  Ultrafast, femtosecond laser pulse interaction with dielectric materials has
shown them to have significantly higher laser fluence threshold requirements,
as compared to metals and semiconductors, for laser material modification, such
as laser ablation. Examples of dielectrics are crystalline materials such as
quartz and sapphire, and amorphous glasses. The interaction between femtosecond
laser pulses, at a wavelength with negligible linear absorption, and a
dielectric has been found to be weak, and multiple pulse irradiation is
therefore typically used in order to see significant and quantifiable effects.
In this study the dielectric is the crystalline, layered, natural mineral
muscovite, a mica with formula KAl_2 (Si_3 Al) O_10 (OH)_2. Muscovite, newly
cleaved, is used in a wide range of technological and scientific applications
including as an insulating material in electronics and as an ultra-flat and
ultra-clean substrate. A single, ~800 nm wavelength, ~6 micron spotsize, ~150
fs laser pulse is found to lead to a systematic range of laser modification
topologies, as a function of the fluence of the single laser pulse, including
bulk removal of material. The fs laser pulse/material interaction is greater
than expected for a standard dielectric at a given fluence. Optical surface
profiling and FESEM are used to characterise the topologies. Contrasting the
results of the two techniques supports the use of optical surface profiling to
characterise the material modification despite its limitations in lateral
resolution as compared to FESEM. The interlayer mineral water content of
natural muscovite is proposed as the primary reason that mica behaves
differently to a standard dielectric when irradiated with a single 800 nm fs
laser pulse.
