In contrast to nanogratings in fused silica, borosilicate shows a significant smaller optical retardance. The cumulative action of several hundreds of laser pulses lead to the formation of individual grating planes with a period of about 60 nm (at an inscribing laser wavelength of 800 nm) while the well-known λ/2n (n-refractive index) period is prevented. This has never been observed for ultrashort pulse induced nanogratings so far.