Tailoring of birefringence and dichroism in anisotropic porous silicon nanostructures with free charge carriers for infrared photonic applications

Abstract

A contribution of free charge carriers in an ensemble of anisotropic silicon nanocrystals to the optical properties in the infrared region has been numerically analyzed. Utilizing a generalized effective medium model that accounts for the shape anisotropy of silicon nanocrystals and mobile charge carriers the reflectance, absorption, and transmittance were found to be strongly dependent on the concentration of free charge carriers. For charge carrier (hole) concentrations ranging from \(10^{16}\) to \(10^{20}\) \(cm^{-3}\), significant birefringence and linear dichroism (absorption anisotropy) are observed, along with a non-monotonic dependence of the refractive index and transmittance coefficients for perpendicular polarization directions. The simulation results align well with experimentally measured transmittance spectra of in-plain anisotropic porous silicon films prepared electrochemically from heavily boron-doped (110)-oriented crystalline silicon wafers. Furthermore, the obtained results are discussed in view potential applications in all-silicon based optical switches and modulators and other active elements of the silicon photonics.