Particle localization and phonon sidebands in GaAs/AlxGa1-xAs multiple quantum wells

Shedding light on exciton's nature in monolayer quantum material by optical dispersion measurements

Strong light-matter interactions based on two-dimensional excitons formed in quantum materials such as monolayer transition-metal dichalcogenides have become a major subject of research in recent years. Particularly attractive is the extraordinarily large oscillator strength as well as binding energy of the excitonic quasiparticles in these atomically-thin crystal lattices. Numerous theoretical studies and experiments have been devoted to the exploration of the excitonic systems that could be exploited in future nano-scaled optoelectronic devices. To obtain unique insight into the exciton's characteristics in an archetype monolayer quantum material, we directly measure the quasiparticle energy-momentum dispersion for the first time optically. Our results for h-BN encapsulated single-layer WSe₂ clearly indicate an emission regime with a dispersion in the meV range in within the light cone at cryogenic temperatures. The amount of dispersion agrees well with calculations for an exciton-polariton based on the material's monolayer exciton, or energetic modifications caused by exciton exchange interactions predicted for this material family. The measurable dispersion slightly weakens for elevated excitation densities, whereas at elevated temperatures, it even becomes immeasurable. The obtained reduction in dispersion is attributed to an enhanced role of uncorrelated charge carriers as well as the formation of phonon sidebands above 100 K.

Exciton emissions in quasi one-dimensional layered KP15

We studied the excitonic states of KP15 nanowires, which have high carrier mobility and in-plane anisotropic electrical and optical properties. Power, thickness, and temperature-dependent photoluminescence (PL) measurements were carried out. We found two neutral exciton emissions from KP15 nanowires. The high energy emission (1.83 eV) seems to have been produced by the surface state, and the lower one (1.67 eV) may have been produced by the original crystal structure of KP15. The KP15 nanowires also exhibited a large exciton binding energy (98 meV), which is one order of magnitude greater than those of common semiconductors. These properties make KP15 nanowires an interesting material for electrical and optical applications.

Resonant photoluminescence studies of carrier localisation in c-plane InGaN/GaN quantum well structures

In this paper we report on changes in the form of the low temperature (12 K) photoluminescence spectra of an InGaN/GaN quantum well structure as a function of excitation photon energy. As the photon energy is progressively reduced we observe at a critical energy a change in the form of the spectra from one which is determined by the occupation of the complete distribution of hole localisation centres to one which is determined by the resonant excitation of specific localisation sites. This change is governed by an effective mobility edge whereby the photo-excited holes remain localised at their initial energy and are prevented from scattering to other localisation sites. This assignment is confirmed by the results of atomistic tight binding calculations which show that the wave function overlap of the lowest lying localised holes with other hole states is low compared with the overlap of higher lying hole states with other higher lying hole states.

High Anisotropy in Tubular Layered Exfoliated KP15

Two-dimensional (2D) materials with high anisotropic properties, such as black phosphorus and ReS₂, show amazing potential for applications in future nanoelectronic and optoelectronic devices. However, degradation of black phosphorus under ambient conditions and the expensiveness of Re block their application. In this study, another layered material, KP₁₅, that has highly anisotropic properties was successfully prepared. The detailed crystal structure and electron-density distribution calculation reveal that KP₁₅ exhibits an anisotropic layered structure with two rows of P tubes connected by K atoms that are antiparallel in a single layer. Outstanding chemical stability, angular dependence of the Raman response, excitation, and exciton emission at room temperature have been found in exfoliated KP₁₅ nanoribbons. Importantly, the exciton emission at room temperature suggests the existence of a large exciton binding energy. Our results indicate that, because this layered material, KP₁₅, has high anisotropic properties and ultrachemical stability and is derived from abundant raw materials, it has great potential for applications in optoelectronic devices.

Probing the origin of excitonic states in monolayer WSe2

Two-dimensional transition metal dichalcogenides (TMDCs) have spurred excitement for potential applications in optoelectronic and valleytronic devices; however, the origin of the dynamics of excitons, trions, and other localized states in these low dimensional materials is not well-understood. Here, we experimentally probed the dynamics of excitonic states in monolayer WSe₂ by investigating the temperature and polarization dependent photoluminescence (PL) spectra. Four pronounced PL peaks were identified below a temperature of 60 K at near-resonant excitation and assigned to exciton, trion and localized states from excitation power dependence measurements. We find that the localized states vanish above 65 K, while exciton and trion emission peaks remain up to room temperature. This can be explained by a multi-level model developed for conventional semiconductors and applied to monolayer TMDCs for the first time here. From this model, we estimated a lower bound of the exciton binding energy of 198 meV for monolayer WSe₂ and explained the vanishing of the localized states. Additionally, we observed a rapid decrease in the degree of circular polarization of the PL at increasing temperatures indicating a relatively strong electron-phonon coupling and impurity-related scattering. Our results reveal further insight into the excitonic states in monolayer WSe₂ which is critical for future practical applications.

Observations of exciton-surface plasmon polariton coupling and exciton-phonon coupling in InGaN/GaN quantum wells covered with Au, Ag, and Al films

The coupling of excitons to surface plasmon polaritons (SPPs) and longitudinal optical (LO) phonons in Au-, Ag-, and Al-coated InxGa1-xN/GaN multiple and single quantum wells (SQWs) was studied with time-resolved cathodoluminescence (CL) and CL wavelength imaging techniques. Excitons were generated in the metal-coated SQWs by injecting a pulsed high-energy electron beam through the thin metal films, which is found to be an ideal method of excitation for plasmonic quantum heterostructures and nanostructures which are opaque to laser/light excitation. The Purcell enhancement factor (Fp) at low temperatures was obtained by the direct measurement of changes in the carrier lifetime caused by the SQW exciton-SPP coupling. The deposition of thin films of Al, Ag, and Au on an InGaN/GaN QW enabled a comparison of exciton-SPP coupling for energy ranges in which the surface plasmon energy is greater than, approximately equal to, and less than the QW excitonic transition energy. We investigated the temperature dependence of the Huang-Rhys factors for exciton-to-LO phonon coupling for the metal-covered and bare samples. CL imaging and spectroscopy with variable excitation densities are used to examine the spatial correlations between CL emission intensity, carrier lifetime, QW excitonic emission energy, and the Huang-Rhys factor, all of which are strongly influenced by local fluctuations in the In composition and formation of InN-rich centers.

Terahertz-induced optical emission of photoexcited undoped GaAs quantum wells

Intense terahertz (THz) pulse induces photoluminescence (PL) flash from undoped high-quality GaAs/AlGaAs quantum wells under continuous wave laser excitation. The number of excitons increases 10,000-fold from that of the steady state under only laser excitation. The THz electric field dependence and the relaxation dynamics of the PL flash intensity suggest that the strong electric field of the THz pulse ionizes impurity states during the 1 ps period of the THz pulse and release carriers from a giant reservoir containing impurity states in the AlGaAs layers.

GaN nanorods grown on Si (111) substrates and exciton localization

We have investigated exciton localization in binary GaN nanorods using micro- and time-resolved photoluminescence measurements. The temperature dependence of the photoluminescence has been measured, and several phonon replicas have been observed at the lower energy side of the exciton bound to basal stacking faults (I1). By analyzing the Huang-Rhys parameters as a function of temperature, deduced from the phonon replica intensities, we have found that the excitons are strongly localized in the lower energy tails. The lifetimes of the I1 and I2 transitions were measured to be < 100 ps due to enhanced surface recombination.PACS: 78.47.+p, 78.55.-m, 78.55.Cr, 78.66.-w, 78.66.Fd.

Localized suppression of longitudinal-optical-phonon-exciton coupling in bent ZnO nanowires

Using confocal micro-Raman and photoluminescence spectroscopy, we studied bending effects on optical properties of individual ZnO nanowires. Raman spectroscopy shows that local tensile strain can be introduced by bending the nanowire. The strain is expected to reduce the band gap on the bent part and modify the local phonon-exciton interaction. The corresponding micro-photoluminescence spectra indicate local suppression of the longitudinal-optical (LO) phonon-exciton interaction, which is determined by the intensity ratio of the second-order LO-phonon replica of the free exciton (FX-2LO) to the first-order process (FX-1LO). Our results may provide insight into the modulation of local electrical and optical properties by deforming the nanostructures.