Optical elements for polarization control are one of the main parts in advanced laser systems. The state and intensity of polarized light is typically controlled by optical elements, namely waveplates and polarizers. At high powers, both elements suffer low resistivity to laser radiation. Also, standard waveplates, made from solid or liquid crystals, are fragile and difficult to use in small systems, while Brewster-type polarizers leads to light beam displacement in the optical path. Introducing optical anisotropy to thin film coating structures allows to control the polarized light, therefore, evaporated optical elements do not indicate earlier mentioned drawbacks.
In this work serial bi-deposition technique was employed to form nano-structured anisotropic layers evaporating amorphous silica material. Placing the substrate at glancing angle self-shadowing effect is induced, which causes the growth of columnar nano-structure with elliptical shape cross-sections. Anisotropic properties can be improved by rotating the substrate half-turn around surface normal every 5 seconds. Principal illustration and SEM image of anisotropic and dense layers are shown in Fig.1. Combination of porous birefringent and dense isotropic thin films was used to form two spectrally separated Bragg reflection zones for perpendicular polarizations (S-parallel to shadowing direction and P-perpendicular to it). Modeled spectrum of multi-layer design of normal incidence polarizator is shown in Fig.2. Marked transmission district perfectly indicates polarizer properties: high‑transmission for one polarization, and high‑reflection for perpendicular polarization. Furthermore, the combination of birefringent nano-structured and isotropic layers allows to form highly transparent (T ~ 99 %) waveplates. Both optical elements can be manufactured using only one material by changing only its structural morphology. In this work, silica material was used, which has wide band gap and, therefore, high resistivity to laser radiation in UV region. Optical resistance was tested by measuring laser induced damage threshold of both optical elements at the wavelength of 355 nm in ns regime.
Full scale investigation was accomplished to determine optimal optical and structural characteristics of anisotropic nano-structured and isotropic dense silica layers. A novel approach was shown that the combination of both morphologies allows to produce polarizers and wave‑plates with superior resistivity to laser radiation.
