Ultrafast laser-induced self-organized conical micro/nano surface structures and their origin

We have investigated the origin and subsequent evolution of self-organized conical micro/nano structures in silicon surface upon irradiation of multiple ultrafast laser pulses in the gaseous environments of SFG and He. Localized circular surface waves are observed in ultrafast laser exposed regions under various states of polarization These localized circular surface waves interfere with each other to produce scattering sites for subsequent pulses. Additional exposure gives rise to ripples The period of the ripple structures is usually on the order of the wavelength of the irradiating laser pulse (800 nm) in the central region of the laser spot while towards the edge of the spot, ripple periods are much smaller (similar to 500 nm). Also, we have observed that the period of the localized circular surface wave appear to be unrelated to the laser wavelength. With further laser exposure, the initial ripple structure breaks down into surface roughness due to further formation of localized circular surface waves on already produced ripples. This initial surface roughness evolves Into sharp and regular array of self-organized conical micro/nano surface structures due to self-masking phenomenon. We have also demonstrated that for laser assisted conical micro/nano structure formation in silicon, presence of a reactive gas is not necessary, although It enhances the etching process as well as makes the structures sharper. (C) 2010 Elsevier Ltd. All rights reserved