Sub-wavelength Uniform Periodic Surface Structures Induced by Low Femtosecond Laser Fluence on Silicon with Optical Glass Covering

Thus far, femtosecond laser-induced periodic surface structures (LIPSS) on the surfaces of solid materials have been studied in air and liquids. However, few studies have investigated LIPSS formation on a solid surface through a solid transparent medium material. In the present study, LIPSS are formed on a silicon wafer surface covered with optical glass (glass environment) and compared with those formed in air. The periodicities of low and high spatial frequency LIPSS (LSFL and HSFL, respectively) formed in the two environments are 650 and 200 nm, respectively. In glass environment, LSFL can be induced by laser fluence of 410 mJ/cm(2), which is much lower than that of 570 J/cm(2) used in air. Moreover, LIPSS formed in glass environment are rough, contradicting a previous report on LIPSS formation in liquid environments. The possible mechanisms forming the different structures in the two environments are discussed.