In this new research project, both spherical and cylindrical nanostructures will be fabricated by atomic layer deposition (ALD) for photonic applications. These radially symmetric structures exhibit an effective radial anisotropy of the refractive index due to a defined sequence of isotropic layers. The optical properties can be precisely adjusted by the layer sequence, the respective layer thicknesses and the material selection. In addition to the already established approach of a subsequent coating of already prepared photonic structures, this project will also use a rotational reactor for ALD to achieve a conformal, complete outer coating of the spherical particles with multiple material layers before the actual assembly. For this purpose, known standard oxide processes (SiO2, TiO2, Al2O3) will be transferred to the rotary reactor and optimized for particle coating. The radially anisotropic particles will then be assembled into three-dimensional photonic crystals and glasses with tailored optical properties in collaboration with C4. The combination of prior multilayer coating with ALD and subsequent assembly forms a new approach to fabricate tailored photonic structures. In addition to three-dimensional opals, two-dimensional photonic crystals based on radial multilayer cylinders embedded in hexagonally ordered tailored nanoporous membranes will also be realized. In this context, multilayer ALD allows the tuning of the electromagnetic scattering properties of the single (hollow) cylinder - the elementary functional unit of the two-dimensional photonic crystal.
Furthermore, the use of functional materials in photonic crystals and glasses will be investigated, which allow switching the optical properties between two states. By using vanadium and niobium dioxide, which exhibit a temperature-dependent transition from a dielectric to a metallic state, the electronic and thus optical properties of a photonic structure can be changed. The method of ALD in combination with switching between the insulating (low temperature) phase and the metallic (high temperature) phase of these materials allows the realization of "active" tunable and switchable photonic crystals and glasses. In particular, not only the changes of the electronic and thus the optical properties of the photonic structures via the insulator-metal phase transition will be investigated, but also to what extent external stimuli, e.g. current heat or mechanical stress, and chemical doping can be used to induce or influence the transition.
|Dr. rer. nat. Robert Zierold, |
|Prof. Dr. rer. nat. Robert H. Blick,|
ALD phase transition