Sub-wavelength ripples in fused silica after irradiation of the solid/liquid interface with ultrashort laser pulses

Laser-induced backside wet etching (LIBWE) is performed using ultrashort 248?nm laser pulses with a pulse duration of 600?fs to obtain sub-wavelength laser-induced periodic surface structures (LIPSS) on the back surface of fused silica which is in contact with a 0.5?mol?l(-1) solution of pyrene in toluene. The LIPSS are strictly one-dimensional patterns, oriented parallel to the polarization of the laser radiation, and have a constant period of about 140?nm at all applied laser fluences (0.33-0.84?J?cm(-2)) and pulse numbers (50-1000 pulses). The LIPSS amplitude varies due to the inhomogeneous fluence in the laser spot. The LIPSS are examined with scanning electron microscopy (SEM) and atomic force microscopy (AFM). Their power spectral density (PSD) distribution is analysed at a measured area of 10?μm × 10?μm. The good agreement of the measured and calculated LIPSS periods strongly supports a mechanism based on the interference of surface-scattered and incident waves.