Ellipsometric Study of Liquid Crystal Infiltrated Porous Silicon

Abstract

Porous silicon (PS) based devices are nowadays under an intense and widespread investigation in the optoelectronic and sensing fields. Recently, the range of possible applications has been widened by the use of the liquid crystals (LCs), which can be infiltrated in the PS sponge-like structure. The large changes of the optical properties, as exhibited by LCs under the action of electrical or thermal fields, allow developing a new family of optical devices like tunable PS based multilayer mirrors, microcavities and optical filters.

In this work, we have optically characterized the LC-PS heterogeneous composite as a guest-host system by means of the variable angle spectroscopic ellipsometry (VASE). A PS layer, 450 nm thick, has been infiltrated with the nematic LC 5CB, and the main optical parameters, anisotropic refractive indices and thickness of both materials, have been estimated below and above the isotropic transition temperature, at 27.0°C and 38.0°C respectively. We have found a clear indication that the LC molecules tend to align parallel to the direction of the pore columns.

Keywords:

• anisotropy
• ellipsometry
• liquid crystal
• porous silicon
• refractive index

Acknowledgments

We are deeply grateful to Dr. Thomas Wagner, by Lot-Oriel Europe (Germany), for technical support about the WVASE32^TM software, and to Mr. Alfredo Pane, by CalcTec Srl (Italy), for technical support in the LC infiltration.