Localized defect modes in dual-periodical multilayer structures based on porous silicon

We present an experimental study of the resonance modes in dual-periodical multilayer structures based on porous silicon. These multilayered structures are composed by stacking N times two substructures A, and B, i.e., (AnBm)N. The An and Bm are in turn composed of two different period units, a and b, respectively, where subscripts n and m are the period number in the a and b substructures. Both substructures a and b consist of a pair of alternating layers with high and low refractive indices n1 and n2, respectively. The thickness parameters of the dielectric layers in a and b are all different. We observe several resonance transmission peaks due to the periodical repetition of the AnBm structure. The number of resonance peaks, their full width at half-maximum (FWHM), etc., can be controlled by selecting the structural parameters of the system. The experimental data are in good agreement with those calculated using the transfer matrix method. These optical superlattices are very promising, since they can be designed so that the reflectance response presents a determined number of resonance modes in the most important window for optical communications, making them good candidates for direct applications.