PhD Defence by Yujing Wang

Supervisors

• Principal supervisor: Professor Niels Gregersen, DTU Electro, Denmark.
• Co-supervisor: Assistant Professor Luca Vannucci, DTU Electro, Denmark.

Assessment committee

• Associate professor Sanshui Xiao, DTU Electro, Denmark (chair).
• Professor Jonathan Finley, University of Munich, Germany.
• Associate professor Loïc Lanco, Center for Nanoscience and Nanotechnology, France.

Master of the ceremony

• Assistant Professor, Battulga Munkhbat, DTU Electro, Denmark

Abstract:
As a basement for quantum computing, quantum cryptography, and quantum information processing, the research on a quantum dot (QD)-based, highly-efficient single-photon source (SPS) becomes the main topic of this thesis. Although this Ph.D. project is entirely based on numerical analyses, we still consider the experimental perspectives and aim to propose beneficial cavity designs contributing to more excellent SPSs.

The first part of this thesis is devoted to illustrating the motivation of our work by introducing the applications and requirements for SPSs, the superior properties and challenges of QDs, the implementation of nano-cavities for the improvement of SPS, and the importance of emission tuning and charge control for realizing the SPSs with more practical values. The main methods adopted to analyze the physical problems in our works are also disclosed in this part in a general way.

At the start of the central part, three types of vertical-emission SPSs are investigated. We propose a design procedure for the nanopost cavity and discuss the spatial misalignment of the emitter. We also explain the asymmetric behavior that appears on the spectrum. To get a high-quality cavity, our attention is turned to the new-type bullseye structures, where we demonstrate the optimization strategies in the hole-bullseye device and apply the optimizer to a chirped-bullseye cavity. The last vertical-emission device is developed in a fiber-coupled tunable open double cavity, which consists of a bottom planar cavity and a top metal/dielectric mirror. The suitable cavity length is selected to provide a Gaussian-like far-field profile, aiming for a reasonable single-mode fiber collection efficiency.

The second section is launched from a design procedure for the champion nanobeam cavity with a nearunity on-chip coupling efficiency. The problems in the cavity design in scale-up waveguides are appropriately analyzed and solved, according to which other fabrication-tolerant cavities are carried out. Collaborating with the experimental Ph.D. student in the MSCA QUDOT-TECH project, we implement the superconducting single-photon detector to the on-chip waveguide and provide our predictions of detector length via numerical simulations.

Finally, from a realistic consideration, the electrical contact strategies for QD emission tuning and charge environment stabilization are investigated in different nano-cavities, including Micropillar, bullseye, nanopost, and on-chip nanobeam structures. We provide perspectives on the impacts of additional sections for implementing the contacts in the abovementioned nano-cavities on the SPS performances.