Thursday, 24 October 2013, 10:30 a.m.
Sala Grande Palazzina B via alla Cascata 56/C
Some materials aspects in photonic structures: infiltration in multilayers, photonic crystals made with four compounds, inhomogeneous optical media
Dipartimento di Fisica, Politecnico di Milano,
Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Milano
We illustrate the fabrication and the optical characterization of a photonic crystal made by metal oxide nanoparticles, silicon dioxide and titanium dioxide, infiltrated with a nematic liquid crystal. By infiltrating the liquid crystal in the porous structure, we have observed a red shift of the photonic band gap, due to the increase of an effective refractive index of the multilayer. We have thus reported a shift towards shorter wavelengths of 8 nm, with a relatively low voltage of 8 V, corresponding approximately to an electric field of 3.4 V/µm, one order of magnitude smaller with respect to previously reported tunable liquid crystal infiltrated photonic crystals. These results could be very interesting for optoelectronic switches and for low cost displays.
We also present the study of the light transmission properties of four-material based one-dimensional photonic crystals. Their properties have been theoretically investigated by means of the transfer matrix method, that is a general technique that is widely used in optics for the description of stacked layers. With respect to a conventional one-dimensional photonic crystal, the four-material system shows an additional photonic band gap at longer wavelengths. This new gap could be ascribed to a longer-order periodicity that is not occurring in conventional 1DPCs. Furthermore, an interesting behaviour of the four-material system is that intensity and shape of the photonic band gaps are strongly related to the position of the four materials in the sequence constituting the unit cell of the photonic lattice.
Finally, we show how inhomogeneity photonic media affects their light transmission properties. Structures with different disorder are realized by concentrating high refractive index domains in certain unit cells of the original ordered photonic crystals. We have then correlated the disorder to a diversity index used in statistics and information entropy, called Shannon index (the more ordered and uniform the structure, the higher the Shannon index). In 1D, average light transmission decreases by increasing the Shannon index up to a certain value, while it is growing with a Shannon index above this value. Instead, in 2D we have observed an interesting linear behaviour of the average transmission that increases as a function of the Shannon index in the whole range.