06 - Holography based disordered structures for tailored scattering

We propose a holographic approach to the formation of disordered photonic structures. Holographic methods were used previously to manufacture 3D photonic crystal structures using multiple beam interference or sequential two beam exposure. We propose to use the second technique and add disorder to it by choosing random directions of the counter propagating plane waves and random difference between the phases of two respective waves. The disordered photonic structure can be obtained directly by index variation in the exposed photorefractive material or by selective etching or dissolution of a photoresist thus providing index contrast between air filled pores and the undissolved material. For example, we show that sequential exposure with counter propagating plane waves with random orientation and the same wavelength can lead to a disordered structure resembling that obtained by spinodal decomposition.

The main part of the approach is the tailored scattering of the incident light from the previously written multiple random gratings in the structure. This scattering is visualized using the Ewald sphere construction usually applied in X-ray diffraction. The main objective of the proposal can be separated in three partial goals. First, the application of the Ewald sphere construction will be studied to describe the problem of optical scattering in disordered holographic structures. Second, experimental methods to write disordered holograms will be developed. The photoresist or the photorefractive material will be exposed to a sequence of two wave interferences or a single exposure by a plane wave and diffuse light. Third, the following optical effects should be demonstrated: non-iridescent structural colour and frequency and direction dependent scattering elements for diffuse reflection or light trapping. The three effects will be characterised with respect to their spectral and directional properties.The theoretical results of this proposed project will lead to development of a simple model for scattering in disordered structures with small refractive index contrast. The experimental work will provide a technological contribution to the development of structural colour, transparent displays and light trapping.