Public defence in Micro- and nanosciences, M.Sc. Xiaoqi Cui

The integration of novel materials and platforms unlocks the photon, a quantum of light, for broader engineering and advanced applications.
- Public defence from the Aalto University School of Electrical Engineering, Department of Electronics and Nanoengineering
Doctoral hat floating above a speaker's podium with a microphone

The title of the thesis: Integrated Optoelectronic Devices with van der Waals Materials

Thesis defender: Xiaoqi Cui
Opponent: Prof. Yun-Feng Xiao, Peking University, China
Custos: Prof. Zhipei Sun, Aalto University School of Electrical Engineering, Department of Electronics and Nanoengineering

vdW materials have inspired continuous research interest since the success of the first exfoliated graphene in 2004. These low-dimensional layered materials exhibit unique optical and electrical properties from their bulk form to atomic 2D monolayers (widely known as 2D materials), enabling great potential in photonics and optoelectronics. The integration of vdW materials through material stacking (constructing homojunctions and heterojunctions) and waveguide integration has led to various novel devices (e.g., photodetectors and modulators) with enriched functions and enhanced performance. However, challenges remain with the integration of these promising materials, such as the large device footprint when using evanescent field light coupling, realizing homojunction-based optoelectronic spectroscopic applications, and achieving the on-chip integration of these miniaturized spectroscopic devices. 

In this thesis, we explore novel optoelectronic operation principles and functions with vdW material integrated devices. An optically configurable AAT/bi-AAT photoresponse is demonstrated. The AAT behavior results from charge trapping and detrapping processes, assisted by manually introduced trap states. A symmetric device configuration is employed to achieve the AAT photoresponse. Two sources of carrier detrapping lead to wavelength-dependent AAT/bi-AAT photoresponses. Furthermore, a vdW homojunction with configurable photoresponses empowers a micro-spectroscopy function, and its on-chip integration is investigated. Material identification is realized via this compact optoelectronic spectroscopy. Additionally, a nanostructured vdW material waveguide is employed for high-efficiency photodetection. 

These demonstrations introduce optical wavelength as a trigger for advanced photoresponses, emphasizing optical wavelength (also considered as photon energy) as an important parameter (often ignored) in optoelectronic device engineering. Furthermore, the success in functionalizing nanostructured vdW materials as compact photodetectors opens the avenue for vdW materials as a novel material platform for photonic integrated circuits, facilitating the realization of more photonic and optoelectronic devices, such as vdW material-based micro-resonators.

Keywords: Nanophotonics, Optoelectronics, Photodetection, Spectrometers

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