Realization of ultrawide omnidirectional photonic band gap in multiple one-dimensional photonic crystals

Theoretical and experimental investigations have shown that the omnidirectional photonic band gap (omni-PBG) of one-dimensional photonic crystals (1D PCs) can be enlarged by including multiple-periodic structures. These structures, consisting of Si-SiO2 alternating layers with multiple periods, Lambda(1), Lambda(2),...,Lambda(M), have great potential for ultrawide omnireflectors operating in the infrared frequency range. To confirm the effect of multiple periods in the 1D PCs, we prepared two types of 16-pair, Si/SiO2 1D PCs with a single period of Lambda(1)=426.9 nm and Lambda(2)=306.9 nm, and one type of 16-pair 1D PC with double periods-the combination of eight-pair Lambda(1) and eight-pair Lambda(2). Theoretically, the normalized frequency range for omni-PBG (Delta omega) in double PC is enhanced by approximately twice that in the single PC. That is, Delta omega increases from 0.086 to 0.166, which corresponds to the wavelength range (Delta lambda) from 520 to 980 nm for Lambda=407.7 nm. Measured reflectance (R) spectra are in a good agreement with the calculated results. For example, the R spectrum of Si/SiO2 double 1D PC (8-pair Lambda(1)+8-pair Lambda(2): Lambda(1)=426.9 nm, Lambda(2)=306.9 nm, and filling factor eta=0.406) exhibits an ultrawide PBG over the wavelength range of 1050-2500 nm for TE polarization at the incident angle of 5 degrees. (c) 2006 American Institute of Physics.