Xiuming Dou

Tuning and Identification of Interband Transitions in Monolayer and Bilayer Molybdenum Disulfide Using Hydrostatic Pressure

Few-layer molybdenum disulfide (MoS2) is advantageous for application in next-generation electronic and optoelectronic devices. For monolayer MoS2, it has been established that both the conduction band minimum (CBM) and the valence band maximum (VBM) occur at the K point in the Brillouin zone. For bilayer MoS2, it is known that the VBM occurs at the Γ point. However, whether the K valley or the Λ valley forms the CBM and the energy difference between them remain disputable. Theoretical calculations have not provided a conclusive answer. In this paper, we demonstrate that a direct K-K to an indirect Λ-K interband transition in bilayer MoS2 can be optically detected by tuning the hydrostatic pressure. A changeover of the CBM from the K valley to the Λ valley is observed to occur under a pressure of approximately 1.5 GPa. The experimental results clearly indicate that the K valley forms the CBM under zero strain, while the Λ valley is approximately 89 ± 9 meV higher in energy.

Probing Spin–Orbit Coupling and Interlayer Coupling in Atomically Thin Molybdenum Disulfide Using Hydrostatic Pressure

In two-dimensional transition-metal dichalcogenides, both spin-orbit coupling and interlayer coupling play critical roles in the electronic band structure and are desirable for the potential applications in spin electronics. Here, we demonstrate the pressure characteristics of the exciton absorption peaks (so-called excitons A, B and C) in monolayer, bilayer, and trilayer molybdenum disulfide (MoS2) by studying the reflectance spectra under hydrostatic pressure and performing the electronic band structure calculations based on density functional theory to account for the experimental observations. We find that the valence band maximum splitting at the K point in monolayer MoS2, induced by spin-orbit coupling, remains almost unchanged with increasing pressure applied up to 3.98 GPa, indicating that the spin-orbit coupling is insensitive to the pressure. For bilayer and trilayer MoS2, however, the splitting shows an increase with increasing pressure due to the pressure-induced strengthening of the interlayer coupling. The experimental results are in good agreement with the theoretical calculations. Moreover, the exciton C is identified to be the interband transition related to the van Hove singularity located at a special point which is approximately 1/4 of the total length of Γ-K away from the Γ point in the Brillouin zone.

Self‐Assembled Quantum Dot Structures in a Hexagonal Nanowire for Quantum Photonics

Two types of quantum nanostructures based on self-assembled GaAs quantumdots embedded into GaAs/AlGaAs hexagonal nanowire systems are reported, opening a new avenue to the fabrication of highly efficient single-photon sources, as well as the design of novel quantum optics experiments and robust quantum optoelectronic devices operating at higher temperature, which are required for practical quantum photonics applications.

Single-photon-emitting diode at liquid nitrogen temperature

We report on the study of a single-photon-emitting diode at 77 K. The device is composed of InAs/GaAs quantum dots embedded in the i-region of a p-i-n diode structure. The high signal to noise ratio of the electroluminescence, as well as the small second order correlation function at zero-delay g((2))(0), implies that the device has a low multiphoton emission probability. By comparing the device performances under different excitation conditions, we have, in detail, discussed the basic parameters, such as signal to noise ratio and g((2))(0), and provided some useful information for the future application. (c) 2008 American Institute of Physics.

Single InAs quantum dot coupled to different “environments” in one wafer for quantum photonics

Self assembled small InAs quantum dots (SQDs) were formed in various densities and environments using gradient InAs deposition on a non-rotating GaAs substrate. Two SQD environments (SQD I and SQD II) were characterized. SQD I featured SQDs surrounded by large QDs, and SQD II featured individual SQDs in the wetting layer (WL). Micro-photoluminescence of single QDs embedded in a cavity under various excitation powers and electric fields gave insight into carrier transport processes. Potential fluctuations of the WL in SQD II, induced by charge redistribution, show promise for charge-tunable QD devices; SQD I shows higher luminescence intensity as a single-photon source.

In situ tuning the single photon emission from single quantum dots through hydrostatic pressure

We demonstrate that exciton emission wavelength in InAs/GaAs quantum dots (QDs) can be shifted up to 160 nm using hydrostatic pressure (0.4–4 GPa) in situ in an optical cryostat through an improved diamond anvil cell driven by piezoelectric actuator. It is confirmed that the high pressure does not destroy the photon anti-bunching properties of single QD emitter. Exciton emission intensity is not obviously weakened under the pressure range of 0–4 GPa. Such a tunable QD single photon emitter enables a flexibly tuned source for quantum optical experiments.

Optical study of lateral carrier transfer in (In,Ga)As∕GaAs quantum-dot chains

We have studied the lateral carrier transfer in a specially designed quantum dot chain structure by means of time-resolved photoluminescence (PL) and polarization PL. The PL decay time increases with temperature, following the T-1/2 law for the typical one-dimensional quantum system. The decay time depends strongly on the emission energy: it decreases as the photon energy increases. Moreover, a strong polarization anisotropy is observed. These results are attributed to the efficient lateral transfer of carriers along the chain direction. (c) 2008 American Institute of Physics.

In situ tuning biexciton antibinding-binding transition and fine-structure splitting through hydrostatic pressure in single InGaAs quantum dots

Exciton and biexciton emission energies as well as excitonic fine-structure splitting (FSS) in single (In,Ga)As/GaAs quantum dots (QDs) have been continuously tuned using hydrostatic pressure up to 4.4 GPa. The blue shift of excitonic emission and the increase of FSS are 320 meV and , respectively, which are significantly greater than those that could be achieved by previously reported techniques. We successfully produce a biexciton antibinding-binding transition along with a detailed polarization-resolved emission spectra. It is shown that the biexciton binding energy linearly increases with increasing pressure and tends to be sublinear at high pressure. We have performed atomistic pseudopotential calculations on realistic QDs to understand the pressure-induced effects.

Zinc-blende and wurtzite GaAs quantum dots in nanowires studied using hydrostatic pressure

We report both zinc-blende (ZB) and wurtzite (WZ) crystal phase self-assembled GaAs quantum dots (QDs) embedding in a single GaAs/AlGaAs core-shell nanowires (NWs). Optical transitions and single-photon characteristics of both kinds of QDs have been investigated by measuring photoluminescence (PL) and time-resolved PL spectra upon application of hydrostatic pressure. We find that the ZB QDs are of direct band gap transition with short recombination lifetime (~1 ns) and higher pressure coefficient (75-100 meV/GPa). On the contrary, the WZ QDs undergo a direct-to-pseudodirect bandgap transition as a result of quantum confinement effect, with remarkably longer exciton lifetime (4.5-74.5 ns) and smaller pressure coefficient (28-53 meV/GPa). These fundamentally physical properties are further examined by performing state-of-the-art atomistic pseudopotential calculations.

Photoluminescence studies on self-organized 1.55-μm InAs/InGaAsP/InP quantum dots under hydrostatic pressure

We report an experimental study on the optical properties of the self-organized 1.55-μm InAs/InGaAsP/InP quantum dots (QDs) under hydrostatic pressure up to 9.5 GPa at 10 K. The obtained pressure coefficients of emissions from InGaAsP to InAs QDs are 92 meV/GPa and 76 meV/GPa, respectively. Their photoluminescence intensities are found to decrease significantly with increasing pressure due to the pressure-induced Γ-X mixing of InGaAsP at about 8.5 GPa. The lifetime of excitonic emission from QDs decreases from about 1.15 at zero pressure to about 1.05 ns at 7.41 GPa. The wavelength of QD emission was tuned from 1.55 to 0.9 μm by applying a pressure of 8 GPa, displaying the feasibility for indirectly characterizing the individual InAs/InGaAsP/InP QDs of 1.55-μm emission (at zero pressure) under high-pressure using silicon avalanche photodiode.