| Abstract [eng] |
In recent decades, diffractive optical elements (DOEs) have become essential components in modern photonics, enabling precise spatial modulation of light phase and amplitude. While traditional lithography provides high resolution, it is a complex, multi-step, and expensive process. Direct laser writing using ultrashort pulses offers a flexible, maskless alternative. This master’s thesis aims to investigate and optimize the fabrication process of surface phase DOEs on transparent dielectrics using ultraviolet (UV) femtosecond laser pulses (𝜆 = 257 nm), and to evaluate the effects of chemical etching and thermal annealing on the fabricated structures. A systematic parametric analysis of laser ablation was conducted on soda-lime glass, borosilicate glass (BK7), fused silica (UVFS), and organic glass (PMMA). The results demonstrated that in inorganic glasses, the ablation depth grows linearly with the number of pulses, whereas the crater diameter remains relatively stable. Depth sensitivity analysis revealed that forming 300, 600, and 900 nm phase steps with a strict ±15 nm tolerance requires precise pulse energy control, with the permissible energy window narrowing from 1.4 𝜇J for the first level to 0.4 𝜇J for the deepest level. Chemical post-processing in a 10% hydrofluoric acid (HF) solution yielded distinct morphological modifications depending on the material. In fused silica, an advantageous polishing effect was observed, reducing surface roughness. In BK7 glass, HF etching successfully transformed Gaussian-shaped craters into flat-bottomed rectangular profiles with steeper walls, which is highly beneficial for phase DOEs. Conversely, soda-lime glass suffered from rapid isotropic expansion and increased roughness, indicating that weaker (1–2%) HF concentrations are necessary for optimal control. To scale up the fabrication, a galvanometric scanner equipped with a telecentric F-theta lens ( 𝑓 = 160 mm) was implemented. This setup maintained high depth control while drastically increasing the writing speed—fabricating 3.8×3.8 mm holograms in just 25 seconds at an optimal 20 kHz repetition rate. Optical characterization revealed that maximum diffraction efficiency requires at least a 30% spatial overlap of the ablated craters (e.g., 14.5 𝜇m craters with a 10 𝜇m pitch). Finally, thermal annealing at 300◦C for 1 hour successfully annihilated UV-induced color centers, restoring glass transparency. This thermal treatment increased the overall diffraction efficiency by a factor of 1.15, reaching a maximum of 51% (approaching the ∼ 60% theoretical limit for 4-phaselevel DOEs). The successful realization of complex structures, such as the Vilnius University crest and multi-channel beam splitters, confirms that the optimized direct UV femtosecond writing method is highly reliable and well-suited for the rapid prototyping of diffractive optics. |
