Near 100% valley coherence in molybdenum disulphide

Coherence is a property that allows the determination of the phase of a system at any point in space and time. For example, monochromatic light sources exhibit interference only when they are coherent. Similarly, microscopic particles in semiconductors can also demonstrate coherence, for example, excitons in sub-nanometer-thick semiconductors (such as monolayer MoS2). An exciton is a bound state of an electron and a hole, wherein the two oppositely charged particles are bound to each other through coulomb interaction. Information can be embedded in the form of valley pseudospin (a form of angular momentum) of excitons in these materials. In order to preserve and manipulate this information for practical quantum applications, it is essential to maintain the valley coherence of the exciton for a long duration.

Valley coherence can be generated using polarised light excitation on MoS2. Unfortunately, as time progresses, excitons quickly (within sub-picoseconds) lose such coherence due to a strong exchange interaction between the electron and the hole spin within the exciton. This valley decoherence timescale is much shorter than the lifetime of the exciton.

In a new paper published in Light: Science & Applications, a team of scientists, led by Kausik Majumdar, Professor in the Department of Electrical Communication Engineering, IISc, and co-workers have found a solution to this longstanding problem. In this work, the authors electrostatically weaken this exchange interaction through screening by encapsulating the monolayer MoS2 with graphene flakes on the top and bottom. This helped increase the valley coherence time by at least 10-fold beyond what is reported in literature to date, allowing them to demonstrate ~100% valley coherent excitons in a steady-state photoluminescence experiment.

According to the researchers, enabling the optical readout of such perfect valley coherence through achieving longer-than-lifetime valley coherence time of excitons in monolayer semiconductors can have far reaching implications in new experiments, as well as in possible applications such as quantum light emission and quantum information processing.

Gupta G, Watanabe K, Taniguchi T, Majumdar K, Observation of ~100% valley-coherent excitons in monolayer MoS2 through giant enhancement of valley coherence time, Light: Science & Applications (2023).