Micro-volumetric analysis of complex fs laser processed sites using optical surface profilometry (OSP)

Abstract

Ultrafast laser pulse processing is an important tool used to modify the structural and morphological properties of a target material. Muscovite, a naturally occurring nano-layered mineral, is a material that has exhibited a complex systematic variation in the topography of laser processed sites as a function of the fluence of a single 150 fs, 800 nm laser pulse. These sites include depressed craters; raised bumps; bumps in craters, both with and without rims; and ultimately the onset of what would be identified as standard ablation. A means of quantifying the sequence of observed topographies of these micron sized modifications; which have height features ranging from a few, to a few hundred, nanometers; is required. We present the quantification of the sub-cubic-micrometer laser-modified-volumes in muscovite using full 3-D profiles obtained from optical surface profilometry. This includes quantifying modification volumes above and below the original surface plane separately. This gives information about redistribution of material. The net volume change indicates whether there is a net or apparent volume increase due to rarefraction, delamination, or cavitation, or a net volume removal. Thus, the volumetric analysis in combination with the morphological knowledge of the modified sites emerges as a potential indicator of the likely physical phenomena at play. This is illustrated by the volumetric analysis reported here-in for muscovite, exposed to single pulses in the fluence range 1.5–7 J/cm2. The OSP technique is commonly used to characterize surface roughness, and, morphology of processed surfaces, including laser processed surfaces. But, to our knowledge, this is the first time a detailed micro-volume analysis has been reported in a laser processing context. This can be applied universally to different laser processed materials, especially when multiple physical processes are at play. The resolution of the measurements is limited by the lateral and vertical resolution specifications of the OSP instrument.