Cross-sectional study of periodic surface structures on gallium phosphide induced by ultrashort laser pulse irradiation

We present cross-sectional transmission electron microscopy studies of laser induced periodic surface structures (LIPSS) formed on gallium phosphide after irradiation with ultrashort laser pulses with a pulse duration of 150fs and centered at a wavelength of 800nm. LIPSS with spatial periods (∼165nm) substantially smaller than the incident wavelength are found to have vertical profiles as high as ∼960nm measured from the crest to the trough. The present observations provide important insights into the formation mechanisms of subwavelength LIPSS.