Thin film photonic crystals

Torsten Maka (a), Sergei G. Romanov (a), Clivia M. Sotomayor Torres(a),
Manfred Müller(b) and Rudolf Zentel (b)

(a) Institute of Materials Science and Dept. of Electrical Engineering, University of Wuppertal, Gaußstr. 20, 42097 Wuppertal, Germany; email: maka@uni-wuppertal.de
(b) Institute of Materials Science and Dept. Chemistry, University of Wuppertal, Gaußstr. 20, 42097 Wuppertal, Germany

Opal crystals are known as a model system of tree-dimensional photonic crystals. Opals can be processed further as templates to form inverted structures. We have extended this approach to thin film photonic structures based on opaline (fcc) packages of PMMA balls.
PMMA balls for polymeric photonic crystals were synthesised and thin film photonic crystals of several cm2 area and a thickness of 30-50 monolayers were prepared on glass substrates.
Due to the low refractive index contrast PMMA opals posses an incomplete PBG and thus anisotropic optical properties. The possibility to impregnate the polymeric balls with fluorescent dyes offers an easy way to incorporate efficiently light emitting molecules into the photonic structures. By changing the dye these structures offer flexibility to match the emission band with the photonic stop band.
By infilling of the PMMA-opaline films with the high-refractive-index semiconductor SnS2 and subsequent dissolving of the polymer, inverse opals have been produced. These replicas show high refractive index contrast and a volume fraction to favour the opening of an omnidirectional PBG. So far SnS2 is the only material of sufficiently high refractive index, allowing to prepare inverted opals with full PBG that work in the red part of the visible spectrum.
Angular resolved spectroscopy has been used to study the PBG effects in particular directions of the photonic crystal. Both PMMA and inverted opal show distinct photonic behaviour. Observations of the photonic bandgap-related dip in photoluminescence spectra and self-focusing of the emission has proven the merit of quasi-3-dimensional photonic films in the control of light emission.