Tunable Microcavity Based on Macroporous Silicon: Feasibility of Fabrication

Simultaneous electrochemical etching of deep pores and trenches in silicon was used to fabricate a two-dimensional, photonic crystal slab (PCS). The structure consists of five rows of macro-pores on both sides of a trench-defect, filled with a nematic liquid crystal. Polarized reflection and transmission spectra from the fabricated structure were investigated in the mid-infrared spectral range and were compared with spectra calculated using a scattering matrix method. In order to obtain agreement between the experimental and calculated spectra, a model structure with a complex refractive index of silicon was introduced. This enabled us to take into account losses related to light scattering at the inner surfaces of pores and trenches within the structure. The influence of these losses on the amplitude of the defects and surface Tamm states was analysed using this model. The Tamm states originate from the unstructured Si layer at the interface of the structure and the external medium, air in this case. A quantitative evaluation of the losses was performed by extracting a coefficient from a fit to the experimental spectra. This coefficient was utilised to determine the dependence of the micro-cavity parameters on the number of periods in the PCS. We conclude that a micro-cavity based on macro-porous silicon should not have more than three periods on each side of a defect.