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Light-enabled transport phenomena in van der Waals heterostructures

Periodic Reporting for period 1 - OptoTransport (Light-enabled transport phenomena in van der Waals heterostructures)

Reporting period: 2019-04-01 to 2021-03-31

One of the fundamental goals of solid state physics is to study and discover emergent phenomena in electronic systems. Atomically thin crystals, that fall into the broader category of two-dimensional (2D) materials, have generated a tremendous amount of interest in the last decade due to their remarkable properties which result in a plethora of new effects, such as unconventional superconductivity and correlated insulating phases. In particular, a new family of 2D semiconductors, known as Transition Metal Dichalcogenides (TMD), have garnered attention as they not only offer possibilities to study electronic states, but also optically excited particles such as electron-hole pairs known as excitons. In this context, the broader objective of our project was to explore the interplay between optical and electronic excitations, and study collective of their mixtures. Specifically, we were motivated by the question, can we modify the electronic properties of the system, and induce new effects by shining light on it? To achieve this, we proposed to harness the quantum mechanical interactions between excitons and electrons in TMD monolayers to create new hybrid light-matter states. One of the main goals of the project was to achieve light-induced transport effects in a TMD monolayer system. This could potentially have revolutionary impact in the field, both from a fundamental physics and technology perspective. New devices can be envisioned where incident photons switch the collective state of the system, which may have implications for sensors and quantum information platform.
To achieve these goals, we had to first solve several technical problems and establish technological know-how of the system, which are as follows:

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.
During the course of the project, we explored several new territories and discovered new effects. In addition, the work led to many ideas for future work, both within our team and with our collaborators. The main experimental results of the project are listed below:

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.
A conceptual illustration of quantum confinement of neutral excitons in lateral p-i-n diodes.