Optical Thin Films with Very Low Refractive Index and Their Application in Photonic Devices

The 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 nano-rod layer with vapor incident angle thetas = 85deg. 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 les 50 nm, i.e. much smaller than the wavelength of visible light, and thus sufficiently small to make scattering negligibly 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