Metallic assisted guided-mode resonances in slab waveguide gratings for reduced optical switching intensity in bistable devices

A thin metal layer is introduced into a slab waveguide grating with guided-mode resonances to reduce the switching intensity in its bistable operational mode. A dielectric grating put on top of the metal layer plays the role of a coupling element between the normally incident light and the guided mode in the slab waveguide grating. The presence of the metal layer increases the reflectivity of the optical device that as a consequence exhibits high-reflection side bands and close-to-zero reflectivity drops. Several structures are designed by changing the grating depths and the metal layer thicknesses, and their influences on linear and nonlinear characteristics are analysed using the finite-difference time-domain method. We found that the metal layer increases the quality factor of guided-mode resonance filters. Numerical results show that the quality factor improves 5.6 times and the switching intensity is reduced 45 times when these devices are compared to typical slab waveguide gratings in the same working conditions, in terms of polarization and operating wavelength.