The GdF3/MgF2 bilayer as an antireflective narrow-band ultraviolet filter

The effective work of optical systems, applied in advanced optic and optoelectronic devices, requires antireflective coatings. In this work we focus on designing the bilayer system dedicated to work as a narrow-band filter within UV range of electromagnetic spectrum. Such coatings are applied in lasers where reduction in reflection to a small value at any single wavelength is needed. The bilayer system is based on GdF3 and MgF2 used as materials with middle and low refractive index. The multilayers were obtained by thermal evaporation on a highly purified CaF2 substrates. The spectral dispersion of the refractive index of single layers has been determined by ellipsometric measurements. Thicknesses of single layers included in the bilayer system, aimed to work at specific wavelength, have been optimized based on optical characteristics simulation, including experimentally measured values of the refractive indices. During the deposition, layer thickness and deposition rate were controlled with Inficon XTC/2 thickness measuring system. Optical properties of obtained GdF3/MgF2 bilayer systems have been determined based on spectral dependences of reflectance and transmittance measured with the application of a spectrophotometer. The crystal structure and phase composition of the films have been examined by X-ray diffraction. The result of studies revealed that proper optimization of thicknesses of individual layers creating GdF3/MgF2 bilayer systems makes it possible to obtain the antireflective coating for desired wavelength of electromagnetic spectrum. The GdF3/MgF2 antireflective bilayer will be applied as a narrow-band filter for 238 nm irradiation produced by gas-ion/BBO crystal laser.