Quantum-confined Stark Shift Research Articles

Harmonic to anharmonic tuning of moiré potential leading to unconventional Stark effect and giant dipolar repulsion in WS2/WSe2 heterobilayer

Excitonic states trapped in harmonic moiré wells of twisted heterobilayers is an intriguing testbed for exploring many-body physics. However, the moiré potential is primarily governed by the twist angle, and its dynamic tuning remains a challenge. Here we demonstrate anharmonic tuning of moiré potential in a WS2/WSe2 heterobilayer through gate voltage and optical power. A gate voltage can result in a local in-plane perturbing field with odd parity around the high-symmetry points. This allows us to simultaneously observe the first (linear) and second (parabolic) order Stark shift for the ground state and first excited state, respectively, of the moiré trapped exciton - an effect opposite to conventional quantum-confined Stark shift. Depending on the degree of confinement, these excitons exhibit up to twenty-fold gate-tunability in the lifetime (100 to 5 ns). Also, exciton localization dependent dipolar repulsion leads to an optical power-induced blueshift of ~ 1 meV/μW - a five-fold enhancement over previous reports.

Electronic and Optical Properties of Two-Dimensional GaN from First-Principles.

Gallium nitride (GaN) is an important commercial semiconductor for solid-state lighting applications. Atomically thin GaN, a recently synthesized two-dimensional material, is of particular interest because the extreme quantum confinement enables additional control of its light-emitting properties. We performed first-principles calculations based on density functional and many-body perturbation theory to investigate the electronic, optical, and excitonic properties of monolayer and bilayer two-dimensional (2D) GaN as a function of strain. Our results demonstrate that light emission from monolayer 2D GaN is blueshifted into the deep ultraviolet range, which is promising for sterilization and water-purification applications. Light emission from bilayer 2D GaN occurs at a similar wavelength to its bulk counterpart due to the cancellation of the effect of quantum confinement on the optical gap by the quantum-confined Stark shift. Polarized light emission at room temperature is possible via uniaxial in-plane strain, which is desirable for energy-efficient display applications. We compare the electronic and optical properties of freestanding two-dimensional GaN to atomically thin GaN wells embedded within AlN barriers in order to understand how the functional properties are influenced by the presence of barriers. Our results provide microscopic understanding of the electronic and optical characteristics of GaN at the few-layer regime.

Strain- and surface-induced modification of photoluminescence from self-assembled GaN/Al0.5Ga0.5N quantum dots: strong effect of capping layer and atmospheric condition

We report on the influence of a capping layer on the photoluminescence properties of self-assembled GaN quantum dots grown on an Al0.5Ga0.5N template. Self-assembled GaN quantum dots show a large quantum confined Stark shift and long carrier recombination time due to strong built-in spontaneous and piezoelectric polarization fields. Nevertheless, owing to strong carrier localization and suppressed nonradiative processes, these quantum dots have a high-quantum efficiency even at room temperature. Here, we show that the capping thickness has an important role on the optical properties of the GaN quantum dots. The radiative and nonradiative recombination processes of quantum dots are strongly affected by adjusting the capping thickness, and the GaN quantum dots with 12 monolayers-thick Al0.5Ga0.5N capping layer show a remarkably high internal quantum efficiency of more than 80% at room temperature. We also studied photoluminescence quenching and enhancement for surface (uncapped) quantum dots caused by photoadsorption and photodesorption of oxygen.

Electro-optic properties of GaInAsSb/GaAs quantum well for high-speed integrated optoelectronic devices

The electro-optic properties of strained GaInAsSb/GaAs quantum wells (QWs) are investigated. A single QW p-i-n sample was grown by molecular beam epitaxy with antimony (Sb) pre-deposition technique. We numerically predict and experimentally verify a strong quantum confined Stark shift of 40 nm. We also predict a fast absorption recovery times crucial of high-speed optoelectronic devices mainly due to strong electron tunneling and thermionic emission. Predicted recovery times are corroborated by bias and temperature dependent time-resolved photoluminescence measurements indicating (≤30 ps) recovery times. This makes GaInAsSb QW an attractive material particularly for electroabsorption modulators and saturable absorbers.

Nanoengineering the built-in electric field of a photonic device by interstitial-ion diffusion

We use focused laser annealing to activate the diffusion of Mn-interstitial ions (Mn${}_{i}$${}^{2+}$) from a $p$-(GaMn)As layer towards the intrinsic GaAs/AlAs quantum well (QW) region of a p-i-n light emitting diode (LED). The random clustering of the Mn${}_{i}$${}^{2+}$ ions creates a complex potential landscape $U$ and electric field ${\mathbf{F}}^{\mathrm{Mn}}=\ensuremath{-}\ensuremath{\nabla}U$ in the QW plane, which we probe with nanoscale precision by monitoring the quantum-confined Stark shift of the ``natural'' quantum dots formed in the QW. The use of focused laser annealing to form electric field landscapes at predetermined positions is potentially applicable to other material systems containing mobile dopant atoms and is relevant to research on nanophotonics and manipulation of quantum devices.

Stark spectroscopy and radiative lifetimes in single self-assembled CdTe quantum dots

We present studies on Coulomb interactions in single self-assembled CdTe quantum dots. We use a field effect structure to tune the charge state of the dot and investigate the impact of the charge state on carrier wave functions. The analysis of the quantum confined Stark shifts of four excitonic complexes allows us to conclude that the hole wave function is softer than electron wave function, i. e. it is subject to stronger modifications upon changing of the dot charge state. These conclusions are corroborated by time-resolved photoluminescence studies of recombination lifetimes of different excitonic complexes. We find that the lifetimes are notably shorter than expected for strong confinement and result from a relatively shallow potential in the valence band. This weak confinement facilitates strong hole wave function redistributions. We analyze spectroscopic shifts of the observed excitonic complexes and find the same sequence of transitions for all studied dots. We conclude that the universality of spectroscopic shifts is due to the role of Coulomb correlations stemming from strong configuration mixing in the valence band.

Quantum-Confined Stark Effect in Ensemble of Colloidal Semiconductor Quantum Dots

The presence of a strong, changing, randomly-oriented, local electric field, which is induced by the photo-ionization that occurs universally in colloidal semiconductor quantum dots (QDs), makes it difficult to observe the quantum-confined Stark effect in ensemble of colloidal QDs. We propose a way to inhibit such a random electric field, and a clear quantum-confined Stark shift is observed directly in close-packed colloidal QDs. Besides the applications in optical switches and modulators, our experimental results indicate how the oscillator strengths of the optical transitions are changed under external electric fields.

Electric field modulation of exciton recombination in InAs/GaAs quantum dots emitting at 1.3μm

Changing the electric field applied to InAs quantum dots embedded in a p-i-n diode was found to modulate the radiative recombination rate of excitons in the dots. The quantum dots were capped with a strain-reducing layer to realize 1.3 μm photoemission and a large dipole moment to the exciton states. The exciton states in a quantum dot were investigated by measuring the quantum-confined Stark shift for various applied electric fields and were compared with the theoretical electron and hole wave functions calculated using an eight-band k⋅p model. When the absolute value of the applied electric field was reduced from −82.4 kV/cm to 0, the radiative recombination rate increased from 0.88 to 1.11 ns−1. Comparison of the experimental rate with the calculated one revealed that the increase in the radiative recombination rate was due to a decrease in the overlap integral between the electrons and holes. These optical characteristics of InAs quantum dots are especially important for developing optical devices that use single photons and single charges because the contribution of nonradiative processes is smaller than that of the radiative process.

Manipulating quantum-confined Stark shift in electroluminescence from quantum dots with side gates

Single quantum dot (QD) light-emitting diodes were fabricated with side gates in a lateral p-i-n structure. The electroluminescence (EL) energy from the QDs can be controlled independently by the side gates and by forward bias. Stark shifts in EL have been observed up to 0.4 meV as a function of forward injection current, and around 0.7 meV by applying an electric field of 36 kV cm−1 across the QDs. The independent control of the QD emission energy is an important step towards electrically tuning the coupling between QDs and cavities, and generating entangled-photon sources.

Suppression of the internal electric field effects inZnO/Zn0.7Mg0.3O quantum wells by ion-implantation induced intermixing

Strong suppression of the effects caused by the internal electric field in ZnO/ZnMgOquantum wells following ion-implantation and rapid thermal annealing, is revealed byphotoluminescence, time-resolved photoluminescence, and band structure calculations. Theimplantation and annealing induces Zn/Mg intermixing, resulting in graded quantum wellinterfaces. This reduces the quantum-confined Stark shift and increases electron–holewavefunction overlap, which significantly reduces the exciton lifetime and increases theoscillator strength.

Analysis of Compressively Strained GaInAsP–InP Quantum-Wire Electro-Absorption Modulators

Performance of compressively strained (CS) GaInAsP-InP quantum-wire (QWR) electro-absorption modulators (EAMs) is theoretically studied using an eight-band kldrp model. An empirical relationship is proposed for the quantum-confined Stark shift in QWR EAMs. The accuracy of this relationship is verified by comparing with numerical data. The effects of the variation of different device parameters on the absorption spectra are investigated. The absorption peaks are found to be stronger in narrower QWRs with strain-compensating barriers. Comparison of the extinction ratio with that of similar quantum-well EAMs show that, in spite of the lower in-plane filling factor, QWR EAMs exhibit a higher extinction ratio. Effect of fluctuation of wire width on the absorption spectrum of QWRs has been studied. The proposed QWR EAMs are suitable for photonic integrated circuits (PICs) fabricated by electron-beam lithography, reactive-ion etching, and two-step epitaxial growth. Due to the nature of the integration in such structures, the QWR EAMs are not required to be polarization-insensitive. On the contrary, the QWR EAMs are naturally tuned to the polarization of the output of the CS QWR lasers, fabricated on the same PIC, leading to an enhancement of the absorption strength. Moreover, the QWR EAMs, integrated with QWR lasers, offer low insertion loss.

Electroabsorption and Electrorefraction in an InAs Quantum-Dot Waveguide Modulator

Optical transmission measurements are reported in the 1.3-mum region in an InAs quantum-dot (QD) waveguide modulator using a supercontinuum generator. A quantum-confined Stark shift of 15 nm was demonstrated in a five-layer QD stack with a field of 270 KV/cm. The electrically induced change in absorption has been measured and the change in refractive index (up to 0.001) has been estimated via the Kramers-Kronig relations. This highlights the potential of such a device as an electroabsorption or electrooptic phase modulator at telecommunication wavelengths.

Microscopic electroabsorption line shape analysis for Ga(AsSb)∕GaAs heterostructures

A series of Ga(AsSb)∕GaAs∕(AlGa)As samples with varying GaAs spacer width are studied by electric-field modulated absorption (EA) and reflectance spectroscopy and modeled using a microscopic theory. The analysis of the Franz–Keldysh oscillations of GaAs capping layer and of the quantum-confined Stark shift of the lowest quantum well (QW) transitions shows the strong inhomogeneity of the built-in electric field indicating that the field modulation due to an external bias voltage differs significantly for the various regions of the structures. The calculations demonstrate that the line shape of the EA spectra of these samples is extremely sensitive to the value of the small conduction band offset between GaAs and Ga(AsSb) as well as to the magnitude of the internal electric field changes caused by the external voltage modulation in the QW region. The EA spectra of the entire series of samples are modeled by the microscopic theory. The good agreement between experiment and theory allows us to extract the strength of the modulation of the built-in electric field in the QW region and to show that the band alignment between GaAs and Ga(AsSb) is of type II with a conduction band offset of approximately 40 meV.

Fluorescence and infrared spectroscopy of electrochemically self assembled ZnO nanowires: evidence of the quantum confined Stark effect

We report room temperature fluorescence (FL) and infrared absorption (IR) spectra of spatially ordered two-dimensional arrays of vertically standing ZnO nanowires. The wires are produced by selective electrodeposition of Zn in 10-, 25- and 50-nm pores of a porous anodic alumina film, followed by chemical oxidation. Wires of different diameters show distinctly different FL emission characteristics associated with either deep level traps, or exciton recombination. The intensity of the peak caused by exciton recombination is larger than that caused by deep level traps, which is unusual in nanostructures, and attests to the high structural purity. We also see an anomalous red-shift in the FL emission spectrum which appears to be evidence of quantum confined Stark shift caused by built-in electric fields in the alumina template. The IR absorption spectra are mostly featureless and show no significant peaks indicating the absence of shallow level traps.

Unconventional quantum-confined Stark effect in a singleGaNquantum dot

We show that a peculiar excitonic effect and a confining potential in self-assembled hexagonal $\mathrm{GaN}∕\mathrm{AlN}$ quantum dots produce an unconventional quantum-confined Stark effect. In contrast to the conventional quantum-confined Stark shift, the emission line from a single $\mathrm{GaN}$ dot under the applied electric field perpendicular to the growth direction blueshifts nearly symmetrically with respect to the direction of the field. The field dependence of the emission lines is reproduced in a charge self-consistent effective mass calculation, taking into account strain, piezoelectric charge, and pyroelectric charge. The unconventional blueshift is attributed to a significant variation of the exciton binding energy, made obvious by a cancellation between the energy shifts of electron and hole confined states.

Quantum-confined Stark shift in electroreflectance of a cylindrical GaN quantum dot

The electroreflectance (ER) spectra in the presence of the modulated electric field have been employed to study the fine structure of a cylindrical GaN quantum dot (QD), including the light hole and heavy hole interband transitions, and the ER spectra exhibit Franz–Keldysh oscillation characteristics with abscissa of energy (E−Eg). The quantum-confined Stark shift (QCSS) happened when the electric field intensity increased and the light and heavy holes dependent characteristics have been shown. The three-dimensional Schrödinger equation of QD has been calculated within the framework of effective-mass approximation, and the ER indices have been obtained from modulation absorption coefficients using the Seraphin coefficients and the Kramer–Kroning relation.

Photocurrent spectroscopy of single InAs/GaAs quantum dots

In this work, we present a carrier escape study from InAs/GaAs self assembled QD's by the use of photocurrent measurements. As a function of the applied field, we detect a shift of the exciton ground state transition due to the quantum-confined Stark shift. ¿From the measured Stark shift S = 4:3 meV we deduce a exciton dipole moment of p = (4.3 ± 0.2) × 10–29 Cm. The tunneling time, which is directly related to the observed photocurrent linewidth due to τ ∼ ħ/(2Γ), changes by a factor of five in the photocurrent regime. The measured linewidth dependency on the electric field is modelled by a simple 1D WKB approximation for the tunneling process, which shows that the energetic position of the wetting layer is important for the measured tunneling time out of the dot. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical study on the absorption and dispersion in asymmetrically coupled qua ntum well structure

The absorption and dispersion behaviors of an asymmetrically coupled quantum well (ACQW) structure are studied based on V-type system. We show that the quantum-confined Stark shift due to an electric field applied along the growth direction of ACQW leads to the detuning absorption, and the corresponding linear refraction is dramatically decreased in a smaller negative-bias voltage region. This means that refraction quenching induced by an external electric field can occur in an ACQW structure. However, the absorption and the dispersion behaviors become the same as those of the lowest excitonic state in a single quantum well structure as the negative-bias voltage further increases due to the disappearance of quantum-confined Stark effect. This means that the ACQW structure has controllable dispersion by using an external field.

Theoretical study of quantum confined Stark shift in InAs/GaAs quantum dots

The quantum confined Stark effect (QCSE) of the self-assembled InAs/GaAs quantum dots has been investigated theoretically. The ground-state transition energies for quantum dots in the shape of a cube, pyramid or ``truncated pyramid'' are calculated and analysed. We use a method based on the Green function technique for calculating the strain in quantum dots and an efficient plane-wave envelope-function technique to determine the ground-state electronic structure of them with different shapes. The symmetry of quantum dots is broken by the effect of strain. So the properties of carriers show different behaviours from the traditional quantum device. Based on these results, we also calculate permanent built-in dipole moments and compare them with recent experimental data. Our results demonstrate that the measured Stark effect in self-assembled InAs/GaAs quantum dot structures can be explained by including linear grading.

Quantum-confined Stark shifts of charged exciton complexes in quantum dots

We probe the permanent excitonic dipole of neutral and positively charged excitons in individual ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ self-assembled quantum dots using Stark effect perturbation spectroscopy. A systematic reduction of the permanent excitonic dipole is found as excess holes are controllably added to individual dots containing a single exciton $({X}^{0})$. Calculations of the few-body states show that this effect arises from a strong, Coulomb-mediated, spatial redistribution of the few-body wave function upon charging. By investigating correlations between the permanent dipole, polarizability, and the emission energy of ${X}^{0}$ for many dots, we also show that the strength of the $\mathrm{In}:\mathrm{Ga}$ composition gradient is related to the absolute In content.