Comparative study of femtosecond laser ablation of glass using ultrathin gold and nickel films as sensitizers.

It is known that an ultrathin Au film can sensitize femtosecond ablation of dielectric substrates by reducing the ablation threshold reduction and enhancing the ablation rate. In this work, we report the first observation that an ultrathin Ni film can also serve as a sensitizer for glass, although it is not as effective as Au. A comparative study of Au and Ni sensitizers provides clear evidence that metal-sensitized ablation of dielectric substrates is mediated by hot electrons in the metal film through interfacial electron-phonon coupling rather than phonon-phonon coupling. To understand the contrasting behaviors of these two metals, we present a simple thermodynamic model that examines the energy partitioning of hot electrons in metals between interfacial phonons and bulk phonons. We show that the relative strength of these energy-transfer pathways, rather than the absolute strength of the individual processes, plays a key role in determining the effectiveness of metal sensitizers. This relative strength is greater for Au than for Ni, making Au a superior sensitizer for optically induced metal-assisted dielectric breakdown.