Carbon-nanofibre-based tunable photonic crystals

Licentiatavhandling, 2008

Photonic crystals are materials with periodically varying refractive indices. In conventional photonic crystal design it is hard to achieve tunable structures in the visible range. That is, structures with changeable optical properties. Carbon nanofibres are relatives to carbon nanotubes with a larger radius, approximately 25 nm, that can be fabricated vertically free-standing. The possibility to use carbon nanofibres as the basic building block for tunable two-dimensional photonic crystals is investigated. By growing nanofibres in a lattice pattern and keeping neighbouring fibres at different electrostatic potentials, the nanofibres can be bent electrostatically. This changes the lattice basis, which in turn modifies the optical properties of the photonic crystal. A finite-difference time-domain method is used to model a photonic crystal with a changeable basis. It is shown that the optical transmission through a photonic crystal slab can, at a certain frequency, be switched from almost 100% to approximately 1% with only a few rows of nanofibres in the light propagation direction. Many features in the transmission can be attributed to changes in the bandstructure. Carbon nanofibre photonic crystals are fabricated using catalytic DC plasma enhanced chemical vapour deposition. An optical measuring setup is being developed and preliminary results are presented. Future plans and possible directions of the project are discussed. Possible technical applications of this kind of system, and problems that need to be overcome in fabrication are considered. Finally further theoretical investigations of carbon nanofibre based photonic crystals are discussed.