Here, we show that deep trapped "dark" exciton states are responsible for the surprisingly long lifetime of band-edge photoluminescence in acid-treated single-layer MoS2. Temperature-dependent transient photoluminescence spectroscopy reveals an exponential tail of long-lived states extending hundreds of meV into the band gap. These sub-band states, which are characterized by a 4 microsecond radiative lifetime, quickly capture and store photogenerated excitons before subsequent thermalization up to the band edge where fast radiative recombination occurs. By intentionally saturating these trap states, we are able to measure the "true" 150 ps radiative lifetime of the band-edge exciton at 77 K, which extrapolates to ~600 ps at room temperature. These experiments reveal the dominant role of dark exciton states in acid-treated MoS2, and suggest that excitons spend > 95% of their lifetime at room temperature in trap states below the band edge. We hypothesize that these states are associated with native structural defects, which are not introduced by the superacid treatment; rather, the superacid treatment dramatically reduces non-radiative recombination through these states, extending the exciton lifetime and increasing the likelihood of eventual radiative recombination.
Optical trapping and manipulation of neutral particles has led to a variety of experiments from stretching DNA-molecules to trapping and cooling of neutral atoms. An exciting recent outgrowth of the technique is an experimental implementation of atom Bose-Einstein condensation. In this paper, we propose and demonstrate laser induced trapping for a new system--a gas of excitons in quantum well structures. We report on the trapping of a highly degenerate Bose gas of excitons in laser induced traps.
For quantum technologies based on single excitons and spins, the deterministic placement and control of a single exciton is a long-standing goal. MoSe2-WSe2 heterostructures host spatially indirect interlayer excitons (IXs) which exhibit highly tunable energies and unique spin-valley physics, making them promising candidates for quantum information processing. Previous IX trapping approaches involving moiré superlattices and nanopillars do not meet the quantum technology requirements of deterministic placement and energy tunability. Here, we use a nanopatterned graphene gate to create a sharpl…
A simple method to create and control magnetic potentials onto coupled quantum wells is demonstrated for indirect-exciton magnetic confinement. Localized inhomogeneous magnetic potentials with periodically distributed local minima and maxima, known as magnetic lattices, are projected into the plane of the coupled quantum wells and used for the spatially distributed two-dimensional indirect-exciton trapping, in which case localized indirect-excitons are observed. The indirect-exciton trapping mechanism is examined by controlling external magnetic field bias resulting in a shift of the localized…
We present compelling experimental evidence for a successful electrostatic trapping of two-dimensional dipolar excitons that results in stable formation of a well confined, high-density and spatially uniform dipolar exciton fluid. We show that, for at least half a microsecond, the exciton fluid sustains a density higher than the critical density for degeneracy if the exciton fluid temperature reaches the lattice temperature within that time. This method should allow for the study of strongly interacting bosons in two dimensions at low temperatures, and possibly lead towards the observation of …
Transition metal dichalcogenide heterostructures can host interlayer excitons (IXs), which consist of electrons and holes spatially separated in different layers. IXs possess permanent dipoles and have proven to offer a wealth of novel physics. We develop a discrete, random-walk model which includes annihilation and repulsion interactions among IXs. Using this model, we simulate the trapping of IXs in traps of different depths, densities, and shapes. Our results show that dipole-dipole interactions play an important role in regulating IX trapping. The effects of dipole interactions can be miti…
Metal-organic species can be designed to self-assemble in large-scale, atomically defined, supramolecular architectures. Hybrid quantum wells, where inorganic two-dimensional (2D) planes are separated by organic ligands, are a particular example. The ligands effectively provide an intralayer confinement for charge carriers resulting in a 2D electronic structure, even in multilayered assemblies. Air-stable metal organic chalcogenides hybrid quantum wells have recently been found to host tightly bound 2D excitons with strong optical anisotropy in a bulk matrix. Here, we investigate the excited c…
Interlayer excitons (IXs) in 2D semiconductors have long lifetimes and spin-valley coupled physics, with a long-standing goal of single exciton trapping for valleytronic applications. In this work, we use a nano-patterned graphene gate to create an electrostatic IX trap. We measure a unique power-dependent blue-shift of IX energy, where narrow linewidth emission exhibits discrete energy jumps. We attribute these jumps to quantized increases of the number occupancy of IXs within the trap and compare to a theoretical model to assign the lowest energy emission line to single IX recombination.
Atomically-thin transition metal dichalcogenide crystals (TMDCs) hold great promise for future semiconductor optoelectronics due to their unique electronic and optical properties. In particular, electron-hole pairs (excitons) in TMDCs are stable at room temperature and interact strongly with light. When TMDCs are embedded in an optical microcavity, the excitons can hybridise with cavity photons to form exciton polaritons (polaritons herein), which display both ultrafast velocities and strong interactions. The ability to manipulate and trap polaritons on a microchip is critical for future appli…
We study the localization of dressed Dirac electrons in a cylindrical quantum dot (QD) formed on monolayer and bilayer graphene by spatially different potential profiles. Short lived excitonic states which are too broad to be resolved in linear spectroscopy are revealed by cross peaks in the photon-echo nonlinear technique. Signatures of the dynamic gap in the two-dimensional spectra are discussed. The effect of the Coulomb induced exciton-exciton scattering and the formation of biexciton molecules are demonstrated.
Huazhong University of Science and Technology（华中科技大学）；Tsinghua University（清华大学）；Nanyang Technological University（南洋理工大学）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）
In this letter, we present a study of the condensation of exciton-polaritons in large etched pillar structures that exhibit shallow edge trapping. The $\approx$ 100 $μ$m $\times$ 100 $μ$m pillars were fabricated using photolithography and a BCl$_3$/Cl$_2$ reactive ion etch. A low energy region emerged along the etched edge, with the minima $\approx$ 7 $μ$m from the outer edge. The depth of the trap was 0.5-1.5 meV relative to the level central region, with the deepest trapping at the corners. We were able to produce a Bose-Einstein condensate in the trap near the edges and corners by pumping n…
In condensed-matter physics, remarkable advances have been made with atomic systems by establishing a thorough control over cooling and trapping techniques. In semiconductors, this method may also provide a deterministic approach to reach the long standing goal of harnessing collective quantum phenomena with exciton gases. While long-lived excitons are simply cooled to very low temperatures using cryogenic apparatus, engineering confining potentials has been a challenging task. This degree of control was only achieved recently with devices realized by highly demanding nano-fabrication processe…
A textbook example of quantum mechanical effects is the coupling of two states through a tunnel barrier. In the case of macroscopic quantum states subject to interactions, the tunnel coupling gives rise to Josephson phenomena including Rabi oscillations, the a.c. and d.c. effects, or macroscopic self-trapping depending on whether tunnelling or interactions dominate. Non-linear Josephson physics, observed in superfluid helium and atomic condensates, has remained inaccessible in photonic systems due to the required effective photon-photon interactions. We report on the observation of non-linear …
Exciton-polaritons in semiconductor microcavities have advanced to become a model system for studying dynamical Bose-Einstein condensation, macroscopic coherence, many-body effects, nonclassical states of light and matter, and possibly quantum phase transitions in a solid state. Being low mass bosons, these light-matter quasiparticles can condense at comparably high temperatures up to 300K, while preserving fundamental properties such as coherence in space and time domain even when they are out of equilibrium with the environment. Although the presence of an in-plane polariton confinement pote…
Bosonic condensation of microcavity polaritons is accompanied by their relaxation from the ensemble of excited states into a single quantum state. The excess of energy is transferred to the crystal lattice that eventually involves heating of the structure. Creation of the condensate results in the local increase of the temperature which leads to the red shift of the exciton energy providing the mechanism for polariton self-trapping. By employing the driven-dissipative Gross-Pitaevskii model we predict a new type of a stable localized solution supported by the thermally-induced self-trapping in…
We study experimentally and theoretically polarization-dependent luminescence from an ensemble of quantum-dot-like nanostructures with a very large in-plane shape anisotropy (quantum dashes). We show that the measured degree of linear polarization of the emitted light increases with the excitation power and changes with temperature in a non-trivial way, depending on the excitation conditions. Using an approximate model based on the k.p theory, we are able to relate this degree of polarization to the amount of light hole admixture in the exciton states which, in turn, depends on the symmetry of…
We demonstrate an electrostatic trap for indirect excitons in a field-effect structure based on coupled GaAs quantum wells. Within the plane of a double quantum well indirect excitons are trapped at the perimeter of a SiO2 area sandwiched between the surface of the GaAs heterostructure and a semitransparent metallic top gate. The trapping mechanism is well explained by a combination of the quantum confined Stark effect and local field enhancement. We find the one-dimensional trapping potentials in the quantum well plane to be nearly harmonic with high spring constants exceeding 10 keV/cm^2.
We have succeeded in trapping indirect excitons in coupled quantum wells in a harmonic potential minimum via inhomogeneous applied stress and electric field. These excitons exhibit a strong Stark shift (over 60 meV), long lifetime (100 ns), and high diffusivity (1000 cm$^2$/s). This approach is very promising for obtaining the high exciton density needed for Bose condensation of excitons in two dimensions.
The inherently low photoluminescence (PL) yields in as prepared transition metal dichalcogenide (TMD) monolayers are broadly accepted to be the result of atomic vacancies (i.e. defects) and uncontrolled doping, which give rise to non-radiative exciton decay pathways. To date, a number of chemical passivation schemes have been successfully developed to improve PL in sulphur based TMDs i.e. molybdenum disulphide (MoS2) and tungsten disulphide (WS2) monolayers. Reports on solution based chemical passivation schemes for improving PL yields in selenium (Se) based TMDs are lacking I comparison, with…
Large surface to volume ratios of semiconductor nanocrystals cause susceptibility to charge trapping, which can modify luminescence yields and induce single-particle blinking. Optical spectroscopies cannot differentiate between bulk and surface traps in contrast to spin-resonance techniques, which in principle avail chemical information on such trap sites. Magnetic resonance detection via spin-controlled photoluminescence enables the direct observation of interactions between emissive excitons and trapped charges. This approach allows the discrimination of two functionally different trap state…
Tuning the electronic properties of a material by subjecting it to strain constitutes an important strategy to enhance the performance of semiconducting electronic devices. Using local strain, confinement potentials for excitons can be engineered, with exciting possibilities for trapping excitons for quantum optics and for efficient collection of solar energy. Two-dimensional materials are able to withstand large strains before rupture, offering a unique opportunity to introduce large local strains. Here, we study atomically thin MoS2 layers with large local strains of up to 2.5% induced by co…
We examine a mechanism by which excitons undergo ultrafast relaxation in common monolayer transition metal dichalcogenides. It is shown that at densities $\approx$ 1$\times$ 10$^{11}$ cm$^{-2}$ and temperatures $ \le 60$ K, excitons in well known monolayers (MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$) exist as point-like structureless electron-hole quasi-particles. We evaluate the average rate of exciton energy relaxation due to acoustic phonons via the deformation potential and the piezoelectric coupling mechanisms and examine the effect of spreading of the excitonic wavefunction into the region p…
By considering driven diffusive dynamics of exciton polaritons in an optical microcavity with an embedded molybdenum disulfide monolayer, we determine experimentally relevant range of parameters at which room-temperature superfuidity can be observed. It is shown that the superfluid transitions occurs in a trapped polariton gas at laser pumping power $P>600$ mW and and trapping potential strength $k > 50$ eV/cm$^2$. We also propose a simple analytic model that provides a useful estimate for the polariton gas density, which enables one to determine the conditions for observation of room temperat…
The moiré pattern observed in stacked non-commensurate crystal lattices, such as hetero-bilayers of transition metal dichalcogenides, produces a periodic modulation of their bandgap. Excitons subjected to this potential landscape exhibit a band structure that gives rise to a quasi-particle dubbed moiré exciton. In the case of MoS$_2$/WSe$_2$ hetero-bilayers, the moiré trapping potential has honeycomb symmetry and, consequently, the moiré exciton band structure is the same as that of a Dirac-Weyl fermion, whose mass can be further tuned down to zero with a perpendicularly applied field. Here we…
We theoretically analyze the carrier capture and distribution among the available energy levels of a symmetric semiconductor quantum dot under continuous-wave excitation resonant with the barrier energy levels. At low temperature all the dot level-occupations but one decrease monotonically with energy. The uncovered exception, corresponding to the second (dark) energy level, displays a steady-state carrier density exceeding that of the lowest level more than a factor two. The root cause is not radiative recombination before relaxation to lower energy levels, but at the opposite, carrier trappi…
We have investigated the temperature dependence of the photoluminescence spectra and average photoluminescence decay rate of CdTe quantum dot monolayers of different sizes as a function of concentration in the range 77 K to 296 K. It is shown that a simple three level analytic model involving bright and dark exciton states can only describe the lower temperature data but is unable to satisfactorily fit the data over the full temperature range. An extended model which includes external trap states is necessary to fit the data above approximately 150 K. Parameters for the model are obtained usin…
Wavepacket transport across a nonlinear region is studied numerically at zero and finite temperatures. In contrary to the zero temperature case which demonstrates ballistic transport, finite temperature lattice vibrations suppresses the transport drastically. The interface between the linear and the nonlinear chain plays the role of a high barrier at finite temperature when anharmonicity factor is small compared to the typical inverse cubic interatomic distance. Inverse participation ratio of the central region shows that for small anharmonicity and finite temperatures lattice vibrations give …
We describe photonic crystal microcavities with very strong light-matter interaction to realize room-temperature, equilibrium, exciton-polariton Bose-Einstein condensation (BEC). This is achieved through a careful balance between strong light-trapping in a photonic band gap (PBG) and large exciton density enabled by a multiple quantum-well (QW) structure with moderate dielectric constant. This enables the formation of long-lived, dense 10~$μ$m - 1~cm scale cloud of exciton-polaritons with vacuum Rabi splitting (VRS) that is roughly 7\% of the bare exciton recombination energy. We introduce a w…
