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Inverse opals photonic crystals
The synthesis of a very large scale, silicon based PBG material offers a number of imminent possibilities, involving further infiltration of this highly open structure with light emitting molecules or atoms. The resulting luminescence and lasing characteristics of light emitting species near a three-dimensional photonic band edge are expected to be quite striking. Important low threshold all-optical switching effects and anomalous nonlinear optical response have been predicted . In this regard, it is useful to explore self-assembly methods for creating diamond lattice templates from which a considerably larger PBG may be achieved . It is also important to generalize the template formation procedure to engineer waveguide channels and specified point defects through which light can flow. Methods of soft lithography  may prove effective in realizing such “circuits of light”.
This can be qualitatively seen without recourse to a full band calculation. An increase in the dielectric contrast conveys an enlargement of the scattering strength (er) of the structure. The scattering strength can be defined as the ratio scattering/no scattering of a dielectric periodic structure and is given by the expression:
This expression depends on the dielectric constant distribution of the PX and, for a two-component system, mainly, through the filling factor. It has been shown that for er>1 full photonic gaps begin to emerge provided an adequate structure is granted. The infiltration of semiconductors within the opal has many advantages as compared to other materials. Firstly, because of the large dielectric constant of bulk semiconductors. Secondly, due to the highly efficient band to band emission of the semiconductor. In this scenario the effect of the PX on the luminescent properties of guests can be conveniently studied. If a hybrid system composed by a semiconductor and a full gap PX judiciously coupled is built, spontaneous emission could eventually be wholly inhibited. This is the first step towards a quasi-zero threshold laser. The semiconductor growth inside the opal voids can be achieved by several methods. The most common procedures are Metal Organic Chemical Vapour Deposition (MOCVD), mainly for III-V semiconductors, and Chemical Bath Deposition (CBD), best suited for II-VI compounds. Also, some high dielectric constant insulators as TiO2 have been synthesized through CVD techniques.
FIG. 1. The scattering strength vs. the percentage of the pore volume infilled for different guest materials.
In Fig. 1, the scattering strength dependence on the percent volume of pore filled is shown for several materials (labelled by their dielectric constant). It can
be seen that for materials of e > 12, er reaches the value 1, needed for the optimum photonic crystal performance. In addition, as Fig. 1 shows, even for very low infills, a
huge enlargement of er occurs. At sight of this graph, another interesting conclusion can be drawn. The higher the dielectric contrast is, the lower the opal pore infill needed for photonic gaps opening. Finally, it can be observed that, even if the optimum value of er is not achieved, the infill with a highly dielectric material should give rise to an
enhancement of the Bragg intensities. Some further predictions can be done at sight of the equation. When the opal hosts a semiconductor, the original Bragg peaks should experience
a red shift. If an infilled material is considered, áeñ in the equation can be expressed as:
f, f’, and f’’ being the filling factors for silica (f=0.74), the host material X and air in the voids respectively (f’+f’’= 0.26).
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3. Xia, Y., Whitesides, G.M. Soft Lithography. Angew. Chem. Int. Ed. Engl. 37, 550-575 (1998)