We have investigated theoretically by finite difference time domain (FDTD) method and experimentally by Fourier transform infrared (FT-IR) spectroscopy the effect of grating’s parameters on the reflective properties of ridges and holes’ photonic crystal (PC) membrane made of polycrystalline silicon (PolySi) in the mid-infrared (MIR) range (5-15 µm). We found that the interaction of an incident wave with the ridges and the drilled membrane modify 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 …