Photonics self-assembled colloidal spheres

Photonic Crystals via Self-assembly of Colloidal Spheres... 

Self-assembly of highly charged colloidal spheres can, under the correct conditions, lead to 3D crystalline structures. The highly charged spheres used are either polystyrene beads or silica spheres, which are laid down to give the ordered structures by evaporation from a solvent, by sedimentation or by electrostatic repulsion (Figure 5.34). The structures created with these materials do not show full photonic band gap, due to their comparitively low relative permittivity, although the voids can be in-filled with other materials to modify the relative permittivity. 

Another important method for photonic crystal fabrication employs colloidal particle self-assembly. A colloidal system consists of two separate phases a dispersed phase and a continuous phase (dispersion medium). The dispersed phase particles are small solid nanoparticles with a typical size of 1-1000 nanometers. Colloidal crystals are three-dimensional periodic lattices assembled from monodispersed spherical colloids. The opals are a natural example of colloidal photonic crystals that diffract light in the visible and near-infrared (IR) spectral regions due to periodic modulation of the refractive index between the ordered monodispersed silica spheres and the surrounding matrix. 

Photonic band gap (PBG) materials are a specific class of ordered stmctures containing a periodic variation in refractive index in two or three dimensions and are used in a variety of applications like low-loss waveguides, low threshold lasers to name a few. Colloidal self-assembly provides a very promising approach for the production of micron scale, three dimension photonic crystals with band gaps in the visible or infrared region. The limiting factor in the self-assembly process is the monodispersity of the colloids, as good quality crystals are only achieved with coUoids that have very low size polydispersity (<5 %). Braun et al. (2001, 2002) have recently employed colloidal crystallization of silica spheres to build photonic band gap crystals for different applications. 

In summary, we have proposed here a promising route to fabricate photonic crystals with a diamond or pyrochlore structure through self-assembly of the MgCu2 structure using a binary mixture of colloidal spheres and subsequently, the removal of one of the species. 

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