Cadmium doping-induced dual photonic stopbands in sol-gel SiO2/TiO2 Bragg reflectors: Structural, Raman, and optical investigations

The effect of 2 at. % Cd incorporation on sol-gel-derived SiO2/TiO2 Bragg reflectors was investigated by structural, vibrational, and optical characterization. Cd incorporation preserved the anatase TiO2 phase, reduced the crystallite size from 11.38 to 9.92 nm, increased the unit-cell volume by 0.49%, and shifted the Eg Raman mode from 145.20 to 144.65 cm−1. The undoped multilayer exhibited one reflection-dominated stopband at 406 nm, with a width of 154 nm and a derived reflectance of 97.21%. The Cd-doped multilayer exhibited two reflection-dominated stopbands at 384 and 725 nm, with widths of 70 and 320 nm and derived reflectance values of 97.33% and 97.27%, respectively. The corresponding absorbance values were low, at 1.81% and 0.08%, confirming reflection-dominated behavior. For the undoped structure, m-lines-derived effective parameters predicted a Bragg wavelength of 415.1 nm, within 2.2% of the experimental feature. A homogeneous-layer transfer-matrix-method model reproduced its principal high-reflectance region, whereas an inhomogeneous effective optical profile was required to reproduce the two high-reflectance regions of the Cd-doped multilayer. These results establish a Cd-associated dual reflection-dominated stopband response spanning the violet and red-near-infrared regions, supporting the potential of Cd-modified SiO2/TiO2 multilayers for dual-band optical filtering.