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Femtosecond laser exposure of fused silica in the non-ablative regime can lead to localized modifications of the material structure. In particular, over the last two decades, it has been demonstrated that one can tune the chemical, mechanical and optical properties of dielectrics by tightly focusing a femtosecond laser in their volume. In this work, we focus our attention on the thermo- mechanical properties of the fused silica. In particular, we demonstrate that one can tune the thermal expansion coefficient of the glass by using different exposure conditions and specific exposure patterns.
Specifically, the thermomechanical properties of the homogenous, so-called, Regime I exposure, and the nanogratings formation ( Regime II) are studied by measuring the out-of-deflections of a bimorph structure using digital holographic microscopy, over a temperature range of the 0 – 100 degrees Celsius. The thermomechanical response depends not only on the type of structural change, but also on the induced stress in the pristine material as shown by others. The latter allow us to alter the thermal strain locally by using different type of writing strategies, in order to prestress selectively the material. This study is the first step to control the thermal expansion of transparent material using ultrafast laser, which is in particular interesting to near zero-thermal expansion of opto-mechanical devices.
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