INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY VOL.1, NO. 2, AUGUST 2006 IJMOT-2006-6-153 © ISRAMT 2006 Large Frequency Range of Omnidirectional Reflection in Si-based One-Dimensional Photonic Crystals S. K. Singh*, Khem B. Thapa and S. P. Ojha Department of Applied Physics, Institute of Technology, Banaras Hindu University, Varanasi-221005, INDIA Tel: +91-542-2307040; Fax: +91-542-2368428; E–mail: sks_m2002@yahoo.co.in, spojha@yahoo.com Abstract: We study the omnidirectional photonic band gap in one dimensional photonic crystal of Si/SiO 2 structures. Three structures of Si/SiO 2 multilayer have been studied. The first structure has filling factor 0.333 and the second structure has filling factor 1.50. The third photonic structure is a combination of first and second structures. Using transfer matrix method, the reflectivities of photonic crystals are plotted as a function of normalized frequency and omnidirectional PBGs are measured. We found that the large frequency range of omnidirectional PBG can be obtained from Si/SiO 2 system by combination of 1D photonic band gap structures. For obtaining this large omnidirectional PBG, the directional PBGs of constituent 1D PCs should overlap each other. Index Terms: Omnidirectional PBG, photonic crystal, transfer matrix. I. INTRODUCTION Yablonovitch [1] in 1987 pointed out the possibility of the realization of photonic band gap, localized defect modes, and their applications to various optoelectronic devices. In the same year, John [2] discussed the strong localization of electromagnetic waves in disordered photonic crystals. Ojha et al. [3] observed filtering properties in PBG materials and extended the idea for constructing monochromators [4]. Photonic crystals are periodic dielectric or metallic structures in which propagation of electromagnetic wave is forbidden within a certain frequency. This frequency range is called the photonic band gap which is analogous to the electronic band gap in ordinary materials. These materials have many potential applications in optoelectronics and optical communications, such as resonant cavities [5], optical waveguides [6], light emitting diodes [1], superfast optical switches and detectors [7], etc. Three dimensional (3D) photonic crystals may exhibit absolute photonic band gap in which propagation of light is prohibited regardless of the direction of the wave vector and all polarization. There is a significant progress in the fabrication of 3D photonic crystals to obtain complete band gap from microwave to visible. However, due to technological problem and high cost, the applications of the 3D photonic crystals are limited. The 1D photonic crystals are attractive since their production is more feasible at any wavelength scale and their analytical and numerical calculations are simpler. Besides 2D and 3D photonic crystals, 1D periodic structure such as superlattices can also exhibit the property of omnidirectional reflection [8-10] provided the optical indices of supperlattice constituents are chosen appropriately. Metallic mirrors reflect light over a broad range of frequencies incident from arbitrary angles (i.e. omnidirectional reflector). However, at infrared and optical frequencies, they absorb a small fraction of incident optical power. An omnidirectional dielectric mirror i.e. 1D PC has low loss. These properties make omnidirectional dielectric mirrors ideal candidate for applications in which a beam of light with an unknown polarization is to arrive from any direction and in which loss of power, although very small, is not desirable. In 1998 Fink et al. [8] first reported that one dimensional dielectric lattice displays total omnidirectional reflection for incident light under certain conditions. They 686