DOCTORAL CANDIDATE
Shreyas J. Kashyap

SUPERVISORS
Matthieu Lancry
Prof. Gang-Ding Peng

Femtosecond Lasers; Optical Engineering; Photonics; Sensors; Glasses; Materials Engineering; Fibre Bragg Gratings

High-temperature (HT) materials are building blocks to a wide range of industries, including aviation / space (engine, launcher), manufacturing (3D laser additive manufacturing), or again photonics (optical temperature / pressure /environmental sensing etc.). In most HT applications, refractory crystalline oxide materials are selected such as Al₂O₃ , ZrO₂ , YAG, directionally solidified eutectic ceramics, metal alloys, carbides / nitrides, due to their high melting points, good resistance to oxidation and abrasion. HT photonics applications would benefit from oxide glass-based materials, since they bring mandatory requisites such as compactness, lightness, flexibility, high-transparency, chemical /radioactive / electromagnetic resistance, and complex manufacturing shaping. To make a HT photonic device, the glass must be functionalized (e.g., gratings or waveguides require stable refractive index modulations). The tool of choice for this purpose is femtosecond laser direct writing (FLDW). The latter enables very high light intensities (10-100s TW/cm²) to be deposited on (2D) and inside (3D) a material, creating unique forces to induce oriented transformations such as migration of chemical species, oxide dissociation, or nano-crystallization. To date, induced modifications employed to fabricate thermally stable devices are dedicated to silica or lightly doped-silica glasses, through the formation of porous nanogratings, limiting their use for few hours beyond 1100 °C. The main objective of this proposal is to overcome this technological lock.

Conventional porous glass modifications survival at HT is dictated by the glass viscosity, and from this view silica is undoubtfully the best candidate due to its highest viscosity at HT. To go beyond the restrictions imposed by silica glass, I will explore a novel approach where fs-laser irradiation is used to photo-precipitate refractory nanocrystals inside high-viscosity silicate glasses (such as ZrO₂, Mullite, Sapphire, YAG). As opposed to a purely viscosity-driven erasure mechanism, I foresee that the stability of the fs-laser induced nanostructures will be dominated by other mechanisms due to the incompressible (unlike pores) and refractory nature of the photo-precipitated phases (melting points between 1600 °C and 2700°C). Therefore, we anticipate these nanocrystal/glass nanogratings to outperform nanogratings structures and enable device survival at temperatures >1200°C for months. Moreover, we will master the size, distribution, and morphology of the nanogratings within the focal volume (few µm³) using a combination of light properties (polarization, phase, intensity) and thermal ones (laser heating effects).