FTIR and Raman investigation of vertically etched silicon as 1D photonic crystal

The reflection spectra of composite materials on a base of grooved silicon and grooved silicon infiltrated with nematic liquid crystal (LC) have been calculated using the optimal parameters of a grooved silicon matrix suitable for the infrared range. The grooved silicon structures with different lattice constants (A=16, 12, 8 and 4 μm) have been designed and prepared. An important parameter of these structures is the thickness of the silicon walls (DSi). This has been obtained using simulations of the spectra. This parameter was used for further analysis of the spectra of composite material “grooved Si-LC”. The experimental reflection reaches a maximum of 65% (with signal modulation from maximum to minimum up to 55%) for the composite structures with a small number of lattice periods. This makes these structures very attractive for a number of potential applications. The analysis of the polarised infrared spectra of Si structures infiltrated with LC allows one to determine the orientation and the refractive index (NLC) of the liquid crystal. For the samples with a distance between Si walls of 6-10 μm, it was found that the refractive index was NLC=~ 1.5 for p-polarised light and NLC≠1.5 for s-polarised light. This leads to the conclusion on the planar orientation of liquid crystal molecules with respect to the Si walls. For the samples with a distance between Si walls less than 3 μm, a homeotropic alignment of liquid crystal molecules has been found. Micro-Raman spectroscopy has been applied for analysis of stress in such Si structures. The maximum stress of about 2 GPa was obtained on the top of Si walls (under the silicon dioxide layer).