Assessing colloidal quantum dot optical properties through free standing layers on photonic waveguides

We present a novel way of assessing the dielectric function of thin Qdot mono- to multilayers by creating free standing layers of PbS/CdS core shell Qdots on top of 450nm wide oxide planarized SOI waveguides (PWGs). Through transmission measurements in the PWGs we can sensitively monitor the absorption coefficient at the bandgap energy of thin free standing Qdot layers. Based on effective medium theory including dipolar coupling between neighbouring Qdots we predict a layer dielectric constant yielding simulated waveguide losses that are in excellent agreement with the experimental results. We show an increase of the host permittivity with the thickness of the free standing Qdot layer, indicating a change of the Qdot environment from air to mainly oleic acid ligand molecules. Considering the emerging use of colloidal Qdots due to their optical properties and the facile (wet) processing, PbS/CdS hybrid PWGs can be used as a model system to assess the fundamental Qdot optical properties of other types of colloidal Qdot photonic hybrid systems