Tunable photonic crystals based on carbon nanofibers

Doctoral thesis, 2012

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, i.e. structures with changeable optical properties. Carbon nanofibers have dimensions similar to multi-walled carbon nanotubes, but have the advantage that they can be fabricated vertically free-standing. In this thesis the possibility to use carbon nanofibers as the basic building block for tunable two-dimensional photonic crystals is investigated. By growing nanofibers in a lattice pattern and keeping neighbouring fibers at different electrostatic potentials, the nanofibers can be bent electrostatically. This changes the lattice, which in turn modifies the optical properties of the photonic crystal. A finite-difference time-domain method was used to model a photonic crystal with a changeable basis. It was 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 nanofibers in the light propagation direction. It was shown that many features in the transmission can be attributed to changes in the bandstructure. Both static and tunable carbon nanofiber photonic crystals were fabricated using catalytic DC plasma enhanced chemical vapour deposition. An optical measurement set-up was developed and used for investigating diffraction from the samples. Samples were also investigated using ellipsometry. It was found that ellipsometry is a powerful tool for probing the band structure of 2D photonic crystal slabs. The intensity variations in diffracted beams, as functions of incidence angle, were measured and verified against theory. It was possible to detect carbon nanofiber actuation using both methods on the tunable samples and results are compared to theoretical expectations. The results from static and tunable structures are compared.