Nanophotonic Waveguides by Self-Assembled Quantum Dots

In order to construct ultra-high density photonic integrated circuits, innovative techniques are required to circumvent the fundamental restriction set by Heisenberg’s Uncertainty Principle and the resulting diffraction limit. Traditionally, waveguides adhere to a lower bound in size roughly equal to the propagating wavelength to allow for energy confinement and minimal crosstalk. However, progress in nanomaterial fabrication is enabling advances in energy transfer within nanoscale structures. Specifically, the quantum dot (QD), which is a nanometer sized 3D confined quantum mechanical system made of semiconductor compounds, exhibits high efficiency near-field coupling as well as a gain mechanism and represents an appealing candidate for creation of devices defined and spaced with sub-diffraction limit dimensions. Therefore, a nanophotonic waveguide constructed of self-assembled QDs is proposed. To arrive at a viable structure, the quantum dot behavior under both pulsed and continuous wave optical pumping is first modeled to determine the absorption, emission and gain spectra. A gain coefficient is then derived to find the region of optimal operation in terms of pump power. Subsequently, a quantum dot waveguide propagation model is provided. Throughout the discussion, two material systems, CdSe core QDs and CdSe/ZnS core/shell QDs, are simulated to demonstrate expected values and provide a numerical basis for analysis. The latter is particularly applicable as a fabrication process using CdSe/ZnS QDs is described and implemented to show feasibility. Correspondingly, a progression of self-assembly steps to final patterned structures is confirmed. Ultimately, the design, modeling and fabrication of the QD nanophotonic waveguide is intended to act as a cornerstone for nanoscale photonic integrated circuits. Furthermore, the ability to optically or electrically excite the quantum dot widens its applicability and allows integration with VLSI based circuitry thus expanding the realm of possible uses from communications to sensing and computing.