Optical properties of 2D semiconductors in van der Waals heterostructures

Transition metal dichalcogenides (TMDCs) such as MoS2 and WSe2 are layered materials that are semiconductors with a direct bandgap when thinned down to one monolayer. The current research effort on TMDC structures for optoelectronics and spintronics, often based on monolayer samples obtained by simple exfoliation using adhesive tape, is motivated by several remarkable properties : First, TMDC monolayers strongly interact with light in the visible region of the optical spectrum. The optically generated electrons and holes form excitons with high binding energy (several hundred meV) and high oscillator strength, resulting in optical absorption up to 20 % per monolayer. Second, the valence and conduction spin states are split in energy by a strong Spin Orbit interaction. Third, interband optical selection rules are polarization selective (chiral). This allows addressing non-equivalent valleys in momentum space with polarized lasers for optical spin and valley index manipulation.
To address the intrinsic optical properties of these materials we investigate TMDC monolayers ‘sandwiched’ between ultrathin insulating layers of hexagonal boron nitride (hBN) in van der Waals heterostructures. The optical emission of these encapsulated monolayers is spectrally narrow (down to 1 nm FWHM) comparable to emission from III-V quantum well structures used in today’s optoelectronic devices and approaching the homogenous limit. This allows accessing the optical and spin properties of these materials with unprecedented detail for neutral and charged excitons (trions). As important examples, we are able to compare exciton resonances with transition dipoles in-plane and out-of the monolayer plane, determining the bright-dark exciton splitting.  First signatures of boson scattering for excitons are presented and Valley polarization and coherence measurements are discussed.
 
Examples of recent results and collaborations:
arXiv 1701.05800, 1702.00323, 1704.05341, 1704.05448