Design and analysis of resonant leaky-mode broadband reflectors

Devices based on the guided-mode resonance (GMR) effect are very promising elements in the areas of optics and electromagnetics. They can provide variety of spectral responses only founded on periodically patterning of a single optical layer on a substrate or as a free-standing membrane. Although the main manifestation of the GMR effect is sharp resonances in the reflection spectra, by proper selection of device parameters, attainment of variety of optical spectral responses such as narrow bandpass/bandstop filters, broadband reflectors, polarizers and so on is possible. Since in this kind of elements, light is coupled into waveguide leaky modes through a subwavelength grating structure, the device works in the second (leaky) stop-band. Response of such devices is highly dependent on the modal characteristics of the guiding layer. Broadband high reflectors, based on periodically patterned single layers (with substrate or as a membrane), are promising and attractive elements in the area of optical devices and have recently found practical application as top mirror in tunable vertical cavity surface emitting lasers (VCSELs). In this paper, a single layer, strongly-modulated GMR-based broadband high reflector is designed for 1.45-2.0 mu m band for TE and TM polarizations using particle swarm optimization (PSO) technique; a robust, easy to implement evolutionary technique inspired from the behavior of particles in a swarm searching for their requirements resources. A silicon-on-insulator (SOI) structure has been chosen in which the binary patterned silicon layer act as both grating and waveguide. The spectral and modal characteristics of these elements are analyzed utilizing rigorous electromagnetic techniques like rigorous coupled-wave analysis (RCWA) and modal techniques. The designed reflector for TM and TE polarizations provide similar to 520 nm and similar to 125 nm bandwidth at > 99% reflectance, respectively. Also, showing the reflection and transmission maps as well as the mode profiles, we investigate the effect of refractive index modulation and the thickness of the grating on device's spectral response.