Absorption, Reflection and Nonlinear Optical Effects

The interaction of photons with electrons in the porous structure, which is the topic of this section, provides information about the PS band structure, while the excitation of vibrational modes of molecules present in the porous structure by photons gives information about its chemical composition, as discussed in Section 6.6. 

The dependence of the optical properties of silicon on its microstructure is shown by the color figure in the lower left corner of this books cover. Despite the fact that the thickness of all PS samples shown has been chosen to correspond to a 20 pm thick film of bulk silicon, the difference in light transmission is dramatic [Le27]. 

A certain anisotropy of the refractive index along specific crystallographic axes indicates that the microstructures in the porous network are not spherical but somewhat elongated along the PS growth direction [Mi4], This birefringence is below 1% for micro PS, while it may reach values in the order of 10% for meso PS films formed on (110) oriented silicon wafers [Ko22]. 

The refractive index of micro PS, nPS, for wavelengths in the visible or near IR regime is found to decrease with porosity p [Lol] and can be roughly approximated if the refractive index of bulk silicon nsi for the wavelength in question is known, using 

This dependence of nPS on p can be used to fabricate optical superlattices, e.g. Bragg filters, by modulating the porosity of a PS layer, as discussed in Section 10.5. 

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