Active NIR filter with a free-standing porous silicon microcavity containing liquid crystal

We present the design of an interference filter containing a free-standing porous silicon (PS) multilayer film with a microcavity. The estimated pore diameter is 40 - 70 nm. The PS film is installed into a planar glass cell and infiltrated with a nematic liquid crystal (LC) mixture E7. The resonant wavelength of the structure is continuously tuned using thermal dependence of the LC effective refractive index. The reversible tuning of the resonance in the range of 13 nm is demonstrated. Spectra of the structure with the LC in the nematic phase were simulated using the Frank free energy approach with an assumption of the LC molecular configuration being escaped radial (ER). These spectra are very close to corresponding experimental data. Thus, the ER model for nematics in pores narrower then 100 nm is still applicable.