PhD defence by Qiaoling Lin

PhD defence by Qiaoling Lin


26. sep 2023 13:30 - 16:30


Building 358/room 054


DTU Electro

PhD defence by Qiaoling Lin

Title: Exploring excitons in van der Waals heterostructures and their potential for lasing

Principal supervisor: Associate Professor Sanshui Xiao, DTU Electro, Denmark
Co-supervisor: Associate Professor Martijn Wubs, DTU Electro, Denmark
Co-supervisor: Associate Professor Nicolas Leitherer-Stenger, DTU Electro, Denmark

Evaluation Board
Professor Peter Uhd Jepsen, DTU Electro, Denmark
Professor Harri Kalevi Lipsanen, Aalto University, Finland
Professor Uriel Levy, The Hebrew University of Jerusalem, Israel

Master of the Ceremony
Senior Researcher Radu Malureanu, DTU Electro, Denmark

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have gained intense attention in the field of optoelectronics and nanophotonics due to their unique excitonic properties. TMDs can act as efficient gain materials for lasing in combination with photonic cavities. Nanolasers based on TMDs have been extensively studied and offer promising prospects for energy-efficient lasers. Interlayer excitons (IXs) in type-II van der Waals heterostructures hold promise for energyefficient, silicon-integrated, tunable, and electrically pumped lasers. The moiré potentials formed in heterostructures create additional in-plane confinement on IXs, thus potentially providing large optical gain at low pumping levels. However, there is still a lack of research regarding the moiré exciton at room temperature, and the influence of moiré superlattices on laser performance remains unexplored.

In this PhD project, we build a deterministic assembly system for transferring 2D materials. Starting with debugging an old spectrometer, we construct a multifunctional optical measurement setup working from the visible to the near-infrared (NIR) spectrum with high sensitivity. The success in platform construction serves as the cornerstone for our upcoming experiments.

We fabricate high-quality MoS2/WSe2 heterobilayers and study the optical properties of excitons and their dependence on twist angles and pump power. We observe a significant energy shift (> 200 meV) of the IXs by varying twist angles. When the twist angle is close to zero, the IX resonance shows a notable blue-shift as we increase the pump power and the absorption peak of WSe2 A exciton splits. These observations indicate the existence of moiré excitons at room temperature.

In addition, we study the impact of the moiré potential on the excitons by analyzing the time-resolved PL dynamics. We find that heterobilayers with strong interlayer coupling exhibit a lower IX energy and a longer IX lifetime than that with weak coupling. Our observation reveals that moiré excitons could be an efficient gain medium for energy-efficient and high-performance nanolasers.

Furthermore, we couple moiré IXs to silicon topological cavities and study their light-matter interaction. We fabricate silicon high-Q cavities at DTU Nanolab and the hBN-encapsulated MoS2/WSe2 heterostructures are then transferred onto the photonic cavities by using our home-built transfer setup. By conducting powerdependent PL measurements, we observe low-threshold lasing-like emission in the optical fiber communication O-band (1260-1360 nm). Our device shows the highest spectral coherence (∼0.1 nm) compared to all 2D material laser systems and an impressive spontaneous-emission suppression ratio (SESR) of ∼10 dB, similar to the value achieved by the TMD-based vertical-cavity surface-emitting laser (VCSEL).

Our works encourage studying novel exciton physics in moiré superlattices at room temperature and open new avenues for using these artificial quantum materials in high-performance device applications.