21 research outputs found
Spin-order-dependent magneto-elastic coupling in two dimensional antiferromagnetic MnPSe observed through Raman spectroscopy
Layered antiferromagnetic materials have emerged as a novel subset of the
two-dimensional family providing a highly accessible regime with prospects for
layer-number-dependent magnetism. Furthermore, transition metal phosphorous
trichalcogenides, MPX3 (M = transition metal; X = chalcogen) provide a platform
for investigating fundamental interactions between magnetic and lattice degrees
of freedom providing new insights for developing fields of spintronics and
magnonics. Here, we use a combination of temperature dependent Raman
spectroscopy and density functional theory to explore
magnetic-ordering-dependent interactions between the manganese spin degree of
freedom and lattice vibrations of the non-magnetic sub-lattice via a
Kramers-Anderson super-exchange pathway in both bulk, and few-layer, manganese
phosphorous triselenide (MnPSe). We observe a nonlinear temperature
dependent shift of phonon modes predominantly associated with the non-magnetic
sub-lattice, revealing their non-trivial spin-phonon coupling below the
N{\'e}el temperature at 74 K, allowing us to extract mode-specific spin-phonon
coupling constants.Comment: 20 pages, 4 figures, Submitted to ACS Nano Letter
Van der Waals Nanoantennas on Gold as Hosts for Hybrid Mie-Plasmonic Resonances
Dielectric nanoresonators have been shown to circumvent the heavy optical
losses associated with plasmonic devices, however they suffer from less
confined resonances. By constructing a hybrid system of both dielectric and
metallic materials, one can retain the low losses of dielectric resonances,
whilst gaining additional control over the tuning of the modes with the metal,
and achieving stronger mode confinement. In particular, multi-layered van der
Waals materials are emerging as promising candidates for integration with
metals owing to their weak attractive forces, which enable deposition onto such
substrates without the requirement of lattice matching. Here we use layered,
high refractive index WS exfoliated on gold, to fabricate and optically
characterize a hybrid nanoantenna-on-gold system. We experimentally observe a
hybridization of Mie resonances, Fabry-P\'erot modes, and surface
plasmon-polaritons launched from the nanoantennas into the substrate. We
achieve experimental quality factors of Mie-plasmonic modes of up to 20 times
that of Mie resonances in nanoantennas on silica, and observe signatures of a
supercavity mode with a Q factor of 263 28, resulting from strong mode
coupling between a higher-order anapole and Fabry-P\'erot-plasmonic mode. We
further simulate WS nanoantennas on gold with an hBN spacer, resulting in
calculated electric field enhancements exceeding 2600, and a Purcell factor of
713. Our results demonstrate dramatic changes in the optical response of
dielectric nanophotonic structures placed on gold, opening new possibilities
for nanophotonics and sensing with simple-to-fabricate devices.Comment: 21 + 11 pages, 5 + 7 figure
Stark Spectroscopy and Radiative Lifetimes in Single Self-Assembled CdTe Quantum Dots
We present studies on Coulomb interactions in single self-assembled CdTe
quantum dots. We use a field effect structure to tune the charge state of the
dot and investigate the impact of the charge state on carrier wave functions.
The analysis of the quantum confined Stark shifts of four excitonic complexes
allows us to conclude that the hole wave function is softer than electron wave
function, i. e. it is subject to stronger modifications upon changing of the
dot charge state. These conclusions are corroborated by time-resolved
photoluminescence studies of recombination lifetimes of different excitonic
complexes. We find that the lifetimes are notably shorter than expected for
strong confinement and result from a relatively shallow potential in the
valence band. This weak confinement facilitates strong hole wave function
redistributions. We analyze spectroscopic shifts of the observed excitonic
complexes and find the same sequence of transitions for all studied dots. We
conclude that the universality of spectroscopic shifts is due to the role of
Coulomb correlations stemming from strong configuration mixing in the valence
band.Comment: sent to Physical Review
Understanding the impact of heavy ions and tailoring the optical properties of large-area Monolayer WS2 using Focused Ion Beam
Focused ion beam (FIB) has been used as an effective tool for precise
nanoscale fabrication. It has recently been employed to tailor defect
engineering in functional nanomaterials such as two-dimensional transition
metal dichalcogenides (TMDCs), providing desirable properties in TMDC-based
optoelectronic devices. However, the damage caused by the FIB irradiation and
milling process to these delicate atomically thin materials, especially in the
extended area, has not yet been elaboratively characterised. Understanding the
correlation between lateral ion beam effects and optical properties of 2D TMDCs
is crucial in designing and fabricating high-performance optoelectronic
devices. In this work, we investigate lateral damage in large-area monolayer
WS2 caused by the gallium focused ion beam milling process. Three distinct
zones away from the milling location are identified and characterised via
steady-state photoluminescence (PL) and Raman spectroscopy. An unexpected
bright ring-shaped emission around the milled location has been revealed by
time-resolved PL spectroscopy with high spatial resolution. Our finding opens
new avenues for tailoring the optical properties of TMDCs by charge and defect
engineering via focused ion beam lithography. Furthermore, our study provides
evidence that while some localised damage is inevitable, distant destruction
can be eliminated by reducing the ion beam current. It paves the way for the
use of FIB to create nanostructures in 2D TMDCs, as well as the design and
realisation of optoelectrical devices on a wafer scale
Nonlinear interactions of dipolar excitons and polaritons in MoS2 bilayers
Nonlinear interactions between excitons strongly coupled to light are key for
accessing quantum many-body phenomena in polariton systems. Atomically-thin
two-dimensional semiconductors provide an attractive platform for strong
light-matter coupling owing to many controllable excitonic degrees of freedom.
Among these, the recently emerged exciton hybridization opens access to
unexplored excitonic species, with a promise of enhanced interactions. Here, we
employ hybridized interlayer excitons (hIX) in bilayer MoS2 to achieve highly
nonlinear excitonic and polaritonic effects. Such interlayer excitons possess
an out-of-plane electric dipole as well as an unusually large oscillator
strength allowing observation of dipolar polaritons(dipolaritons) in bilayers
in optical microcavities. Compared to excitons and polaritons in MoS2
monolayers, both hIX and dipolaritons exhibit about 8 times higher
nonlinearity, which is further strongly enhanced when hIX and intralayer
excitons, sharing the same valence band, are excited simultaneously. This gives
rise to a highly nonlinear regime which we describe theoretically by
introducing a concept of hole crowding. The presented insight into many-body
interactions provides new tools for accessing few-polariton quantum
correlations
Resonant band hybridization in alloyed transition metal dichalcogenide heterobilayers
Bandstructure engineering using alloying is widely utilised for achieving
optimised performance in modern semiconductor devices. While alloying has been
studied in monolayer transition metal dichalcogenides, its application in van
der Waals heterostructures built from atomically thin layers is largely
unexplored. Here, we fabricate heterobilayers made from monolayers of WSe
(or MoSe) and MoWSe alloy and observe nontrivial tuning of
the resultant bandstructure as a function of concentration . We monitor this
evolution by measuring the energy of photoluminescence (PL) of the interlayer
exciton (IX) composed of an electron and hole residing in different monolayers.
In MoWSe/WSe, we observe a strong IX energy shift of
100 meV for varied from 1 to 0.6. However, for this shift
saturates and the IX PL energy asymptotically approaches that of the indirect
bandgap in bilayer WSe. We theoretically interpret this observation as the
strong variation of the conduction band K valley for , with IX PL
arising from the K-K transition, while for , the bandstructure
hybridization becomes prevalent leading to the dominating momentum-indirect K-Q
transition. This bandstructure hybridization is accompanied with strong
modification of IX PL dynamics and nonlinear exciton properties. Our work
provides foundation for bandstructure engineering in van der Waals
heterostructures highlighting the importance of hybridization effects and
opening a way to devices with accurately tailored electronic properties.Comment: Supporting Information can be found downloading and extracting the
gzipped tar source file listed under "Other formats
Emergence of Highly Linearly Polarized Interlayer Exciton Emission in MoSe<sub>2</sub>/WSe<sub>2</sub> Heterobilayers with Transfer-Induced Layer Corrugation
The availability of accessible fabrication methods based on deterministic
transfer of atomically thin crystals has been essential for the rapid expansion
of research into van der Waals heterostructures. An inherent issue of these
techniques is the deformation of the polymer carrier film during the transfer,
which can lead to highly non-uniform strain induced in the transferred
two-dimensional material. Here, using a combination of optical spectroscopy,
atomic force and Kelvin probe force microscopy, we show that the presence of
nanometer scale wrinkles formed due to transfer-induced stress relaxation can
lead to strong changes in the optical properties of MoSe/WSe
heterostructures and the emergence of the linearly polarized interlayer exciton
photoluminescence. We attribute these changes to the local breaking of crystal
symmetry in the nanowrinkles, which act as efficient accumulation centers for
the interlayer excitons due to the strain-induced interlayer band gap
reduction. The surface potential images of the rippled heterobilayer samples
acquired using Kelvin probe force microscopy reveal the variation of the local
work function consistent with the strain-induced band gap modulation, while the
potential offset observed at the ridges of the wrinkles shows a clear
correlation with the value of the tensile strain estimated from the wrinkle
geometry. Our findings highlight the important role of the residual strain in
defining optical properties of van der Waals heterostructures and suggest novel
approaches for interlayer exciton manipulation by local strain engineering
Understanding the impact of heavy ions and tailoring the optical properties of large-area monolayer WS2 using focused ion beam
Abstract Focused ion beam (FIB) is an effective tool for precise nanoscale fabrication. It has recently been employed to tailor defect engineering in functional nanomaterials such as two-dimensional transition metal dichalcogenides (TMDCs), providing desirable properties in TMDC-based optoelectronic devices. However, the damage caused by the FIB irradiation and milling process to these delicate, atomically thin materials, especially in extended areas beyond the FIB target, has not yet been fully characterised. Understanding the correlation between lateral ion beam effects and optical properties of 2D TMDCs is crucial in designing and fabricating high-performance optoelectronic devices. In this work, we investigate lateral damage in large-area monolayer WS2 caused by the gallium focused ion beam milling process. Three distinct zones away from the milling location are identified and characterised via steady-state photoluminescence (PL) and Raman spectroscopy. The emission in these three zones have different wavelengths and decay lifetimes. An unexpected bright ring-shaped emission around the milled location has also been revealed by time-resolved PL spectroscopy with high spatial resolution. Our findings open up new avenues for tailoring the optical properties of TMDCs by charge and defect engineering via focused ion beam lithography. Furthermore, our study provides evidence that while some localised damage is inevitable, distant destruction can be eliminated by reducing the ion beam current. It paves the way for the use of FIB to create nanostructures in 2D TMDCs, as well as the design and realisation of optoelectrical devices on a wafer scale