Modulation of the refractive properties of 1D and 2D photonic crystal polycrystalline silicon-based membranes in the MIR frequency range

We have investigated, theoretically using finite difference time domain method, and experimentally using Fourier transform infrared spectroscopy, the effect of grating's parameters on the reflective properties of ridges and holes' photonic crystal membrane made of polycrystalline silicon in the mid-infrared (MIR) range (5-15 mu m). We found that the interaction of an incident wave with the ridges and the drilled membrane modifies the reflective spectrum in a similar way. For the two geometries we first found Fabry-Perot effect due to the variation of the effective index of the membrane. Then, in both cases, we have been able to highlight the occurrence of dips in the reflection spectrum which are due to the structuration of the membrane. We showed that for the ridge structure, the mode is confined inside the ridges while for the drilled layer, the mode is spread over the whole membrane. Reflectivity tuning in the MIR frequency range can be achieved and we have calculated that the reflectivity of the photonic membrane can be modulated from 20% to at least 40%. It opens new perspectives in the fields of thermal insulation of buildings, high reflective optical mirrors and coatings and personal thermal management.