1. Department of General and Applied Physics, Ivano-Frankivsk National Technical University of Oil and Gas, Ivano-Frankivsk, Ukraine
2. Department of Optics Physical, Yuriy Fedkovych Chernivtsi National University, Chernivtsi, Ukraine
3. Department of Solid State Physics and Chemistry, Vasyl Stefanyk Precarpathian National University, Ivano-Frankivsk, Ukraine