Advisors

Effects of polarization and scattering in disordered photonic crystal structures

Optical microcavities form in defects of 2D photonic crystals where the light field remains confined by Bragg reflection. Alternatively, optical microcavities can form by scattering (strong localization) in random media. Depending on these two confinement mechanisms, Bragg reflection or random scattering, the optical properties of the localized light fields are expected to differ significantly. Here, we will investigate a photonic crystal structure that confines light through the interplay of Bragg reflection AND random scattering.  The lattice of the 2D photonic crystals is disordered which leads to strongly localized light fields resulting in high Q cavities, spectrally observed at the cut-off frequency of the waveguide mode. These microcavities are distributed randomly along the ~ 80 mm waveguide. We will characterize disordered W1 2D silicon photonic crystal waveguides. First, scanning electron microscope (SEM) imaging will be utilized to extract lattice parameters such as hole size, fill factor, lattice constant, lattice shape and thickness of the silicon membrane. These parameters will be used in numerical plain wave expansion (PWE) calculations to predict the band edge resonance of the extended structure. In parallel, we will experimentally observe band edge resonances of the photonic crystal structures using a super-continuum laser source with tunable filters and by collecting scattered light in reflection mode microscopy. Next, we will use an external cavity laser tuned to the band edge resonance to spectrally resolve random high Q microcavities by excitation with a free space beam and a cross-polarization measurement.  Q factors are expected in the 10^5 range. The experimental data will be analyzed for polarization and k-spectrum distribution of the scattered light, and numerical methods will be utilized to relate back to the near field. These studies will lay the foundation for application of disordered photonic crystal structures in biosensing applications, as well as for designing random lasers.

Contact:

frank.vollmer@mpl.mpg.de