48 research outputs found
Exciton Diamagnetic Shifts and Valley Zeeman Effects in Monolayer WS and MoS to 65 Tesla
We report circularly-polarized optical reflection spectroscopy of monolayer
WS and MoS at low temperatures (4~K) and in high magnetic fields to
65~T. Both the A and the B exciton transitions exhibit a clear and very similar
Zeeman splitting of approximately 230~eV/T (), providing
the first measurements of the valley Zeeman effect and associated -factors
in monolayer transition-metal disulphides. These results complement and are
compared with recent low-field photoluminescence measurements of valley
degeneracy breaking in the monolayer diselenides MoSe and WSe. Further,
the very large magnetic fields used in our studies allows us to observe the
small quadratic diamagnetic shifts of the A and B excitons in monolayer WS
(0.32 and 0.11~eV/T, respectively), from which we calculate exciton
radii of 1.53~nm and 1.16~nm. When analyzed within a model of non-local
dielectric screening in monolayer semiconductors, these diamagnetic shifts also
constrain and provide estimates of the exciton binding energies (410~meV and
470~meV for the A and B excitons, respectively), further highlighting the
utility of high magnetic fields for understanding new 2D materials.Comment: 9 pages, 5 figure
Magneto-reflection spectroscopy of monolayer transition-metal dichalcogenide semiconductors in pulsed magnetic fields
We describe recent experimental efforts to perform polarization-resolved
optical spectroscopy of monolayer transition-metal dichalcogenide
semiconductors in very large pulsed magnetic fields to 65 tesla. The
experimental setup and technical challenges are discussed in detail, and
temperature-dependent magneto-reflection spectra from atomically thin tungsten
disulphide (WS) are presented. The data clearly reveal not only the valley
Zeeman effect in these 2D semiconductors, but also the small quadratic exciton
diamagnetic shift from which the very small exciton size can be directly
inferred. Finally, we present model calculations that demonstrate how the
measured diamagnetic shifts can be used to constrain estimates of the exciton
binding energy in this new family of monolayer semiconductors.Comment: PCSI-43 conference (Jan. 2016; Palm Springs, CA
Magneto-Optics of Exciton Rydberg States in a Monolayer Semiconductor
We report 65 tesla magneto-absorption spectroscopy of exciton Rydberg states
in the archetypal monolayer semiconductor WSe. The strongly field-dependent
and distinct energy shifts of the 2s, 3s, and 4s excited neutral excitons
permits their unambiguous identification and allows for quantitative comparison
with leading theoretical models. Both the sizes (via low-field diamagnetic
shifts) and the energies of the exciton states agree remarkably well with
detailed numerical simulations using the non-hydrogenic screened Keldysh
potential for 2D semiconductors. Moreover, at the highest magnetic fields the
nearly-linear diamagnetic shifts of the weakly-bound 3s and 4s excitons provide
a direct experimental measure of the exciton's reduced mass, .Comment: To appear in Phys. Rev. Lett. Updated version (25 jan 2018) now
includes detailed supplemental discussion of Landau levels, Rydberg exciton
energies, exciton mass, Dirac Hamiltonian, nonparabolicity, and dielectric
effect
Probing the Dark Exciton in Monolayer MoS by Quantum Interference in Second Harmonic Generation Spectroscopy
We report resonant second harmonic generation (SHG) spectroscopy of an
hBN-encapsulated monolayer of MoS. By tuning the energy of the excitation
laser, we identify a dark state transition (D) that is blue detuned by +25 meV
from the neutral exciton X. We observe a splitting of the SHG spectrum into
two distinct peaks and a clear anticrossing between them as the SHG resonance
is tuned through the energy of the dark exciton D. This observation is
indicative of quantum interference arising from the strong two-photon
light-matter interaction. We further probe the incoherent relaxation from the
dark state to the bright excitons, including X and localized excitons LX,
by the resonant enhancement of their intensities at the SHG-D resonance. The
relaxation of D to bright excitons is strongly suppressed on the bare substrate
whilst enabled when the hBN/MoS/hBN heterostructure is integrated in a
nanobeam cavity. The relaxation enabled by the cavity is explained by the
phonon scattering enhanced by the cavity phononic effects. Our work reveals the
two-photon quantum interference with long-lived dark states and enables the
control through nanostructuring of the substrate. These results indicate the
great potential of dark excitons in 2D-material based nonlinear quantum
devices
Selective Exciton-Phonon-Phonon Coupling and Anharmonicity with Cavity Vibrational Phonons and MoS Lattice Phonons in Hybrid Nanobeam Cavities
We report selective coupling between neutral excitons X, vibrational
phonon modes of a freestanding nanobeam cavity and lattice phonons of a MoS
monolayer fully encapsulated by hBN. Our experimental findings demonstrate that
the cavity vibrational phonons selectively couple to neutral excitons (X),
and the coupling to negatively charged trion (X) being significantly
weaker. We establish this result by studying the lattice temperature induced
broadening of exciton linewidths, where the contribution from the X-cavity
phonon coupling is clearly observed while the X-cavity phonon coupling is
not. Furthermore, when the Raman modes of MoS lattice phonons A and
2LA are tuned into an outgoing resonance with exciton emissions, we observe the
X-cavity phonon-lattice phonon coupling which inherits the characteristics
rule the of X-cavity phonon coupling. As a result, X-induced Raman
scatterings are enhanced, while X-induced scatterings are suppressed,
revealed by the detuning-dependent Raman intensities and the ratio of
X/X emission intensities. The phonon anharmonicity from the coupling
between cavity vibrational phonons and MoS lattice phonons is further
demonstrated by the observed Raman linewidth. Such hybrid couplings between
materials and nanostructures enable the control of phonon-induced processes in
nanophotonic and nanomechanical systems incorporating 2D semiconductors
Charged exciton kinetics in monolayer MoSe near ferroelectric domain walls in periodically poled LiNbO
Monolayers of semiconducting transition metal dichalcogenides are a strongly
emergent platform for exploring quantum phenomena in condensed matter, building
novel opto-electronic devices with enhanced functionalities. Due to their
atomic thickness, their excitonic optical response is highly sensitive to their
dielectric environment. In this work, we explore the optical properties of
monolayer thick MoSe straddling domain wall boundaries in periodically
poled LiNbO. Spatially-resolved photoluminescence experiments reveal
spatial sorting of charge and photo-generated neutral and charged excitons
across the boundary. Our results reveal evidence for extremely large in-plane
electric fields of 3000\,kV/cm at the domain wall whose effect is manifested in
exciton dissociation and routing of free charges and trions toward oppositely
poled domains and a non-intuitive spatial intensity dependence. By modeling our
result using drift-diffusion and continuity equations, we obtain excellent
qualitative agreement with our observations and have explained the observed
spatial luminescence modulation using realistic material parameters.Comment: 29 pages, 6 figures, submetted materia
Electrical control of orbital and vibrational interlayer coupling in bi- and trilayer 2H-MoS
Manipulating electronic interlayer coupling in layered van der Waals (vdW)
materials is essential for designing opto-electronic devices. Here, we control
vibrational and electronic interlayer coupling in bi- and trilayer 2H-MoS
using large external electric fields in a micro-capacitor device. The electric
field lifts Raman selection rules and activates phonon modes in excellent
agreement with ab-initio calculations. Through polarization resolved
photoluminescence spectroscopy in the same device, we observe a strongly
tunable valley dichroism with maximum circular polarization degree of in bilayer and in trilayer MoS that are fully consistent
with a rate equation model which includes input from electronic band structure
calculations. We identify the highly delocalized electron wave function between
the layers close to the high symmetry points as the origin of the tunable
circular dichroism. Our results demonstrate the possibility of electric field
tunable interlayer coupling for controlling emergent spin-valley physics and
hybridization driven effects in vdW materials and their heterostructures.Comment: Main manuscript: 10 pages, 4 figures ; Supplemental material: 14
pages, 9 figure
Proximity-enhanced valley Zeeman splitting at the WS/graphene interface
The valley Zeeman physics of excitons in monolayer transition metal
dichalcogenides provides valuable insight into the spin and orbital degrees of
freedom inherent to these materials. Being atomically-thin materials, these
degrees of freedom can be influenced by the presence of adjacent layers, due to
proximity interactions that arise from wave function overlap across the 2D
interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B-
excitons in monolayer WS, systematically encapsulated in monolayer
graphene. While the observed variations of the valley Zeeman effect for the A-
exciton are qualitatively in accord with expectations from the bandgap
reduction and modification of the exciton binding energy due to the
graphene-induced dielectric screening, the valley Zeeman effect for the B-
exciton behaves markedly different. We investigate prototypical WS/graphene
stacks employing first-principles calculations and find that the lower
conduction band of WS at the valleys (the band) is strongly
influenced by the graphene layer on the orbital level. This leads to variations
in the valley Zeeman physics of the B- exciton, consistent with the
experimental observations. Our detailed microscopic analysis reveals that the
conduction band at the point of WS mediates the coupling between
and graphene due to resonant energy conditions and strong coupling to the Dirac
cone. Our results therefore expand the consequences of proximity effects in
multilayer semiconductor stacks, showing that wave function hybridization can
be a multi-step process with different bands mediating the interlayer
interactions. Such effects can be exploited to resonantly engineer the
spin-valley degrees of freedom in van der Waals and moir\'e heterostructures.Comment: 14 pages, 6 figures, 3 table