Oscillator Strengths Of Optical Transitions Research Articles

Local defects in two-dimensional gallium sulfide as potential single-photon emitters: First-principles evaluation

The single-photon emitters (SPEs) are essential components of emerging quantum communication technology. It has been shown that the local defects in the wide-bandgap semiconductors can be sources of sharp zero-phonon lines (ZPLs) of emission, which is favorable for applications. Most of the reported SPEs emit photons in the visible light region, while fiber-based quantum communication applications require sources of near-infrared (NIR) emission. Therefore, the purpose of this paper is to search for defects which are promising candidates for SPEs in the NIR region by applying computational methods. To this end, based on an educated guess, 12 local doping substitutional defects in two-dimensional (2D) gallium sulfide were selected for consideration. The selected defects are ${Y}_{X}$, where $X=\mathrm{Ga}$ or S, and $Y=\mathrm{C}$, Si, Ge, N, P, As. Here, $X$ stands for a substituted element and $Y$ for a dopant. To evaluate the stability of the defects, their formation energy, binding energy of the dopants, and phonon spectra were calculated within the density functional theory (DFT) approximation. Three defects, ${\mathrm{Ge}}_{\mathrm{Ga}}, {\mathrm{N}}_{\mathrm{Ga}}$, and ${\mathrm{P}}_{\mathrm{Ga}}$, are found to have imaginary-frequency vibration modes, indicating their dynamical instability. They were removed from further consideration. A preliminary analysis of the DFT densities of electronic states (DOS) revealed six remaining defects, ${\mathrm{As}}_{\mathrm{Ga}}, {\mathrm{Ge}}_{\mathrm{S}}, {\mathrm{Si}}_{\mathrm{S}}, {\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$, with DOS favorable for formation of local optical excitations in the NIR region. Next, for these six defects, the state-of-the-art linear response GW and the Bethe-Salpeter equation (BSE) methods were used to obtain their electronic structures, in terms of the independent quasiparticle (IQP) states, and optical excitation spectra, in terms of the frequency-dependent dielectric function and the dipole optical transition oscillator strength. The calculation results indicate that the ${\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$ defects have intense sharp excitation peaks in the NIR region. The analysis of the contributions of the IQP states to the corresponding BSE eigenstates brings us to the conclusion that these excitations can result in emission with dominating narrow ZPL peaks. I thus propose here the ${\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$ defects in 2D GaS as promising candidates for SPEs in the NIR emission region.

Combined Experimental and Ab Initio Methods for Rationalization of Magneto-Luminescent Properties of YbIII Nanomagnets Embedded in Cyanido/Thiocyanidometallate-Based Crystals.

The ab initio calculations were correlated with magnetic and emission characteristics to understand the modulation of properties of NIR-emissive [YbIII(2,2′-bipyridine-1,1′-dioxide)4]3+ single-molecule magnets by cyanido/thiocyanidometallate counterions, [AgI(CN)2]− (1), [AuI(SCN)2]− (2), [CdII(CN)4]2–/[CdII2(CN)7]3– (3), and [MIII(CN)6]3– [MIII = Co (4), Ir (5), Fe (6), Cr (7)]. Theoretical studies indicate easy-axis-type ground doublets for all YbIII centers. They differ in the magnetic axiality; however, transversal g-tensor components are always large enough to explain the lack of zero-dc-field relaxation. The excited doublets lie more than 120 cm–1 above the ground one for all YbIII centers. It was confirmed by high-resolution emission spectra reproduced from the ab initio calculations that give reliable insight into energies and oscillator strengths of optical transitions. These findings indicate the dominance of Raman relaxation with the power n varying from 2.93(4) to 6.9(2) in the 4–3–5–1–2 series. This trend partially follows the magnetic axiality, being deeper correlated with the phonon modes schemes of (thio)cyanido matrices.

Electrical tuning of the oscillator strength in type II InAs/GaInSb quantum wells for active region of passively mode-locked interband cascade lasers

Two designs of active region for an interband cascade laser, based on double or triple GaInSb/InAs type II quantum wells (QWs), were compared with respect to passive mode-locked operation in the mid-infrared range around 4 µm. The layer structure and electron and hole wavefunctions under external electric field were engineered to allow controlling the optical transition oscillator strength and the resulting lifetimes. As a result, the investigated structures can mimic absorber-like and gain-like sections of a mode-locked device when properly polarized with opposite bias. A significantly larger oscillator strength tuning range for triple QWs was experimentally verified by Fourier-transform photoreflectance.

Novel design of type-II quantum wells for mid-infrared emission with tensile-strained GaAsSb layer for confinement of holes

We theoretically investigate the electronic structure properties of modified design GaSb-based type-II W-shaped quantum wells predicted for applications in interband cascade lasers. The results show that introducing a tensely-strained GaAsSb layer to confine the holes offers an additional tuning knob for the active optical transition oscillator strength and emission wavelength. Moreover, such a solution allows exploiting designs with thicker quantum well layers, which translates into a decreased sensitivity of the system to the growth inaccuracies, making it more robust and thus easier to reproduce.

Blue and green electroluminescence from CdSe nanocrystal quantum-dot-quantum-wells

CdS/CdSe/ZnS quantum dot quantum well (QDQW) nanocrystals were synthesized using the successive ion layer adsorption and reaction technique, and their optical properties were tuned by bandgap and strain engineering. 3-monolayer (ML) CdSe QWs emitted blue photoluminescence at 467 nm with a spectral full-width-at-half-maximum of ∼30 nm. With a 3 ML ZnS cladding layer, which also acts as a passivating and strain-compensating layer, the QDQWs acquired a ∼35% quantum yield of the QW emission. Blue and green electroluminescence (EL) was obtained from QDQW light-emitting devices with 3–4.5 ML CdSe QWs. It was found that as the peak blueshifted, the overall EL was increasingly dominated by defect state emission due to poor hole injection into the QDQWs. The weak EL was also attributed to strong field-induced charge separation resulting from the unique QDQW geometry, weakening the oscillator strength of optical transitions.

Oscillator strength for optical transitions in a cylindrical quantum wire with an inverse parabolic confining electric potential

Evaluations of oscillator strength for single electron transitions in a cylindrical quantum wire are carried out. In particular, effect of a potential that varies inversely with square of radial distance on the oscillator strength is investigated. Oscillator strength generally decreases as confining potential increases in strength. For l > 0 transitions, oscillator strengths go through local minima, as strength of inverse parabolic potential increases. These local minima percolate from interaction integral and are due to squashing of electron wave functions as confining potential gets stronger.

Spectroscopic and electronic structure properties of CdSe nanocrystals: spheres and cubes

In this work we study the electronic structure of CdmSem quantum dots of various sizes and different shapes such as spheres and cubes using DFT, TDDFT, and CIS methods. This work requires a careful computational analysis where a proper exchange-correlation functional has to be chosen to fit the experimental optical gap. We find some differences in the optical and HOMO-LUMO gap values between spheres and cubes. In general, the gaps for the cubes have higher values than those for the spheres. We also calculate optical absorption spectra using the data for energy levels and oscillator strengths for optical transitions. We find that DFT yields some discrepancy in the density of states for the spheres and cubes. However, the density of states calculated by TDDFT and CIS provide better agreement. The results of the calculation can be useful for quantum dots synthesized in laser ablation experiments.

Increasing the optical transition oscillator strength in GaSb-based type II quantum wells

The oscillator strength of the fundamental optical transition in GaSb-based type II quantum well structures as one of the crucial parameters for the performance of interband cascade lasers was investigated. Modulation spectroscopy, supported by eight-band k·p calculations, has been employed as a sensitive probing technique allowing to determine the transition intensities of samples with various layer structures. The results show that altering the composition of the valence band well in a type II system can efficiently enhance the transition oscillator strength. Especially, the utilization of a quaternary GaInAsSb material for hole confinement turned out to be highly beneficial.

Carrier dynamics in type-II GaAsSb/GaAs quantum wells

Time-resolved photoluminescence (PL) characteristics of type-II GaAsSb/GaAs quantum wells are presented. The PL kinetics are determined by the dynamic band bending effect and the distribution of localized centers below the quantum well band gap. The dynamic band bending results from the spatially separated electron and hole distribution functions evolving in time. It strongly depends on the optical pump power density and causes temporal renormalization of the quantum well ground-state energy occurring a few nanoseconds after the optical pulse excitation. Moreover, it alters the optical transition oscillator strength. The measured PL lifetime is 4.5 ns. We point out the critical role of the charge transfer processes between the quantum well and localized centers, which accelerate the quantum well photoluminescence decay at low temperature. However, at elevated temperatures the thermally activated back transfer process slows down the quantum well photoluminescence kinetics. A three-level rate equation model is proposed to explain these observations.

Oscillator strengths of optical transitions of a cylindrical shell: Crossed static electric and magnetic fields

The single-electron eigenstates of a cylindrical shell are determined as functions of the applied crossed electric and magnetic fields in the effective-mass approximation. The system considered consists of donor charges taken to be uniformly distributed within an inner core of infinitely long length. The core is concentrically enveloped by a semiconducting material of finite thickness; which is essentially the host material. This configuration of the donor charges sets up a spatially varying electric field nonetheless with only the radial component. In addition, a uniform magnetic field is applied parallel to the axis of symmetry of the inner core. As is well known, the axial applied magnetic field lifts the double degeneracies of the electron’s subbands characterized by the same azimuthal quantum numbers which differ only in sign. The main effect of increasing the external electric field is to elevate the various energy subbands, more or less to the same extent, to higher values. Further, evaluations of the oscillator strengths of optical transitions of the cylindrical shell are carried out within the dipole approximation. The radiation field is taken to be that of circularly polarized light incident along the axis of the core. The oscillator strengths of optical transitions are found to increase with an increase of the applied magnetic field, particularly in the regime of small magnetic fields. In contrast, the oscillator strengths of these optical interactions become suppressed as the donor charge density is increased.

Effect of the Built-In Strain on the In-Plane Optical Anisotropy of m-Plane GaN/AlGaN Quantum Wells

We study theoretically the influence of the anisotropic biaxial strain originating from the lattice mismatch between the m-plane GaN/AlGaN quantum wells structure and the substrate on the optical anisotropy of such systems. It is demonstrated that the oscillator strengths for optical transitions with polarization of light parallel and perpendicular to the crystal axis c strongly depend on strain to such an extent that, by increasing the concentration of Al in the substrate from x = 0 to x = 0.5 one can change the polarization of the emitted light with respect to the c-axis by 90 degrees.

Effects of symmetry-breaking perturbations on excitonic states bound to systems of reduced symmetry

Using an invariant expansion, we build an Hamiltonian describing the influence of the crystal-field, the electron-hole exchange interaction, and any symmetry-breaking perturbations on the fine structure of excitons bound to systems of reduced symmetry: D2d, C3v, and C2v. Several perturbations are considered, including, but not limited to, an electric field, a magnetic field, a strain field, and their combinations. For each symmetry system considered, symmetrized excitonic wave functions, build from heavy- and light-holes states, are used to expand the Hamiltonian in the form of matrices, whose eigenvalues directly provide the energy of the excitonic states and whose eigenstates can be used to determine oscillator strengths of optical transitions. Using this model, we satisfactorily reproduce the excitonic emission observed from nitrogen dyads in GaAs and tellurium dyads in strained ZnSe. We also present Hamiltonians for independent heavy- and light-hole subsystems. Comparing the two models, we demonstrate that the splitting observed in strained quantum dots of C2v symmetry does not necessarily imply a significant anisotropic exchange interaction. This splitting can be produced by a weak coupling between heavy- and light-hole bands.

Calculations of electronic and optical properties of diamond quantum films

Energy gaps and oscillator strengths (lifetimes) for (0 0 1) ultrathin quantum films of diamond are calculated by the extended Hückel-type nonorthogonal tight-binding method. For the diamond films with monohydrogenated dimers on the surface, a lowest unoccupied surface state (LUSS) occurs just below the conduction-band minimum of the diamond bulk at the center of the (0 0 1) surface Brillouin zone, and that a direct energy gap is formed between the LUSS and the film valence-band maximum. Calculations of the density of states show that the LUSS extends into the deeper C layers up to the ninth layer, and that a significant s-orbital character is incorporated into the LUSS by the formation of the monohydrogenated dimer on the film surface, which is responsible for significant oscillator strength for optical transitions across the energy gap. It is shown that calculated lifetimes across the energy gap are less than 100 μs, suggesting that the quantum films of diamond studied have potential as light-emitting materials.

Large vacuum Rabi splitting in a multiple quantum well GaN-based microcavity in the strong-coupling regime

An $\mathrm{Al}\mathrm{In}\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{N}$ microcavity (MC) containing a $\mathrm{Ga}\mathrm{N}∕{\mathrm{Al}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$ multiple quantum well (MQW) structure is investigated through room temperature (RT) photoluminescence and reflectivity experiments. A vacuum Rabi splitting as large as $50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at RT is reported for this MC structure, the highest value reported so far for a semiconductor MC containing QWs. This is shown to result from a geometry combining a state of the art nitride-based MC with a MQW system where the built-in electric field has a reduced impact on the oscillator strength of optical transitions. The contribution of Bragg modes seen in optical spectra is well accounted for by transfer matrix simulations. In addition, the sensitivity of the present system to the tuning between the various optical components of the microcavity (bottom and top Bragg reflectors and active cavity region) to maximize the strong-coupling regime is also shown through simulations. Prospects regarding the nonlinear polariton emission from such a structure indicate that this type of MCs could potentially sustain ultrafast polariton parametric amplification up to $440\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, thanks to an increased exciton binding energy. More generally, it is predicted that, owing to a large exciton saturation density in excess of $1\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ per QW, such a MC structure would be suitable for the observation of nonlinear effects associated with cavity polaritons (polariton lasing and polariton Bose-Einstein condensates) at RT and above.

Taking Account of the Nonstationarity in Analyzing the Optically Stimulated Electron Emission of Irradiated Dielectrics

The distortions of the optically stimulated electron emission (OSEE) spectra resulting from the exponential decay in the recording time have been analyzed. It has been shown that taking account of the emission current nonstationary in analyzing the emission spectra permits obtaining isochronal temperature dependences of intensity for selective OSEE bands. The procedure for calculating the quantum yield of the OSEE and the activation barriers of thermal ionization of the excited states of defects is described. Analysis of the photoemission kinetics permits estimating the concentration of active OSEE centers with unknown parameters (optical transition oscillator strength, quantum yield). For diamagnetic oxygen-deficient centers in glassy SiO2, the photoemission kinetics has been calculated and the thermal ionization barrier of the photoexcited singlet state and the quantum yield of the OSEE have been determined.

Anisotropic optical properties of mechanically aligned single-walled carbon nanotubes in polymer

We have prepared aligned single-walled carbon nanotubes in polymer by mechanical stretching. The polarized absorption and Raman spectra of the aligned samples have been investigated. The optical transitions in nanotubes strongly depend on the angle θ between the polarization of the incident light and the nanotube axis: the absorbance measured at the peak energy shows a maximum at θ=0°, and they are suppressed at θ=90°. Raman spectra also show strong polarization dependence, which is mainly determined by the dependence of the oscillator strength of optical transitions in nanotubes.

Photoreflectance investigations of oscillator strength and broadening of optical transitions for GaAsSb–GaInAs/GaAs bilayer quantum wells

GaAsSb–GaInAs/GaAs bilayer quantum wells which consist of two adjacent layers of GaAsSb and GaInAs sandwiched between GaAs barriers have been investigated by photoreflectance (PR) spectroscopy. The oscillator strengths of optical transitions in such multiheterointerface structures have been determined from the experiment and compared with the results of envelope function calculations. Additionally, the broadening of the PR features has been analyzed and a correlation has been found with the character of the transitions: the broadening increases significantly when the type of the transition changes from direct to indirect.

Single-electron states near a current-carrying core

The energy spectrum of an electron confined near a current-carrying core is obtained as a function of the azimuthal applied magnetic field within the effective-mass approximation. The double degeneracy of the non-zero electron's axial wave number ( k z ) states is lifted by the current-induced magnetic field while that of the non-zero azimuthal quantum number ( m) states is preserved. A further analysis is the evaluations of the oscillator strengths for optical transitions involving the lowest-order pair of the electron's energy subbands within the dipole approximation. The radiation field is taken as that of elliptically polarized light incident along the core axis. In this polarization and within the dipole approximation, the allowed transitions are only those governed by the following specific selection rules. The azimuthal quantum numbers of the initial and final states must differ by unity while the electron's axial wave number is conserved. The azimuthal magnetic field is also found to lift the multiple degeneracies of the k z ≠0 interaction integrals as well as those of the oscillator strengths for optical transitions.

Vertically polarized lasing and photoluminescence in a ridge quantum-wire laser

We study vertically polarized lasing and spontaneous emissions in a ridge quantum-wire (QWR) laser. In particular, we find that most of emissions with energies near the band edge are vertically polarized. We make numerical calculation of oscillator strength for optical transitions between various combinations of electron and hole states, and indeed show oscillator strength of vertically polarized transitions is stronger in our QWR structure. We find that the different effective mass of electrons and holes, especially anisotropic effective mass of holes causes different shapes of wave functions between electrons and holes, which results in larger oscillator strength of vertically polarized transition.

Interplay of confinement, strain, and piezoelectric effects in the optical spectrum of GaN quantum dots.

We theoretically investigated excitonic states, energy and oscillator strength of optical transitions in GaN quantum dots characterized by different size, shape, interface, and substrate. On the basis of our multi-band model we determined that the piezoelectric field-induced red shift of the ground state transition, observed in recent experiments, can manifest itself only in strained GaN/AIN dots with the dot height larger than 3 nm. It was also established that the oscillator strength of the red-shifted transitions is small (< 0.05) and decreases fast with increasing the dot size, while the strength of ground state transitions in c-GaN/c-AIN and GaN/dielectric dots is large (approximately 0.4-0.7) and almost independent of the dot size.