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The use of omnidirectional mirrors (an special case of distributed Bragg reflectors) as cladding for planar waveguides is proposed and analyzed. The proposed structure is an all-porous silicon multilayer consisting of a core layer inserted between two omnidirectional mirrors. The transfer matrix method is applied for the modal analysis. The influence of the parameters of the waveguide structure on the guided modes is analyzed. These parameters are the layer thickness and number of periods of the omnidirectional mirror, and the refractive index and thickness of the core layer. Finally, the confinement of the omnidirectional mirror cladding is analyzed with respect to two other different distributed Bragg reflector claddings. [ABSTRACT FROM AUTHOR]

Multilayers consisting of the periodic repetition of two layers with different refractive indices (nH and nL) may have an omnidirectional band gap, the width of which depends on the incidence medium and on the refractive index ratio nH/nL. In porous silicon, this ratio is limited by the material and fabrication characteristics. This article makes a theoretical study of two multilayer structures in order to enlarge the omnidirectional band gap with the same refractive index ratio by varying only the thickness of the period. These structures are named balanced and unbalanced mirrors and consist of a few periodic multilayers stacked together. Porous silicon balanced and unbalanced mirrors with enlarged omnidirectional band gap for 1.55-μm applications are proposed. The results obtained with these mirror structures are compared with the results obtained with a chirped structure with the same characteristics. [ABSTRACT FROM AUTHOR]

We report on the fabrication and characterization of a porous silicon omnidirectional reflector formed by periodic substructures stacked together. For these substructures, a low refractive index contrast has been used, resulting in substructures without omnidirectional reflectivity band. The use of a low refractive index contrast involves the reduction of the requirements to obtain omnidirectional reflectors. We demonstrate the existence of an omnidirectional reflectivity range with a high gap-to-midgap ratio by means of reflectivity spectra measurements for a range of incidence angles. The results are in good agreement with a theoretical model of the reflector. The fabricated structure is the first reported porous silicon reflector suitable for 1.55 μm applications. [ABSTRACT FROM AUTHOR]

Abstract: Two multilayer structures made of nanoporous silicon layers are designed, fabricated and characterized. The layers that form the structures are characterized by spectroscopic ellipsometry to determine their refractive index and etch rate. The first structure is a periodic structure that consists of the repetition of two layers with different refractive indices and thicknesses. The second structure is formed by two different periodic structures stacked together, being their bandgaps centered at different wavelengths and with common ranges of high reflectivity. The reflectivity spectra for different incidence angles of the periodic and the stacked structures are measured and the existence of an omnidirectional bandgap is analyzed. A model of the stacked structure is realized and its simulated results are compared with the measured reflectivity spectra. [Copyright &y& Elsevier]

Abstract: We have designed and fabricated a porous silicon (PS) microcavity that shows a reflectivity resonance around 3μm in between two spectral regions with high reflectivity values. The microcavity has been simulated following the photonic crystals formalism, which results in good agreement with the measured spectrum. The reflectivity spectrum of the microcavity has been analyzed under different humidity conditions. We demonstrate that the reflectivity resonance shifts to higher wavelengths and that the reflectivity decreases when the humidity increases. In addition, the reflectivity spectrum of the as-prepared device is recovered when the humidity returns to the initial laboratory conditions. Finally, the effect on the reflectivity spectrum of a condensed water layer at the surface of the microcavity is also analyzed. [Copyright &y& Elsevier]

Abstract: In this paper we present the AC impedance analysis of Au/porous silicon contacts in order to investigate their conduction mechanisms. The porous silicon layer was obtained by electrochemical etching of the p-Si wafer. The measurements were made between 5Hz and 10MHz, at room temperature and in the DC range from 0 to 2V. An electrical equivalent circuit was used to fit the experimental data. The voltage dependence of the fitting parameters led to a deeper understanding of the physical parameters that limit the conduction mechanisms of the Au/PS contacts. [Copyright &y& Elsevier]

Abstract: We present the design and study of waveguide structures based on porous silicon where the light confinement is not due to the usual total reflection effect but to the use of photonic crystals (PCs) as confining walls. These PC are omnidirectional mirrors (OMs), consisting of the periodic repetition of two porous silicon layers with different refractive indices and thicknesses. They reflect the radiation for all angles of incidence within a frequency range called the omnidirectional band gap (OBG). We have followed the PC formalism to investigate the properties of the OM as a multimode waveguide: the number of modes within the band gap, their field spatial distribution and their confinement as a function of the frequency and the core thickness. [Copyright &y& Elsevier]