37 research outputs found
Valley Polarization-Electric Dipole Interference and Nonlinear Chiral Selection Rules in Monolayer WSe
In monolayer transition metal dichalcogenides time-reversal symmetry,
combined with space-inversion symmetry, defines the spin-valley degree of
freedom. As such, engineering and control of time-reversal symmetry by optical
or magnetic fields constitutes the foundation of valleytronics. Here, we
propose a new approach for the detection of broken time-reversal symmetry and
valley polarization in monolayer WSe based on second harmonic generation.
Our method can selectively and simultaneously generate and detect a valley
polarization at the valleys of transition metal dichalcogenides at room
temperature. Furthermore, it allows to measure the interference between the
real and imaginary parts of the intrinsic (electric dipole) and valley terms of
the second order nonlinear susceptibility. This work demonstrates the potential
and unique capabilities of nonlinear optics as a probe of broken time-reversal
symmetry and as a tool for ultrafast and non-destructive valleytronic
operations.Comment: 27 pages 6 figure
Interlayer exciton mediated second harmonic generation in bilayer MoS2
Second harmonic generation (SHG) is a non-linear optical process, where two
photons coherently combine into one photon of twice their energy. Efficient SHG
occurs for crystals with broken inversion symmetry, such as transition metal
dichalcogenide monolayers. Here we show tuning of non-linear optical processes
in an inversion symmetric crystal. This tunability is based on the unique
properties of bilayer MoS2, that shows strong optical oscillator strength for
the intra- but also inter-layer exciton resonances. As we tune the SHG signal
onto these resonances by varying the laser energy, the SHG amplitude is
enhanced by several orders of magnitude. In the resonant case the bilayer SHG
signal reaches amplitudes comparable to the off-resonant signal from a
monolayer. In applied electric fields the interlayer exciton energies can be
tuned due to their in-built electric dipole via the Stark effect. As a result
the interlayer exciton degeneracy is lifted and the bilayer SHG response is
further enhanced by an additional two orders of magnitude, well reproduced by
our model calculations.Comment: main paper and supplemen
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Efficient phonon cascades in WSe 2 monolayers
Abstract: Energy relaxation of photo-excited charge carriers is of significant fundamental interest and crucial for the performance of monolayer transition metal dichalcogenides in optoelectronics. The primary stages of carrier relaxation affect a plethora of subsequent physical mechanisms. Here we measure light scattering and emission in tungsten diselenide monolayers close to the laser excitation energy (down to ~0.6 meV). We reveal a series of periodic maxima in the hot photoluminescence intensity, stemming from energy states higher than the A-exciton state. We find a period ~15 meV for 7 peaks below (Stokes) and 5 peaks above (anti-Stokes) the laser excitation energy, with a strong temperature dependence. These are assigned to phonon cascades, whereby carriers undergo phonon-induced transitions between real states above the free-carrier gap with a probability of radiative recombination at each step. We infer that intermediate states in the conduction band at the Î-valley of the Brillouin zone participate in the cascade process of tungsten diselenide monolayers. This provides a fundamental understanding of the first stages of carrierâphonon interaction, useful for optoelectronic applications of layered semiconductors
Kapitza-resistance-like exciton dynamics in atomically flat MoSe-WSe lateral heterojunction
Being able to control the neutral excitonic flux is a mandatory step for the
development of future room-temperature two-dimensional excitonic devices.
Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with
extremely robust and mobile excitons are highly attractive in this regard.
However, generating an efficient and controlled exciton transport over long
distances is a very challenging task. Here we demonstrate that an atomically
sharp TMD-ML lateral heterostructure (MoSe-WSe) transforms the
isotropic exciton diffusion into a unidirectional excitonic flow through the
junction. Using tip-enhanced photoluminescence spectroscopy (TEPL) and a
modified exciton transfer model, we show a discontinuity of the exciton density
distribution on each side of the interface. We introduce the concept of exciton
Kapitza resistance, by analogy with the interfacial thermal resistance referred
to as Kapitza resistance. By comparing different heterostructures with or
without top hexagonal boron nitride (hBN) layer, we deduce that the transport
properties can be controlled, over distances far greater than the junction
width, by the exciton density through near-field engineering and/or laser power
density. This work provides a new approach for controlling the neutral exciton
flow, which is key toward the conception of excitonic devices
Exciton spectroscopy and unidirectional transport in MoSe2-WSe2 lateral heterostructures encapsulated in hexagonal boron nitride
Chemical vapor deposition (CVD) allows lateral edge epitaxy of transition metal dichalcogenide heterostructures. Critical for carrier and exciton transport is the material quality and the nature of the lateral heterojunction. Important details of the optical properties were inaccessible in as-grown heterostructure samples due to large inhomogeneous broadening of the optical transitions. Here we perform optical spectroscopy of CVD grown MoSe-WSe lateral heterostructures, encapsulated in hBN. Photoluminescence (PL), reflectance contrast and Raman spectroscopy reveal optical transition linewidths similar to high quality exfoliated monolayers, while PL imaging experiments uncover the effective excitonic diffusion length of both materials. The typical extent of the covalently bonded MoSe-WSe heterojunctions is 3ânm measured by scanning transmission electron microscopy (STEM). Tip-enhanced, sub-wavelength optical spectroscopy mapping shows the high quality of the heterojunction which acts as an excitonic diode resulting in unidirectional exciton transfer from WSe to MoSe
Confinement of long-lived interlayer excitons in WS 2 /WSe 2 heterostructures
Abstract: Interlayer excitons in layered materials constitute a novel platform to study many-body phenomena arising from long-range interactions between quantum particles. Long-lived excitons are required to achieve high particle densities, to mediate thermalisation, and to allow for spatially and temporally correlated phases. Additionally, the ability to confine them in periodic arrays is key to building a solid-state analogue to atoms in optical lattices. Here, we demonstrate interlayer excitons with lifetime approaching 0.2 ms in a layered-material heterostructure made from WS2 and WSe2 monolayers. We show that interlayer excitons can be localised in an array using a nano-patterned substrate. These confined excitons exhibit microsecond-lifetime, enhanced emission rate, and optical selection rules inherited from the host material. The combination of a permanent dipole, deterministic spatial confinement and long lifetime places interlayer excitons in a regime that satisfies one of the requirements for simulating quantum Ising models in optically resolvable lattices
Chemical Vapor Deposition of HighâOpticalâQuality LargeâArea Monolayer Janus Transition Metal Dichalcogenides
Oneâpot chemical vapor deposition (CVD) growth of largeâarea Janus SeMoS monolayers is reported, with the asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. The formation of these 2D semiconductor monolayers takes place upon the thermodynamicâequilibriumâdriven exchange of the bottom Se atoms of the initially grown MoSeâ single crystals on gold foils with S atoms. The growth process is characterized by complementary experimental techniques including Raman and Xâray photoelectron spectroscopy, transmission electron microscopy, and the growth mechanisms are rationalized by first principle calculations. The remarkably high optical quality of the synthesized Janus monolayers is demonstrated by optical and magnetoâoptical measurements which reveal the strong excitonâphonon coupling and enable an exciton gâfactor of â3.3