Nondestructive femtosecond laser lithography of Ni nanocavities by controlled thermo-mechanical spallation at the nanoscale

TL;DR

This paper introduces a nondestructive femtosecond laser lithography method to create controlled nanocavities in nickel films, enabling new magnetic and photonic applications by precisely manipulating thermo-mechanical spallation at the nanoscale.

Contribution

It demonstrates a novel laser-based technique for fabricating closed nanocavities in ferromagnetic films with controlled spallation, supported by experimental and molecular dynamics studies.

Findings

Controlled formation of closed nanocavities below ablation threshold

Understanding thermo-mechanical spallation mechanisms

Potential for advanced magnetic and photonic device applications

Abstract

We present a new approach to femtosecond direct laser writing lithography to pattern nanocavities in ferromagnetic thin films. To demonstrate the concept we irradiated 300~nm thin nickel films by single intense femtosecond laser pulses through the glass substrate and created complex surface landscapes at the nickel-air interface. Using a fluence above the ablation threshold the process is destructive and irradiation leads to the formation of 200~nm thin flakes of nickel around the ablation crater as seen by electron microscopy. By progressively lowering the peak laser fluence, slightly below the ablation threshold the formation of closed spallation cavities is demonstrated by interferometric microscopy. Systematic studies by electron and optical interferometric microscopies enabled us to gain an understanding of the thermo-mechanical mechanism leading to spallation at the solid-molten interface, a conclusion supported by molecular dynamics simulations. We achieved a control of the spallation process that enabled the fabrication of closed spallation nanocavities and their periodic arrangements. Due to their topology closed magnetic nanocavities can support unique couplings of multiple excitations (magnetic, optical, acoustic, spintronic). Thereby, they offer a unique physics playground, before unavailable, for magnetism, magneto-photonic and magneto-acoustic applications.