Emission modification in heterostructured opal photonic crystals

Abstract

The major goal of 3-dimensional photonic crystals (PhC) is to put the emission under control. However, functionality of a plain piece of PhC is limited in its capability to confine the light and to direct the emission flux. Further structuring, e.g. insertion of artificial defects, is a conventional way to accomplish the goal. So far, experiments with light emitting 3D PhCs are bounded to self-assembled colloidal crystals, in particular, to various kinds of opals. By the nature of self-assembling, these PhCs possess a high concentration of structural defects that makes senseless the intentional formation of point defects and appeals for the use of defects of higher dimensions. In this communication, we consider the role of interface in bi-layer opals in the modification of radiation from an emitting layer. Opals were prepared as thin films by slow evaporation of the latex sphere suspension on a glass slide. After sintering, a layer-by-layer coating technique was used to cover homogeneously the inner surface of opal with layers of alternatively charged polyelectrolyte and CdTe nanocrystals (NC). CdTe-opal represents a PBG light source. Selection of CdTe NC diameters allows us to tune independently the emission band across the PBG. Consequently, an opal film made from spheres of another diameter was grown on top of the first layer to form the hetero-opal with a PBG interface. Angular-resolved photoluminescence and studies of the emission intensity as a function of excitation power in combination with transmissionireflectance spectroscopy were main experimental tools. The interface between non-commensurable photonic bandgap (PBG) structures introduces a controllable anisotropy of the light propagation due to the eigenmode mismatch between two different PhCs. Numerous consequences of this mismatch include the anisotropy of the emission spectra with respect to the film plane, different emission directionality diagrams on opposite sides of the structure, redirection of the emission flow along the interface and anisotropy in the emission stimulation. Vector diagram technique was used to visualise the de-coupling of propagating modes at the interface and to explain the role of super-refraction in focusing of the emission flow. Peaks in the spectra of the emission saturation rate, which coincide with PBG, were interpreted as the evidence of the radiative recombination acceleration. The role of diffuse scattering upon the radiation propagating in the filter layer is also discussed. The overall impression of emission from the source-filter hetero-opals is the co-existence of trivial features generated by opal layers separately and uncommon features manifesting the mode re-coupling across the heterointerface.