Heavy-hole Exciton Research Articles

Strong high-energy exciton electroluminescence from the light holes of polytypic quantum dots

High-energy exciton emission could allow single-component multi-colour display or white light-emitting diodes. However, the thermal relaxation of high-energy excitons is much faster than the photon emission of them, making them non-emissive. Here, we report quantum dots with light hole-heavy hole splitting exhibiting strong high-energy exciton electroluminescence from high-lying light holes, opening a gate for high-performance multi-colour light sources. The high-energy electroluminescence can reach 44.5% of the band-edge heavy-hole exciton emission at an electron flux density Φe of 0.71 × 1019 s−1 cm−2 − 600 times lower than the photon flux density Φp (4.3 × 1021 s−1 cm−2) required for the similar ratio. Our simulation and experimental results suggest that the oscillator strength of heavy holes reduces more than that of light holes under electric fields. We attribute this as the main reason for strong light-hole electroluminescence. We observe this phenomenon in both CdxZn1-xSe-ZnS and CdSe-CdS core-shell quantum dots exhibiting large light hole-heavy hole splittings.

Chirality in Atomically Thin CdSe Nanoplatelets Capped with Thiol-Free Amino Acid Ligands: Circular Dichroism vs. Carboxylate Group Coordination.

Chiral semiconductor nanostructures and nanoparticles are promising materials for applications in biological sensing, enantioselective separation, photonics, and spin-polarized devices. Here, we studied the induction of chirality in atomically thin only two-monolayer-thick CdSe nanoplatelets (NPLs) grown using a colloidal method and exchanged with L-alanine and L-phenylalanine as model thiol-free chiral ligands. We have developed a novel two-step approach to completely exchange the native oleic acid ligands for chiral amino acids at the basal planes of NPLs. We performed an analysis of the optical and chiroptical properties of the chiral CdSe nanoplatelets with amino acids, which was supplemented by an analysis of the composition and coordination of ligands. After the exchange, the nanoplatelets retained heavy-hole, light-hole, and spin-orbit split-off exciton absorbance and bright heavy-hole exciton luminescence. Capping with thiol-free enantiomer amino acid ligands induced the pronounced chirality of excitons in the nanoplatelets, as proven by circular dichroism spectroscopy, with a high dissymmetry g-factor of up to 3.4 × 10-3 achieved for heavy-hole excitons in the case of L-phenylalanine.

Double resonant Raman scattering and photoluminescence mediated by heavy and light hole excitons in a (Cd, Mn)Te quantum well in external magnetic field

Double resonant Raman scattering and photoluminescence mediated by heavy and light hole excitons in a (Cd, Mn)Te quantum well in external magnetic field

Combined Effects of Intense THz Laser Field and Applied Electric Field on Binding Energy of Exciton in GaAs/GaAlAs Finite Spherical Quantum Dot

This paper describes the combined effects of intense THz laser field and electric field on the ground state binding energy of heavy hole excitons confined in GaAs/ GaAlAs spherical finite quantum dots. The formulation is based on the model of “laser dressed potential” which combines Coulomb interaction and field effect in only one potential as reported in the literature. The calculation is performed by using the variational method in the framework of the single band effective mass theory. Our results show that (i) the laser field increases the electron and hole confinement energy that form the exciton in the QD until they reach a maximum, then they become almost constant for an intense laser field, (ii)the electric field and the laser field lowers the binding energy for all quantum dot radii making the exciton stabilized and clustered near the center of the dot, and iii)the laser field increases the spatial extension of exciton but the electric field decreases it linearly.

Iterative density matrix revisited: Excitonic phenomena on the InAs quantum well as a saturable absorber

By solving the Liouville equation, third-order nonlinear terms of the optical absorptive two quantum-level system are obtained through iterative density matrix calculations. In the improved model, the full frequency range is taken into account instead of just the weak absorptive limit. he process investigated in this study can be likened to saturation fitting for heavy-hole excitons in the InAs quantum well. Additionally, we demonstrate that the iterative method used for analyzing band-to-band excitonic spectra enables easy retrieval of the static dielectric constant.

Quantum beat of heavy-hole and light-hole excitons originating from interband Stark-ladder transitions in a biased GaAs/AlAs superlattice

The letter reports the quantum beat of heavy-hole (HH) and light-hole (LH) excitons originating from interband Stark-ladder transitions in a GaAs/AlAs superlattice. Under the condition that the pump photon energy was well below the LH-related transition energy, only Bloch oscillation was observed. As the pump photon energy was increased, another oscillatory signal with a frequency of ∼6 THz was superimposed. The calculation considering related transitions indicates that the oscillation is due to the quantum beat of HH and LH excitons originating from the interband Stark-ladder transitions between the hole states and the electron state in the first-nearest-neighbor quantum wells.

Measuring the Ultrafast Spectral Diffusion and Vibronic Coupling Dynamics in CdSe Colloidal Quantum Wells using Two-Dimensional Electronic Spectroscopy.

We measure the ultrafast spectral diffusion, vibronic dynamics, and energy relaxation of a CdSe colloidal quantum wells (CQWs) system at room temperature using two-dimensional electronic spectroscopy (2DES). The energy relaxation of light-hole (LH) excitons and hot carriers to heavy-hole (HH) excitons is resolved with a time scale of ∼210 fs. We observe the equilibration dynamics between the spectroscopically accessible HH excitonic state and a dark state with a time scale of ∼160 fs. We use the center line slope analysis to quantify the spectral diffusion dynamics in HH excitons, which contains an apparent sub-200 fs decay together with oscillatory features resolved at 4 and 25 meV. These observations can be explained by the coupling to various lattice phonon modes. We further perform quantum calculations that can replicate and explain the observed dynamics. The 4 meV mode is observed to be in the near-critically damped regime and may be mediating the transition between the bright and dark HH excitons. These findings show that 2DES can provide a comprehensive and detailed characterization of the ultrafast spectral properties in CQWs and similar nanomaterials.

Effects of Pump Photon Energy on Generation and Ultrafast Relaxation of Excitons and Charge Carriers in CdSe Nanoplatelets.

We studied the initial nature and relaxation of photoexcited electronic states in CdSe nanoplatelets (NPLs). Ultrafast transient optical absorption (TA) measurements were combined with the theoretical analysis of the formation and decay of excitons, biexcitons, free charge carriers, and trions. In the latter, photons and excitons were treated as bosons and free charge carriers as fermions. The initial quantum yields of heavy-hole (HH) excitons, light-hole (LH) excitons, and charge carriers vary strongly with photon energy, while thermal relaxation occurs always within 1 ps. After that, the population of LH excitons is negligible due to relaxation to HH excitons or decay into free electrons and holes. Up to the highest average number of about four absorbed photons per NPL in our experiments, we found no signatures of the presence of biexcitons or larger complexes. Biexcitons were only observed due to the interaction of a probe-generated exciton with an exciton produced previously by the pump pulse. For higher pump photon energies, the initial presence of more free charge carriers leads to formation of trions by probe photons. On increasing the number of absorbed pump photons in an NPL, the yield of excitons becomes higher as compared to free charge carriers, since electron-hole recombination becomes more likely. In addition to a TA absorption feature at energy below the HH exciton peak, we also observed a TA signal at the high-energy side of this peak, which we attribute to formation of LH-HH biexcitons or trions consisting of a charge and LH exciton.

Ultrafast quantum-path interferometry of photo-absorption involving excitons in a GaAs multiple-quantum-well structure

Using a pump–probe protocol with pairs of relative-phase-locked femtosecond pulses, we gathered transient reflectivity measurements from a multiple quantum well structure of GaAs/Al0.3Ga0.7As at 10 K. Quantum beating between heavy-hole and light-hole excitons was observed as an oscillation in the transient reflectivity. Its intensity was coherently controlled by controlling the delay between two successive pump pulses. We found characteristic interference fringes arising from the interference of the photo-induced excitons traversing entangled quantum paths. As established in theory, these fringes are peculiar to this dual-pulse excitation via photo-absorption.

Effects of strain and hydrostatic pressure on exciton properties in asymmetric zinc-blende (In,Ga)N/GaN coupled double quantum wells

Within the effective mass approximation, we studied the light-hole and heavy-hole exciton states and their optical properties, such as oscillator strength and radiative decay time, in asymmetric zinc-blende GaN/InxGa1−xN/GaN/InyGa1−yN/GaN coupled double quantum wells. The variational method was performed under strain and hydrostatic pressure. The light-hole and heavy-hole exciton binding energies as a function of the left-well width and/or right-well width, middle-barrier thickness, and indium concentration x(y) of (In,Ga)N material are analyzed in detail. The light-hole and heavy-hole exciton binding energies show an increasing trend with increase of hydrostatic pressure. The hydrostatic pressure not only results in an increase in exciton binding energy, interband transition energy, and oscillator strength but also a decrease in the emission wavelength and radiative decay time. In addition, the biaxial strain dependence of the emission wavelength, radiative decay time, and interband transition energy of the exciton is very remarkable.

Effects of external perturbations on light absorption by light/heavy hole excitons in a semi-parabolic quantum well

The effects of electric, magnetic, and intense terahertz laser fields on optoelectronic properties are investigated in a GaAs/AlxGa1-xAs semi-parabolic quantum well. The binding energy, exciton light absorption coefficients, interband transition energies, and dipole elements in the matrix are studied within the structure of the parabolic band and effective-mass approximations. The transition energy difference between the ground-state electron energy level and the ground-state heavy hole (light hole) energy level and matrix elements are carried out as functions of external perturbations with a constant well size. The results exhibit that the exciton binding energies of heavy and light holes reduce as long as the laser dressing parameter is larger. The optical absorption of light hole exciton in the system is stronger than the heavy hole exciton. And the excitonic energies, transition energies, and absorption coefficients are much more sensitive and controlled by changing the external perturbations.

Heavy-hole–light-hole exciton system in GaAs/AlGaAs quantum wells

Light-hole (lh) excitons (Xlhs) in quantum wells (QWs) are hardly studied compared with heavy-hole (hh) excitons (Xhhs), mainly due to the difficulties of their experimental observation. In this paper, a comprehensive study of both types of excitons in high-quality GaAs/AlGaAs QWs of different widths is performed. We focus on the energy positions of exciton resonances and the exciton-light interaction. The effect of mixing lhs and hhs in GaAs-based structures on these exciton characteristics is investigated. The corrections to the exciton energy due to mixing are only a fraction of millielectronvolts for wide QWs, in which the energy levels of Xhhs and Xlhs are close to each other. It is also experimentally found that the oscillator strength of Xlhs is $\ensuremath{\sim}2.5$ times less than that of Xhhs. This value noticeably deviates from the 3:1 oscillator strength ratio known for optical transitions between free electron and hole states. This deviation originates from the different squeezing of the Xlh and Xhh wave functions due to distinct values of the effective masses of the Xlh and Xhh in the heterostructure. Mixing of hh and lh valence bands is not so important.

Quantum beats of light-hole and heavy-hole excitons in reflection spectra of GaAs/AlGaAs quantum well

In this work we experimentally study dynamics of the heavy-hole and light-hole exciton states in a quantum well under coherent excitation of both excitonic resonances using co- and cross-linear and circular polarizations of pump and probe beams. Oscillations of basic parameters of excitonic resonances are observed. Their dependence on the polarization of the pump and probe beams is used to determine the structure of states of the heavy-hole and light-hole excitons by means of theoretical modelling of experimental data. Keywords: quantum well, excitons, quantum beats, light-hole exciton, heavy-hole exciton.

Квантовые биения экситонов с легкими и тяжелыми дырками в спектрах отражения в квантовой яме GaAs/AlGaAs

In this work we experimentally study dynamics of states of light- and heavy-hole excitons in a quantum well under coherent excitation of both excitonic resonances using co- and cross- linear and circular polarizations of pump and probe beams. In the parameters of excitonic resonances quantum beats are observed. Their polarization-dependent existence is used to determine the structure of states of the light- and heavy-hole exciton system by means of theoretical modelling of experimental data.

Non-trivial dependence of spectral characteristics of excitons in quantum wells on the resonant optical excitation power

Basic exciton parameters, such as the energy of exciton transition and the radiative and nonradiative broadenings, are experimentally studied by means of reflectance spectroscopy for a heterostructure with a 14-nm GaAs/AlGaAs quantum well. Particular attention is paid to the nonradiative broadening which is sensitive to densities of free carriers and long-lived nonradiative excitons. A sublinear increase of the broadening of the heavy-hole and light-hole exciton resonances is observed when the light-hole exciton resonance is excited with increasing power. A simple model is developed, which allows one to well reproduce the observed dependence. Keywords: exciton, quantum well, nonradiative broadening.

Exciton-exciton and exciton–charge carrier interaction and exciton collisional broadening in GaAs/AlGaAs quantum wells

Wave functions of heavy-hole excitons in GaAs/Al$_{0.3}$Ga$_{0.7}$As square quantum wells (QWs) of various widths are calculated by the direct numerical solution of a three-dimensional Schr\"odinger equation using a finite-difference scheme. These wave functions are then used to determine the exciton-exciton, exciton-electron and exciton-hole fermion exchange constants in a wide range of QW widths (5-150 nm). Additionally, the spin-dependent matrix elements of elastic exciton-exciton, exciton-electron and exciton-hole scattering are calculated. From these matrix elements, the collisional broadening of the exciton resonance is obtained within the Born approximation as a function of the areal density of excitons, electrons and holes respectively for QW widths of 5, 15, 30 and 50 nm. The obtained numerical results are compared with other theoretical works.

Stability of Excited Exciton States in Semiconductor Quantum Dots Under a Lateral Electric Field

The effect of the lateral electric field (LEF) on the excited and ground state stability of an exciton ([Formula: see text]) confined in a parabolic cylindrical quantum dot (QD) was estimated in this study. The calculation was performed in the framework of single-band effective mass theory using a variational approach. Our results revealed that the ground state binding energy of [Formula: see text] decreases with increasing the cylindrical QD radius until the exciton stability is lost at moderate LEF strength. By increasing the LEF strength, the excited heavy-hole ([Formula: see text]) can create an excited state [Formula: see text] or excited state [Formula: see text] of [Formula: see text], and the results indicate that the first state is more stable. In contrast, when an excited electron ([Formula: see text]) combines with an excited hole ([Formula: see text]) or unexcited hole ([Formula: see text]), it contains no split excited states for [Formula: see text] with less binding energy than the state [Formula: see text]. Comparing our previous results of donor impurity [Formula: see text] with [Formula: see text], we found that [Formula: see text] has a more stable ground state than [Formula: see text]. Moreover, the excited [Formula: see text] states are more stable than the excited states of [Formula: see text]. The quantum Stark shift (QSS) of the light- and heavy-hole exciton energy was explored, and a blue-shifted and quadratic QSS was observed. In contrast, for single particles (electron, heavy-hole and light hole), a red-shifted and linear QSS was observed.

Modulation of exciton states through resonant excitation by continuous wave lasers in a GaAs/AlAs multiple quantum well

Interactions between excitons with different energies are significant in device application, particularly in the emission of intense terahertz (THz) waves caused by difference frequency mixing. In this paper, the interactions of excitons generated by two resonant continuous-wave laser beams are assessed via the change in reflectivity. While calculations suggest that the reflectivity change depends on the exciton number, experimental results measured for various excitation powers, probe powers, and excitation energies (heavy-hole (HH) and light-hole (LH) excitons) show different profiles. Furthermore, the signals probed at a HH exciton differ from those at a LH exciton. These results indicate the importance of careful control of excitation levels for THz wave generation.

Emission characteristics variation of GaAs0.92Sb0.08/Al0.3Ga0.7As strained multiple quantum wells caused by rapid thermal annealing

Rapid thermal annealing is an effective way to improve the optical properties of semiconductor materials and devices. In this paper, the emission characteristics of GaAs0.92Sb0.08/Al0.3Ga0.7As multiple quantum wells, which investigated by temperature-dependent photoluminescence, are adjusted through strain and interfacial diffusion via rapid thermal annealing. The light-hole (LH) exciton emission and the heavy-hole (HH) exciton emission are observed at room temperature. After annealing, the LH and HH emission peaks have blue shift. It can be ascribed to the variation of interfacial strain at low annealing temperature and the interfacial diffusion between barrier layer and well layer at high annealing temperature. This work is of great significance for emission adjustment of strained multiple quantum wells.

Dark exciton preparation in a quantum dot by a longitudinal light field tuned to higher exciton states

Several important proposals to use semiconductor quantum dots in quantum information technology rely on the control of the dark exciton ground states, such as dark exciton based qubits with a $\mu$s life time. In this paper, we present an efficient way to occupy the dark exciton ground state by a single short laser pulse. The scheme is based on an optical excitation with a longitudinal field component featured by, e.g., radially polarized beams or certain Laguerre-Gauss or Bessel beams. Utilizing this component, we show within a configuration interaction approach that high-energy exciton states composed of light-hole excitons and higher dark heavy-hole excitons can be addressed. When the higher exciton relaxes, a dark exciton in its ground state is created.