Colloidal semiconductor quantum dots : from synthesis to photonic applications

In this research project we synthesized colloidal semiconductor nanocrystals, more specifically PbS and PbSe quantum dots (Qdots) with a typical diameter of 2-10nm, using wet chemical techniques. These materials are promising for telecom applications, as their band gap depends on the particle size and is tunable over the entire near-infrared spectral range. The main aim of the project was to investigate whether the high nonlinear refractive index can be used in photonic devices on a silicon platform. The colloidal Qdot synthesis typically produces a suspension of particles, stabilized by organic ligands. Understanding the nature and dynamics of this ligand shell is essential for a further processing of the Qdots, and it was studied with nuclear magnetic resonance spectroscopy. Next, we developed methods to quantify the linear optical properties of the Qdots. This knowledge helped us to determine and comprehend the nonlinear refractive index. A comparison of the results with typical semiconductors, such as Si and AlGaAs, showed that the Qdots are efficient nonlinear materials. Hereafter, the application-oriented research was initiated by covering a Silicon-on-Insulator microring with a Qdot doped polymer film and investigating the linear transmission characteristics of this hybrid device. A clear interaction between the Qdots and the light in the ring was observed, and the transmission spectrum was strongly influenced by the Qdot size and concentration. A nonlinear study on the microrings is currently under development; preliminary results are already presented