Effective Optical Properties of Nanoporous Silicon

Nanoporous materials consist of nanosize voids embedded in a solid matrix. The pores can be closed or open and have various shapes and sizes. Their applications range from optical and optoelectronics devices to biosensors. In order to effectively utilize and characterize nanoporous media for these various applications, models that describe their effective optical properties are necessary. Numerous effective medium models have been proposed. However, validations of these models against experimental data are often contradictory and inconclusive. This issue was numerically investigated by solving the two-dimensional Maxwell’s equations in absorbing nanoporous silicon thin-films. All interfaces are assumed to be optically smooth and characteristic pore size is much smaller than the wavelength of incident radiation so electromagnetic wave scattering by pores can be safely neglected. The envelope method was then used to retrieve the effective index of refraction and absorption index from the computed transmittance. The numerical results agree very well for both the index of refraction and the absorption index with a recent model obtained by applying the Volume Averaging Theory (VAT) to the Maxwell’s equations. However, commonly used models such as the Maxwell-Garnett, Bruggeman, parallel, and series models systematically and sometimes significantly underpredict the numerical results.