- J. C. Hulteen, R. P. Van Duyne, J. Vac
- Technol. A 1995, 13,
áeñ = 2.22 in the infiltrated one (reducing the contrast at the same time). After inversion the mean dielectric constant decreases to áeñ = 1.41 and, accordingly, the L pseudogap energy shifts upwards (see Fig. 5). The peak width is a function of both the dielectric contrast and the filling factor of the structure. Bare opals present a contrast eSiO2/eair = 2.1 that shifts to epolymer/eSiO2 = 1.24 when infiltration takes place. So, the pseudogap width is largely decreased, as is observed in both the experiment and band structure calculation. When inversion occurs, the dielectric contrast is increased up to epolymer/eair = 2.6. It has to be noticed that, although bare and inverse opal have similar values of the refractive index contrast, inverse opals show a much broader pseudogap than the direct opal structure which reflects the fact that inverse structures are more powerful scatterers (see Fig. 5A). When the sample is tilted with respect to normal incidence, the k vector ceases to be collinear with C±L. For a given direction (tilt angle), at some point of the energy scan, k crosses the Bragg plane and a reflection is obtained. Since L is the closest (to C) point of the Bragg plane, tilting increases both the wavevector length and the energy for which reflection occurs. This pseudogap energy position, can be followed along the L±U (or L±K or L±W) line in the Brillouin zone. In Figure 5C experimental data are superimposed on the band structure diagram by using Snell's law with an average refractive index for calculating the internal angle (with respect to the C±L direction). The theory gives a good account of the behavior of the pseudogap position. In summary, we have obtained and optically analyzed polymer inverse opals with a long-range order. Their photonic crystal behavior has been studied both experimentally and theoretically, and a good agreement between band-structure calculations and experiments was found. From a fundamental point of view, they can be regarded as model systems, where studying the effect of topology and dielectric contrast is possible. Regarding their potential applications, they can be used to modify the emission properties of luminescent species, such as dyes, that can easily be incorporated into the polymer. Polymer inverse opals offer, in turn, the interesting possibility of being used as matrices to obtain new spherical colloidal particles, whose shape cannot be controlled otherwise, from different materials.