Monolayer
transition-metal dichalcogenides (TMDs) manifest exceptional
optical properties related to narrow excitonic resonances. However,
these properties have been so far explored only for structures produced
by techniques inducing considerable large-scale inhomogeneity. In
contrast, techniques which are essentially free from this disadvantage,
such as molecular beam epitaxy (MBE), have to date yielded only structures
characterized by considerable spectral broadening, which hinders most
of the interesting optical effects. Here, we report for the first
time on the MBE-grown TMD exhibiting narrow and resolved spectral
lines of neutral and charged exciton. Moreover, our material exhibits
unprecedented high homogeneity of optical properties, with variation
of the exciton energy as small as ±0.16 meV over a distance of
tens of micrometers. Our recipe for MBE growth is presented for MoSe2 and includes the use of atomically flat hexagonal boron nitride
substrate. This recipe opens a possibility of producing TMD heterostructures
with optical quality, dimensions, and homogeneity required for optoelectronic
applications.
Using ultrafast resonant pump-probe spectroscopy in an unconventional experimental setup we investigate the spectral shape and dynamics of absorption features related to the A exciton in an hexagonal boron nitride (hBN)/MoSe2/hBN van der Waals heterostructure. While in a pure two-level system a pump-probe experiment measures the occupation or the polarization dynamics, depending on the time ordering of the pulse pair, in the transition metal dichalcogenide (TMD) system both quantities get thoroughly mixed by strong exciton–exciton interaction. We find that for short positive delays the spectral lines experience pronounced changes in their shape and energy and they relax to the original situation on a picosecond time scale. For negative delays, distinctive spectral oscillations appear indicating the first-time observation of perturbed free induction decay for a TMD system. The comparison between co-circular and cross-circular excitation schemes further allows us to investigate the rapid inter-valley scattering. By considering a three-level system as a minimal model including the local field effect, excitation-induced dephasing (EID), and scattering between the excited states we explain all phenomena observed in the experiment with excellent consistency. Our handy model can be even further reduced to two levels in the case of a co-circular excitation, for which we derive analytic expressions to describe the detected signals. This allows us to trace back the spectral shapes and shifts to the impact of local field effect and EID thus fully reproducing the complex behavior of the observed effects.
Using four-wave mixing microscopy, we measure coherent response and ultrafast dynamics of excitons and trions in MoSe2 monolayers grown by the molecular beam epitaxy on thin films of hexagonal boron...
Wurtzite CdTe and (Cd,Mn)Te nanowires embedded in (Cd,Mg)Te shells are grown by employing vapour-liquid-solid growth mechanism in a system for molecular beam epitaxy. A combined study involving cathodoluminescence, transmission electron microscopy and microphotoluminescence is used to correlate optical and structural properties in these structures. Typical features of excitonic emission from individual wurtzite nanowires are highlighted including the emission energy of 1.65 eV, polarization properties and the appearance B-exciton related emission at high excitation densities. Angle dependent magneto-optical study performed on individual (Cd,Mn)Te nanowires reveals heavy-hole-like character of A-excitons typical for wurtzite structure and allows to determine the crystal field splitting, Δ CR . The impact of the strain originating from the lattice mismatched shell is discussed and supported by theoretical calculations.
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