Formation and multi-imaging analysis of nascent surface structures generated by femtosecond laser irradiation in silicon

We report on the formation of periodic structures on the (100) crystalline silicon surface after irradiation with a low number of 1055 nm, 850 fs laser pulses, in high vacuum conditions. Our analysis focuses on the nascent stage of surface structures formation. We employ a wide variety of microscopy techniques to retrieve the morphological, optical and structural properties of generated structures. Sample topography is measured by means of an Atomic Force Microscope. Sample structural phase is revealed by performing a Raman micro-analysis through a scanning confocal optical microscope, while confocal and wide-field reflection images of the sample surface are also registered. Our analyses clearly show, besides the topographic ripples, the creation of grating structures of near-wavelength period consisting of alternating amorphous and crystalline periodic lines, with almost no material removal. The gratings originate from defects acting as scattering centers and generating energy modulation patters propagating along the direction of laser polarization.