UC Berkeley

Recently, atomically thin 2D materials have emerged as a new class of nanomaterial with extraordinary physical properties ranging from semi-metal (graphene), semiconductors (transition metal dichalcogenides) to insulators (hBN). Due to its unusual linear electronic spectrum, graphene has been studied as a platform where exotic quantum mechanical phenomena take place and electro-optical property can be greatly tuned by electrostatic gating. On the other hand, in semiconducting transition metal dichalcogenides (TMD), many body particles such as exciton and trion can be readily accessed even at room temperature due to strong Coulomb interaction. There are further exciting opportunities in heterostructures where layer-layer interaction provides new physics and functionalities. In this thesis, I explore light-matter interaction in 2D materials and their heterostructure with laser spectroscopy techniques.

First of all, I study electromagnetic interaction between graphene and optical cavity via Rayleigh scattering spectroscopy. Although light-matter interaction in graphene is extremely strong for atomically thin thickness, overall optical response in macroscopic scale is still limited. Combination of graphene and resonant cavity can amplify the interaction dramatically. Therefore, it is important to understand the electromagnetic interaction between two systems. In this study, I find that the coupling can be explained by real and imaginary part of graphene dielectric constant which affects cavity resonance frequency and quality factor, respectively. In addition to fundamental interest, it also shows that this platform has promising potential for novel sensing application and electro-optical modulator.

Secondly, I study valley-selective dipole interaction of exciton states in a monolayer transition metal dichalcogenides. Due to crystal symmetry, an extra degree of freedom, valley state, is available in this system. In analogy to spin state, it is important to understand and manipulate valley state with light. In this study, I demonstrate that valley excitonic states in a monolayer WSe2 can be manipulated by femtosecond pulse with the control of polarization. Ultrafast pump-probe spectroscopy shows that circularly-polarized femtosecond pulse induces valley-selective optical Stark effect which acts as a pseudomagnetic field. This demonstrates efficient and ultrafast control of the valley excitons with optical light, and opens up the possibility to coherent manipulate the valley polarization for quantum information applications.

Lastly, I study interlayer interaction in heterostructure of MoS2/WS2 where strong exciton binding energy plays an important role. Simple band theory predicts that a heterostructure of two different semiconducting TMD layers forms type-II heterostructure. However, it is not clear how strong Coulomb interaction plays a role in terms of charge transfer dynamics. In this study, I demonstrate ultrafast charge transfer in MoS2/WS2 via both photoluminescence mapping and femtosecond (fs) pump-probe spectroscopy. Despite large exciton binding energy, hole transfer from the MoS2 layer to the WS2 layer takes place within 50 fs after optical excitation. Such ultrafast charge transfer in van der Waals heterostructures indicates that it can enable novel 2D devices for optoelectronics and light harvesting.