Tunable NIR filter based on a free-standing porous silicon film containing nematic liquid crystal

We present the design of a thermally tunable interference filter for the NIR range. The filter is based on a free-standing porous silicon multilayer filled with the nematic liquid crystal mixture E7. The porous silicon stack is constituted by a quarter-wave microcavity sandwiched between two distributed Bragg reflectors. Reversible thermal tuning of the microcavity resonance is obtained in the range of 13 nm. Thermal dependence of the effective refractive index of the liquid crystal in the sample is characterized using variable angle spectroscopic ellipsometry. The non-axial liquid crystal director configuration is found inside silicon mesopores. The distribution of the liquid crystal director within pores is simulated using the Frank's free energy approach. Assuming the molecular configuration of the liquid crystal to be escaped radial, we have determined the value of the surface anchoring strength in the mesoporous silicon to be less than 10(-6) J m(-2). By fitting the transmittance spectra, using an effective medium approximation and a transfer matrix method, the liquid crystal is found to fill 84% of the void inside the porous sample. The good agreement between the experimental data and the theoretical curves justifies the Frank's approach and the hypothesis of escape radial configuration of nematics confined in silicon mesopores.