Pieter Vlugter
The thermomechanical properties of materials are among the most important properties for
designing precision instruments, in particular for devices seeking high-dimensional stability,
invariant temperature-elastic properties or low thermoelastic losses. Dedicated materials
have been created for these purposes, such as Zerodur® and Silinvar®.
In this thesis work, we propose a method for selectively tuning the thermomechanical
behavior of transparent materials using ultrafast lasers. Specifically, we investigate laser-exposed fused silicaâs thermal expansion coefficient and thermoelastic properties. Fused
silicaâs already outstanding physical properties, low-thermal expansion coefficient and such
as low thermoelastic losses, offers a promising starting point for exploring its limits as a high
performance thermomechanical materials.
This study unraveled the preponderance of certain laser induced modifications in this pro-cess. The results are obtained using designed micro-mechanical devices. A bimorph structure
is used to characterize the thermal expansion, while a resonant cantilever is implemented for
investigating thermoelastic properties. Using these micro-instruments, we show that: 1) These
laser-induced modifications can be used to tune silica thermal expansion coefficient perma-nently. In particular, we demonstrate that a given exposure regime leads to a lower thermal
expansion than the pristine material, while other exposure conditions yield the opposite result.
2) The positive temperature-elastic coefficient of silica is reduced by nearly 50% after laser
exposure. Such performance turns fused silica into a good candidate for temperature-stable
resonators. 3) The Youngâs modulus at room temperature can display laser-induced elastic
anisotropy controlled by the laser polarization. 4) The thermomechanical properties can be
fine-tuned with an extra thermal annealing step.
This thesis is a first step towards designing materials with arbitrary thermomechanical
behavior, starting from a given transparent material. This approach stands out from other
methods as the material properties can selectively be transformed at the microscale level with
unique and tailored thermomechanical properties, and this, in arbitrary substrate shapes. It
enables the fabrication of metamaterials with designed thermomechanical response. From
a device point-of-view, this research further expands the capability of femtosecond laser to
tune refractive index and etching selectivity towards thermomechanical properties, forming a
larger palette of possible ultrafast laser-induced functionalities. This addition further enlarges
the potential of femtosecond laser for making highly integrated multifunctional devices. This
is in particular interesting for precision instruments, temperature-responsive sensors and
actuators, as well as temperature-stable optomechanical devices.
EPFL2020
