Photonic Devices Based On Colloidal Quantum Dots

Colloidal quantum dots,also known as nanocrystals,which is synthesized from solutions,refer to the very small semiconductor nanoparticles with size in the range of 1?10 nm in 3 dimensions.Colloidal quantum dots are one of the hot topics in contemporary nanotechnologies.Due to the strong quantum confinement,electrical and optical properties of colloidal quantum dots differ from those of bulk materials such as the size-dependent fluorescence emission energy and the sharper density of states comparing to that of quantum wells.In addition,the synthesis process is less-expensive and time consuming than ordinary semiconductor growth technologies.Meanwhile,their small size allows them to be suspended in solutions,which is compatible with inkjet printing and spin coating processes.Colloidal quantum dots hold much potential in practical applications such as transistors,solar cells,light-emitting diodes,laser diodes,second harmonic generation,quantum computing and medical imaging.This thesis explores applications of colloidal quantum dots in lasing and single photon generation applications.I first reviewed the existing research of colloidal quantum dots for light amplification which also introduce the mechanism of light amplification from colloidal quantum dots.Next,I demonstrated an optical pumped laser based on microsphere resonator covered with colloidal quantum dot on its surface.In this experiment,we observed lasing features from the cavity region where no stable or unstable closed orbit exists.We carried out three-dimensional(3D)numerical simulation on a conical microcavity with a half-angle of 11 degrees.Results showed that there exist high quality modes by introducing a thin high refractive index film on a conical surface.Our study revealed that,rather than the surface profile of the microcavity,the effective radius plays a more crucial role in whether the cavity may support localized modes or not.Specifically,the change of high refractive index film thickness creates an additional angular momentum barrier,so that the conical microcavity may sustain localized high quality modes.In the regard of single photon generation,we reviewed the existing scheme of collecting single photons from quantum emitter and pointed out the drawbacks of these schemes,namely,the narrow bandwidth response and the incompatible outgoing beam profile for commercial fiber networks.We proposed to couple the photons emitted by single quantum dot to the guiding mode of micro-nano fiber and then directly guide them into a commercial single-mode fiber.We demonstrated a single photon source based on a coupled system composed of nanocrystal and nanofiber,which output photons directly from the fiber facet.The experimental results showed that the collection efficiency of nanofiber is more than 30 times higher comparing to collecting with water-immersed objective.The photo-excited single-photon source mentioned above is quite simple.However,electrical-excitation is more preferable for applications.In this paper,we also reviewed all reported electrically driven single-photon sources working at room temperature,then pointed out the main challenge is to concentrate the electron-hole recombination current on the defect or QD of interest rather than on surrounding impurities.We proposed to introduce a thin layer of insulating material with suitable thickness in-between the electron and hole transport layer(ETL,HTL)in the traditional colloidal quantum dot light emitting diode structure,and embedded the isolated colloidal quantum dots in this insulating layer.The insulating layer minimizes the electrical communications between ETL and HTL while still allows efficient generation and effective localization of excitons in individual quantum dots.The experimental results showed that the electroluminescence of the hole transport layer was greatly suppressed in this way.We achieved the first electrically driven single-photon source working at room temperature based on colloidal quantum dots.The optimal g(2)(0)from single-dot electroluminescence breaks the lower g(2)(0)limit of the corresponding photoluminescence.In order to explain this surprising result,we analyzed the differences between the carrier dynamics in photo-excitation and electro-excitation.We found that in the case of single-dot electroluminescence,the carrier dynamics suppresses the rate of forming bi-exciton state,and thus g(2)(0)is smaller than that from single-dot photoluminescence.