Elastic deformation of thin glass by excimer-laser-induced stress modification

Excimer laser irradiation of borosilicate glass enables precise surface modification through both ablation and stress engineering. This becomes particularly important in the field of high-precision optics, especially for thin mirrors, such as those used in X-ray telescopes, which are highly sensitive to stress-induced deformations. In this work, I demonstrate that spatially homogeneous irradiation with an ArF excimer laser (𝜆 = 193 nm) induces tensile stress by forming a stress-bearing melt layer, resulting in parabolic substrate deformation analogous to thin-film-induced bending. The deformed area is confined to the lateral extent of the irradiation pattern, with curvature magnitude determined by the areal coverage, melt layer thickness and stress. This bending behaviour is attributed to the temperature gradient mechanism (TGM), where a laser-induced steep temperature gradient near the surface causes plastic deformation. Upon cooling, the contraction of the upper surface layers result in tensile stress. I show that patterned irradiation introduces anisotropic stress components, enabling distinct deformation modes such as uniaxial bending. A structural mechanics model is developed that incorporates insights from ns-laser-induced material modifications and ablation dynamics. The presented model is validated through finite element simulations, which closely reproduce the experimentally observed anisotropic bending behaviour. The ability to decouple curvature along principal directions offers enhanced flexibility for figure correction. To demonstrate this, correction schemes using both homogeneous and patterned irradiation are developed and applied to thin glass sheets with functional layers, such as reflective chromium films. The resulting surface profiles are analyzed and compared to evaluate the effectiveness of each approach. Since the laser-based approach is limited to the introduction of tensile stress, a complementary method is investigated using compressively stressed substoichiometric SiO𝑥 (1 ≤ 𝑥 ≤ 2) deposited at elevated pressures. The resulting film stress is tunable via the deposition conditions and the films´ ablation behaviour under ArF excimer laser irradiation is characterized. By selectively ablating these coatings, localized stress relief can be achieved. Proof-of-principle experiments demonstrate the potential of this method for figure correction of optics with compressively stressed coatings. Together, these two approaches, laser-induced tensile stress and patternable compressive stress relief, broaden the toolbox for high-precision figure correction of optical components and pave the way for future fabrication of freeform optics.