Periodic Reporting for period 1 - OptoTransport (Light-enabled transport phenomena in van der Waals heterostructures)
Reporting period: 2019-04-01 to 2021-03-31
1) Device fabrication: The first problem we addressed was creating robust TMD heterostructure devices that provide the possibility to perform sensitive optical and transport measurements simultaneously. This requires high-quality electrical contacts to the TMD monolayer, as well as high optical quality. Achieving good electrical contact to 2D materials is an engineering challenge and is the subject of intensive ongoing work in the field. We attempted various strategies for contacting the monolayer, and ultimately took the approach of so-called via-contacts, where a near atomically-flat metallic surface is stacked on a monolayer material. Using this approach, we were able to achieve high-enough contact quality to perform reliable transport measurements. At the same time, we optimized the device fabrication procedure to allow us maximum verstality and reliability of devices.
2) Opto-Transport measurements: With the ability to create high-quality devices, we engineered a device configuration which could offer the possibility to study both optical and electronic excitations and their interplay. Our system consists of a quantum point contact, which is a nano-constriction through which electrons can pass. When the constriction is sufficiently small and the temperature is low, the quantum mechanical properties of electrons kick in, which leads to quantized current through the constriction. We observed this effect in transport measurements of our system. Furthermore, we found that by shining light on the quantum point contact, which creates excitons, we could substantially modify the motion of electrons through the constriction. This led to further theoretical and experiment efforts to understand this effect and to build on it to explore new physics.
3) State-of-the art experimental setup: To study the effects of interplay of excitons and electrons, we developed a new cryogenic experimental setup which allows for advanced optics experiments, such as spectroscopy, correlation measurements and pump-probe measurements, as well as transport measurements. The versatile setup allows for greater stability against ambient vibrations, automated measurements and easier exchange of devices for rapid feedback.
1) The modification of quantized transport of electrons through the quantum point contact due to optical excitation was one of the first main achievements of the project. The focus of our ongoing work is to understand this effect, and a publication on this topic is imminent.
2) While studying the optical modification of transport, we discovered a new effect that solves one of the outstanding problems in photonics and optoelectronics, which has been to realize electrically tunable quantum confined systems. Even though, quantum confinement has been extensively studied for decades, one of the major obstacles towards scaling up these systems has been their lack of tunability. We have recently reported a method to quantum confine excitons at nanoscopic scales with full electrical tunability. The method relies on a novel approach of trapping excitons in a lateral p-i-n junction, and identified fundamentally new kind of confinement mechanism that originates from the quantum interactions between excitons and electrons. A conceptual illustration of our technique is shown below.
We have established a new platform to explore new physics regimes. The unprecedented tunability offered by our approach will allow to realize long-standing goals in the field of photonics - from strongly correlated photonics phases and topological photonics to quantum information. Due to the potential for broad technological impact, we have applied for a EU patent on tunable quantum confined devices.