Fabrication effects in the optical performance of DOEs engraved with femtosecond lasers

Diffractive Optical Elements (DOEs) are amplitude and/or phase masks that can be applied to light beams to modify their phase and/or intensity distribution. They are applied in holography, beam shaping, generation of exotic beams (Hermite-Gauss, Laguerre-Gauss, Gauss-Bessel, accelerating or vortex beams, etc.), generation of custom intensity profiles (top-hat, lines, figures, etc.), atomic physics, quantum optics, etc. They can be implemented using Spatial Light Modulators (SLMs) or micro-structured materials. Femtosecond laser writing is a very promising technique for fabricating photonic and micro-optics devices in metallic and dielectric materials. It consists on the removal (ablation) or modification of the irradiated material. Due to the short pulse duration of fs pulses, the energy is deposited in a localized region by nonlinear absorption mechanisms, allowing a very precise control of the material removal/modification. Compared to other methods, it has many advantages like a reduction of the amount of energy required to fabricate devices, and the absence of pollutant chemicals, becoming one of the most environmentally friendly fabrication techniques. One technique for implementing amplitude modulation DOEs is using dielectric samples covered with a metal thin film (few hundreds of nm thick). Then, the metallic film is selectively removed by laser ablation. This allows the engraving of a binary amplitude mask, where the remaining metallic coating reflects the electric field while the exposed dielectric area supports its transmission. Hence, these masks may work in both transmission and reflection. Although laser processing of DOEs has been successfully proved, some challenges still remain and should be addressed to optimize their behaviour. Several problems may arise during the laser ablation process. One of the problems treated in this contribution is the effect of damage on the dielectric substrate under the metallic coating. This happens since the light used to remove the metallic layer can also affect the dielectric sample, producing damage, variations of the material lattice or compositional changes, thus altering its refractive index. This variation may affect the effect of the DOE when it is used in transmission configuration. Another issue related to the ablation process is the different ablation strategies to engrave a given spatial distribution. Here, the laser is driven to process a matrix of points, or it can work in raster mode across the sample. We will analyze these two effects to properly understand the limitations of the technique and to find some useful strategies to overcome them when engraving DOEs through laser ablation.