AbstractThe refractive index contrast in dielectric multilayer structures, optical resonators and photonic crystals is an important figure of merit, which creates a strong demand for high quality thin films with a very low refractive index. SiO2 nano-rod layers with low refractive indices n = 1.08, the lowest ever reported in thin-film materials, is grown by oblique-angle e-beam deposition of SiO2 with vapor incident angle 85 degree. Scanning electron micrographs reveal a highly porous columnar structure of the low-refractive-index (low-n) film. The gap between the SiO2 nano-rods is ≤50 nm, i.e. much smaller than the wavelength of visible light, and thus sufficiently small to make scattering very small. Optical micrographs of the low-n film deposited on a Si substrate reveal a uniform specular film with no apparent scattering. The unprecedented low index of the SiO2 nano-rod layer is confirmed by both ellipsometry measurements and thin film interference measurements. A single-pair distributed Bragg reflector (DBR) employing the SiO2 nano-rod layer is demonstrated to have enhanced reflectivity, showing the great potential of low-n films for applications in photonic structures and devices.
Low Refractive Index SiO2 Optical Thin Films by Sputtering and Electron Beam Evaporation