Diffraction photonic crystals

Bragg diffraction on crystalline colloidal arrays Photonic crystal material is composed of a crystalline colloidal array that diffracts light at wavelengths determined by the optical lattice spacing, which is affected by the presence of analyte 5,14,15... 

Another nice example of nanostructuring an MIP layer is the work published by Wu et al. [138, 139] who developed a label-free optical sensor based on molecularly imprinted photonic polymers. Photonic crystals were prepared by self-assembly of silica nanospheres. The space between the spheres was then filled with MIP precursor solution. After polymerization, the silica was dissolved, leaving an MIP in the form of a 3D-ordered interconnected macroporous inverse polymer opal (Fig. 15). The authors were able to detect traces of the herbicide atrazine at low concentrations in aqueous solution [139]. Analyte adsorption into the binding sites resulted in a change in Bragg diffraction of the polymer characterized by a color modification (Fig. 15). 

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. 

A colloidal photonic crystal diffracts light (Fig. 4) according to Bragg s law ... 

Fig. 4. Schematic illustration of Bragg diffraction from photonic crystal lattice planes.

An inkjet printing of colloidal crystals was proposed by Frese et describing inkjet printing processes of monodispersed particles which are able to form two- or three-dimensional photonic crystals on the substrate surface by arranging in a closely packed lattice structure on the surface. The particle size was selected so that it will diffract light in the visible spectral region, i.e., particle size of 200-500 nanometers. In this work drop-on-demand inkjet printing techniques are utilized. 

Figure 2. Spectrally dispmai (b) and (d) luminescence images of a single dye doped Z2 pm globule (a) and of a tip of ID colloidal photonic crystal (c) composed of such microspheres. In (b) and (d) the wavelength marks show relative spectral positions, with regard to the reference images (a) and (c) obtained with imaging monochromator tuned to the 0-th diffraction.

Photonic crystals are natural or artificial solids that are able to manipulate light in a predetermined fashion, rather as X-rays are manipulated by ordinary crystals. For this to be possible they must contain an array of scattering centres analogous to the atoms in ordinary crystals. Perhaps surprisingly, the diffraction phenomena are little different than that described for X-ray, electron and neutron diffraction, and the equations given above in this Chapter apply to photonic crystals as well as X-rays. 

Artificial photonic crystals are structures built so that they contain diffracting centres separated by distances that are of the order of the wavelength of light. The interaction with light can be understood in terms of the Bragg equation. However, the terminology employed to describe diffraction in artificial photonic crystals is that of semiconductor physics. The transition from a diffraction description to a physical description can be illustrated with respect to a one-dimensional photonic crystal. 

Three-dimensional opal-VO based photonic crystals were prepared by the chemical bath deposition technique. The x-ray diffraction and Raman spectroscopy confirm the crystalline perfection of VO2 impregnated into synthetic opal pores. It is shown from the optical reflectivity measurements that the photonic bandgap of the opal-V02 based photonic crystals composite is governed by the phase transition in VO2. The shift of Bragg diffraction spectra under the pulsed (10 ns) illumination of YAG.Nd laser has been also observed in the opal-VO2 composites. 

Highly ordered layers of nondispersive colloids may also be formed with the Langmuir-Blodgett technique [39]. The interest in such structures lies in the fact that it is possible to induce the particles to coalesce into a structure analogous to a close-packed crystal. It is possible to obtain repeat distances large enough so that radiation in the optical region can be diffracted, just as X-rays are diffracted in a ordinary crystal. Such photonic crystals may have many practical uses in optoelectronics. 

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