Response of natural muscovite to a single femtosecond laser pulse

Muscovite is a naturally occurring crystalline mineral, a mica, with a unique layered structure with planes of low cleavage energy spaced by ~1.3 nm in the crystal structure. It is a dielectric insulator. Freshly cleaved muscovite surfaces are extremely flat, clean and used in many technical applications of the material. Previous laser ablation study of mica using ultraviolet, nanosecond duration pulses, led to a poor finish at the process sites (K. Rubahn et.al., J. Appl. Phys. 86(5), 2847, 1999). Interest in laser processing of the material, other than CO2 laser cutting of mica sheets, was subsequently, and not surprisingly, curtailed. Here-in we report the morphologies of the laser processed site affected by a single, ~150 fs duration, 800nm wavelength, 6 micron spotsize laser pulse focussed on the surface of a mica substrate. A systematic sequence of the morphology as the fluence of the single pulse is increased is obtained. Optical surface profiling and field emission secondary electron microsocopy are used to characterise the site morphology. Time of flight secondary ion mass specroscopy has been used to map the redistribution of key elements at the process site. Muscovite emerges as a fascinating material in its response to a femtosecond laser pulse. Useful marking without creation of debris beyond the footprint of the laser spotsize is achieved at a flunece as low as 2.4 J/cm2. There is evidence of plasticity and cavitation within the sequence of morphologies found.