Characterization of fused silica specimens exposed to low-energy femtosecond laser pulses using a sub-micron resolution, thermal technique

Low-energy femtosecond laser pulses (typically in the tenth of nJ per pulse regimes at 250kHz) focused into fused silica substrates induce various modifications in the material properties of the base material, including a localized increase of the refractive index. Related sub-microns periodic structures found in the laser-exposed regions have also been recently described by several authors. The characterization of the laser-affected zones is particularly challenging due to their small sizes - typically micron or sub-micron. Experimental methods previously reported have either limited spatial resolution or require additional material processing to reveal the zone of interest, leaving open questions related to the influence of the processing itself. Using an Atomic Force Microscope equipped with a thermal probe, we recently published that low-energy femtosecond laser pulses leave thermal conductivity change footprints. The thermal footprints match very well the zone where a higher refractive index is observed. This novel analytical method does not require any processing of the surface prior to the observation and yields high-quality, sub-micron resolution, maps of the laser affected zones. Furthermore, it also opens new interesting and fundamental questions on the effect of femtosecond laser irradiation on fused silica. In this paper, we report on systematic observations made on fused silica specimens exposed to various pulse energies under different polarization conditions. We analyze and discuss the effect of the laser exposure on the thermal properties of the fused silica substrate.