"Dynamic ultrafast laser-induced structural changes and extreme nanostructuring in hard dielectric materials" by Rajeev Dwivedi

Seminar by Rajeev Dwivedi, Laboratoire Hubert Curien

Abstract

Ultrafast lasers have enabled the controlled fabrication of micro- and nano-scale structures on material surfaces, as well as within bulk transparent materials, with sizes ranging from a few tens of nanometers to,a few micrometres. Non-diffracting fields, like the ultrafast Bessel beam, have enabled new interaction geometries, notably achieving high aspect ratios in glass and crystals. Hard crystals, such as sapphire, have proven to be a key element in a vast range of applications, including industrial, optical, and aerospace research. However, attaining precision in processing them remains an arduous task due to their exceptional tolerance to high pressure and temperature. In this study, we report on quantifying the time-resolved dynamics of laser modifications induced by nondiffractive ultrafast laser beams in bulk sapphire, using both qualitative and quantitative phase-contrast microscopy to link optical changes to thermodynamic and structural evolutions. The final morphological changes of irradiated structures are revealed by high-resolution electron microscopy. Observations confirm that Bessel pulse irradiation transforms the pristine crystalline structure into a homogeneous amorphous phase in tens of ns, via the passage through a liquid phase nucleated at the early stages of the process. This nanosecond-lived liquid phase is subject to cavitation at higher energy concentrations during the cooling phase (100 ns), facilitating the fabrication of nanoscale voids with high aspect ratios. The outcomes strongly support bulk modification without shock assistance, governed instead by thermal relaxation. This determines a robust path for extreme laser structuring down to the nanoscale.

This seminar will be held in english