Optical surface modes in finite photonic crystals based on porous silicon studied by third-harmonic spectroscopy

This work is devoted to the first experimental observation of optical features in spectra of reflectance and third-harmonic generation of the finite one-dimensional photonic crystals based of the porous silicon, which caused by "surface" modes of an electromagnetic field, similarly to surface states of semiconductors and dielectrics. Semiconductor surface states correspond to electronic wave which total reflect from potential barrier crystal-vacuum as well as crystal lattice due to Brag interference. Therefore the electrons can propagate only along the crystal surface. Similar effect can observed in electromagnetic spectrum of photonic crystals. Although ordinary photonic crystals have not potential barrier crystal-vacuum for photons but they have modified density of modes at the surface. The surface waves existed at the interface of two optically different media can be observed using attenuated total reflectance configuration whereas the change of density of modes gives a sensitive tool to observe optical properties in the surface region. For observation optical features associated to surface states of photonic crystal we have to create the photonic crystal with few layers - finite photonic crystal. Also we can observe changes of electromagnetic density of modes by using third-harmonic generation spectroscopy. Dependence of resonances of third-harmonic generation on number of layers in the structure was observed. Increase of Q-factor of photonic crystals lead to wash out optical features of surface modes in a linear spectrum, whereas the nonlinear optics remains sensitive to eigenmodes.