Development and Precision Molding of Optical Glasses with High Refractive Index for Optical Applications

Abstract

The structure and optical properties of phosphate glasses containing bismuth or niobium oxides for subwavelength structure (SWS) optical elements were studied. The glasses containing a large amount of bismuth and niobium oxides had refractive indexes higher than 1.8 and low deformation temperatures. However, these high refractive index glasses were yellowish due to the electronic transition in trivalent bismuth ions and pentavalent niobium ions. The structure of the glasses was investigated by X-ray, IR, and Raman spectroscopy. NbOx and BiOx formed clusters as the bismuth and niobium oxide content increased. The results showed that the local structure around bismuth and niobium ions was related to the coloration. Based on these results, we developed high refractive index glasses that were used for precision molding to fabricate periodic SWSs. One-dimensional periodic SWSs were fabricated on the glass surface by a precision molding method using a SiC mold on which the reverse of the SWS pattern was carved and a flat SiC mold. One-dimensional SWSs with a high aspect ratio were fabricated on one surface of the glass plate. The niobium phosphate glass plates with the one-dimensional SWS showed phase retardation higher than 1/8l between TE- and TM-polarized beams at 400 nm, demonstrating that a wavelength plate can be fabricated by our precision molding technique.