Research on hexagonal boron nitride (hBN) has been intensified recently due to the application of hBN as a promising system of single-photon emitters. To date, the single photon origin remains under debate even though many experiments and theoretical calculations have been performed. We have measured the pressure-dependent photoluminescence (PL) spectra of hBN flakes at low temperatures by using a diamond anvil cell device. The absolute values of the pressure coefficients of discrete PL emission lines are all below 15 meV/GPa, which is much lower than the pressure-induced 36 meV/GPa redshift rate of the hBN bandgap. These PL emission lines originate from atom-like localized defect levels confined within the bandgap of the hBN flakes. Interestingly, the experimental results of the pressure-dependent PL emission lines present three different types of pressure responses corresponding to a redshift (negative pressure coefficient), a blueshift (positive pressure coefficient), or even a sign change from negative to positive. Density functional theory calculations indicate the existence of competition between the intralayer and interlayer interaction contributions, which leads to the different pressure-dependent behaviors of the PL peak shift.

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.

The energy band structures and related room temperature exciton transitions of monolayer and bilayer tungsten diselenide (WSe2) are investigated using photoluminescence (PL) spectra under hydrostatic pressure up to 5.42 GPa. For monolayer WSe2, it is found that the conduction band Λ valley is 70 ± 30 meV higher than the K valley at zero pressure, and the K-Λ valley crossover happens at a pressure of approximately 2.25 GPa. The PL peak of exciton related to the direct K-K interband transition in monolayer and bilayer WSe2 shows a pressure-induced blue-shift at the rates of 31.5 ± 0.6 and 27 ± 1 meV GPa(-1), respectively. The indirect Λ-K interband transition for monolayer and bilayer WSe2 exhibits a distinctly different pressure response. The pressure coefficient is as small as -3 ± 6 meV GPa(-1) for monolayer, but a much larger value of -22 ± 1 meV GPa(-1) for bilayer WSe2, indicating that the interlayer coupling has a strong effect on the electronic states at the Λ valley.

Quantum technologies require robust and photostable single-photon emitters. Here, room temperature operated single-photon emissions from isolated defects in aluminum nitride (AlN) films are reported. AlN films were grown on nanopatterned sapphire substrates by metal organic chemical vapor deposition. The observed emission lines range from visible to near-infrared, with highly linear polarization characteristics. The temperature-dependent line width increase shows T3 or single-exponential behavior. First-principle calculations based on density functional theory show that point defect species, such as antisite nitrogen vacancy complex (NAlVN) and divacancy (VAlVN) complexes, are considered to be an important physical origin of observed emission lines ranging from approximately 550 to 1000 nm. The results provide a new platform for on-chip quantum sources.

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.

Slice analysis is a generalization of the theory of holomorphic functions of one complex variable to quaternions. Among the new phenomena which appear in this context, there is the fact that the convergence domain of f (q) = Σ n∈N (q − p) * n an, given by a σ-ball Σ(p, r), is not open in H unless p ∈ R. This motivates us to investigate, in this article, what is a natural topology for slice regular functions. It turns out that the natural topology is the so-called slice topology, which is different from the Euclidean topology and nicely adapts to the slice structure of quaternions. We extend the function theory of slice regular functions to any domains in the slice topology. Many fundamental results in the classical slice analysis for axially symmetric domains fail in our general setting. We can even construct a counterexample to show that a slice regular function in a domain cannot be extended to an axially symmetric domain. In order to provide positive results we need to consider so-called path-slice functions instead of slice functions. Along this line, we can establish an extension theorem and a representation formula in a slice-domain.

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.

Exciton and biexciton emission energies as well as excitonic fine-structure splitting (FSS) in single InAs/GaAs quantum dots (QDs) have been continuously tuned in situ in an optical cryostat using a developed uniaxial stress device. With increasing tensile stress, the red shift of excitonic emission is up to 5 nm; FSS decreases firstly and then increases monotonically, reaching a minimum value of approximately 10 μeV; biexciton binding energy decreases from 460 to 106 μeV. This technique provides a simple and convenient means to tune QD structural symmetry, exciton energy and biexciton binding energy and can be used for generating entangled and indistinguishable photons.