Optical properties of 2D semiconductors in van der Waals heterostructures
DIPC Seminars
- Speaker
-
Bernhard Urbaszek, CNRS - Toulouse University, France
- When
-
2017/06/09
14:00 - Place
- Donostia International Physics Center
- Add to calendar
- iCal
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