Measurements of far-infrared intersubband absorption linewidths in GaAs/AlGaAs quantum wells as a function of temperature and charge density

Intersubband transitions in doped quantum wells are of fundamental scientific interest, as well as technological interest for potential devices. One of the important characteristics of the transitions is their linewidth. We present the results of linear absorption spectroscopy on a coupled double asymmetric QW structure. Using a backgated sample, we can independently vary the charge density and DC field at the well. The absorption lines appear to be homogeneously broadened. The lines appear to become lifetime broadened for temperatures above 70 K.     

Absorption-free superluminal light propagation in a quantum-dot molecule

The propagation of a weak probe field in a three-level quantum-dot molecule is investigated by employing the tunnel coupling. It is shown that in the absence of tunnel coupling, the propagation of light pulse is superluminal, similar to a simple two-level system. A high-resolution dip appears in optical spectra due to the presence of tunnel coupling. We show that this narrow dip leads to the subluminal light propagation with doublet absorption, so the group velocity of a light pulse can be controlled by interdot tunnel coupling. It is also demonstrated that by applying an indirect incoherent pumping field to the probe transition, the absorption doublet switches to the gain doublet and then the absorption-free superluminal light propagation is appeared.     

Singlet–triplet oscillations and far-infrared spectrum of four-minima quantum-dot molecule

We study ground states and far-infrared spectra (FIR) of two electrons in four-minima quantum-dot molecule in magnetic field by exact diagonalization. Ground states consist of altering singlet and triplet states, whose frequency, as a function of magnetic field, increases with increasing dot–dot separation. When the Zeeman energy is included, only the two first singlet states remain as ground states. In the FIR spectra, we observe discontinuities due to crossing ground states. Non-circular symmetry induces anticrossings, and also an additional mode above ω₊ in the spin-triplet spectrum. In particular, we conclude that electron–electron interactions cause only minor changes to the FIR spectra and deviations from the Kohn modes result from the low-symmetry confinement potential.     

Geometry-induced localization phenomena in semiconductor quantum-dot heterostructures

Conduction-band electrons of semiconductor heterostructures described using the theory obey, for wide-bandgap semiconductors, the one-band effective-mass equation. We present, based on the one-band effective-mass equation, electron-state solutions for a quantum-dot heterostructure composed of two material layers (A and B) and identify localization properties of the groundstate. In particular, we show that the groundstate of two-material layer cylindrical quantum-dot systems can be localized in either material A or B depending on the dimensions of the nanostructure. A structure which is axially stacked (configuration A–B–A) has a certain critical radius below which the electron becomes localized in material A if the total axial length is big enough (A is assumed to be the material with the highest conduction-band edge). Similarly, a structure which is radially stacked (configuration B–A) has a certain critical (axial) length below which the electron becomes localized in the high conduction-band edge material A if the radius is big enough. Although results are presented for cylindrical-shaped heterostructure semiconductors, similar localization inversion of the groundstate may occur in other geometries such as rectangular-shaped quantum-dot heterostructures.     

Optical spectroscopy of CdTe/ZnTe quantum-dot superlattices

Studies of CdTe/ZnTe quantum-dot superlattices (self-assembled quantum-dot multilayers) have been carried out by optical spectroscopy methods in a wide range of temperatures. It has been shown that the ZnTe spacer layer thickness affects the properties of these quantum-dot superlattices due to changes in the elastic strain distribution pattern. An additional luminescence band appearing in the spectrum of the structure with the thinnest ZnTe spacer layer exhibits an anomalous shift of the peak position and an unusual behavior of integral intensity with the temperature increase. We assume that the spectrum of CdTe/ZnTe quantum-dot superlattices with the thinnest ZnTe spacer is caused by two kinds of excitonic states—spatially indirect and spatially direct.     

Theoretical investigation of intersubband transition in AlxGa1−xN/GaN/AlyGa1−yN step quantum well

A modified self-consistent method is introduced for the design of Al_xGa_1−xN/GaN step quantum well (SQW) with the position and energy-dependent effective mass. The effects of nonparabolicity are included. It is shown that the nonparabolicity effect is minute for the lowest subband energy level and grows in size for the higher subband states. The effects of nonparabolicicty have significant influence on the transition energies and the oscillator strengths and should be taken into account in the investigation of the optical transitions. The strong asymmetric property introduced by the step quantum well magnifies the weak intersubband transition from the ground state to the third state (1→3). It is shown that in an appropriate scope, the intersubband transition (1→3) has the comparable oscillator strength with transition from the ground state to the second one (1→2), which suggests the possible application of the two-color photodetectors. The results of this work should provide useful guidance for the design of optically pumped asymmetric quantum well lasers and quantum well infrared photodetectors (QWIPs).     

Colour tunability in a bimodal fluorescent hybrid nanostructure UCNPs@AuNPs@QDs

In the present work, lysine modified NaY_0.78Er_0.02Yb_0.2F₄ upconversion nanoparticles (UCNPs, positively charged) and lysine modified ZnSe:Mn²⁺ quantum dots (QDs, positively charged) are attached onto the surface of citrate reduced gold nanoparticles (AuNPs, negatively charged). The gold nanoparticles not only entangle the QDs and the UCNPs, through electrostatic interaction, but also tune the optical properties of UCNPs through the effect of surface plasmon resonance. The hybrid nanostructure gives green emission both through photoluminescence (under UV excitation) and through photon upconversion (under IR light excitation) process. The colour tuning is observed through variation in the size of QDs and through plasmonic effect of gold nanoparticles. In both the cases, the colour of emission gradually changes from green to red. The colour tunability and bi-modal photon conversion property of this material could be useful for its application in the field of bio-imaging and solar energy harvesting.     

Quantum dots in quantum well structures

Recent progress toward fabricating and characterizing quantum dots in III–V quantum well structures is reviewed. Quantum dots made by use of lithography and etching, including deep-etched, barrier-modulated, strain-induced and interdiffused quantum dots, are described. Quantum dots fabricated by growth, including natural quantum dots, dots on patterned substrates, and self-assembled dots, are discussed. Dot sizes and uniformity, energy-level splittings, and luminescence efficiencies that are now being achieved are discussed. The status of key issues, such as the energy relaxation in quantum dots, is mentioned.     

Effect of the electron population on intraband absorption in InAs/GaAs self-assembled quantum dots

We present a comprehensive study of the intraband transitions in n-type InAs/GaAs quantum dots (QDs) with a filling varying from 0.5 to 4 electrons per dot, using both polarization-dependent absorption and photocurrent spectroscopy. Applying these complementary mid- and far-infrared spectroscopies over a wide energy range allows us to obtain a detailed picture of the intraband transitions and energy levels in self-assembled QDs.     

Control of the entanglement between triple quantum dot molecule and its spontaneous emission fields via quantum entropy

The time evolution of the quantum entropy in a coherently driven triple quantum dot molecule is investigated. The entanglement of the quantum dot molecule and its spontaneous emission field is coherently controlled by the gate voltage and the rate of an incoherent pump field. The degree of entanglement between a triple quantum dot molecule and its spontaneous emission fields is decreased by increasing the tunneling parameter.     

Mid-infrared intersubband emission from optically pumped asymmetric coupled quantum wells

Mid-infrared optical emission due to intersubband transitions between excited conduction subbands of a coupled quantum well structure is studied. The emission process is based on optical pumping of free carriers from the ground subband into the third subband followed by a radiative transition from the third subband into the second subband and a fast phonon assisted relaxation into the ground subband. We have observed spontaneous emission at 14 μm that persists up to room temperature. Our results indicate that population inversion between conduction subbands and large stimulated gain can be achieved.     

Nonuniform energy level broadening in open quantum dots: the influence of the closed dot eigenstates on transport

We present simulations of a realistically modeled quantum dot with soft boundaries and open leads supporting several modes. The wavefunctions of this open dot can be decomposed in terms of the eigenstates of a corresponding closed dot my means of projection. At particular resonances, this decomposition reveals that transport in the open dot can be mediated through a single eigenstate. Thus, the quantization of closed-dot energy levels can be preserved even as the dot is opened, but we find that there is a selection of particular eigenstates that depends strongly on the positions and nature of the contacts.     

Synthesis of stable dispersion of ZnO quantum dots in aqueous medium showing visible emission from bluish green to yellow

Aqueous dispersion of 4-8 nm size stable ZnO quantum dots (QDs) exhibiting luminescence in the visible region have been synthesized by a simple solution growth technique at room temperature. Silica has been used as capping agent to control the particle size as well as to achieve uniform dispersion of QDs in aqueous medium. X-ray diffractometer (XRD) analysis reveals formation phase pure ZnO particles having wurzite (hexagonal) structure. Atomic force microscope (AFM) images show that the particles are spherical in shape, having average crystalline sizes ∼4, 5.5 and 8 nm for samples prepared at pH values of 10, 12 and 14, respectively. From the optical absorption studies, the band gap energy of QDs is found to be blue shifted as compared to bulk ZnO (3.36 eV) due to the quantum confinement effect and is consistent with the band gap calculated by using effective-mass approximation model. The photoluminescence (PL) observed in these QDs has been attributed to the presence of defect centers.     

Entropies associated with electron emission from InAs/GaAs quantum dots

Entropies associated with the transition of electrons into and out of InAs/GaAs quantum dots (QDs) are calculated by considering the temperature dependence of energy eigenvalues due to strain and energy band offset variations. It is found that, for InAs/GaAs quantum dots with base/height dimensions of 20/10 nm, the contribution from the surrounding lattice to entropy is smaller than for the temperature region below 100 K, where most measurements of thermal emission rates are performed. Including the electron degeneracy, the total entropy change has an upper limit of when releasing the first electron from the s-shell, while the second released s-electron is connected with an entropy change not larger than the absolute value of .     

Pressure broadening and shift of argon emission lines

Resonance and van der Waals broadening coefficients, as well as width to shift ratios, have been obtained by scanning argon emission lines produced in a high-pressure (～ 1 atm) glow discharge at a temperature of 1130°K. The resonance broadening coefficients yield absolute oscillator strengths of 0·283 ± 0·024 and 0·076 ± 0·008 for the two resonance lines at 1048 Å and 1067 Å. These oscillator strengths are in good agreement with previous measurements. The importance of higher-order terms in the van der Waals potential expansion is demonstrated by the measured coefficients.     

Formation and atomic structure of GaSb nanostructures in GaAs studied by cross-sectional scanning tunneling microscopy

GaSb nanostructures in GaAs, grown by metalorganic chemical vapor deposition, were studied with cross-sectional scanning tunneling microscopy. Three different samples were examined, containing a thin quantum well, a quantum well near the critical thickness for dot formation, and finally self-organized quantum dots with base lengths of 5–8 nm and heights of about 2 nm. The dots are intermixed with a GaSb content between 60% and 100%. Also small 3D and 2D islands were observed, possibly representing quantum dots in an early growth stage and quantum dot precursors. All GaSb layers exhibit gaps, which are indications of an island-like growth mode during epitaxy.     

Theory of light scattering from semiconductor quantum dots: Excitation frequency dependent emission dynamics

We present a microscopic model for the dynamic scattering and emission spectrum of a semiconductor quantum dot, after coherent optical excitation. We investigate the spectral properties and the emission dynamics of the different scattering and emission contributions considering a V-type semiconductor quantum dot model under resonant conditions and include the coupling to LO-phonons via higher order Born approximations. This theory helps identifying the different contributions to the spectrum via time resolved calculations.     

Correlated photon emission from semiconductor quantum dots grown in inverted pyramids

We demonstrate photon antibunching and deterministic single-photon operation using single InGaAs/AlGaAs semiconductor quantum dots grown on pre-patterned substrates. Additionally, we observe several types of single-photon cascades involving biexcitonic and other excitonic complexes and have modeled this behavior using numerical Monte-Carlo simulations. This method allows us to determine different non-radiative mechanisms otherwise not directly accessible via conventional spectroscopic methods.     

Controlling emission wavelength from InAs self-assembled quantum dots on InP (0 0 1) during MOCVD

We studied the growth of InAs quantum dots on InP (0 0 1) substrates in a low-pressure metalorganic chemical vapor deposition by using a so-called InP ‘double-cap’ procedure. With double-capping, a photoluminescence spectrum is modified into a series of multiple peaks, where the emission peaks arise from several quantum dot families with different heights changing in a step of integer number of an InAs monolayer. Cross-sectional transmission electron micrograph observations revealed that the shape of double-capped dots is dramatically changed into a thin plate-like shape with extremely flat upper and lower interfaces, being consistent with our interpretation of the photoluminescence spectrum. We showed that the procedure was extremely useful for controlling the emission wavelength from quantum dots in an InAs/InP (0 0 1) system.     

Low-temperature and solution-processable inorganic hole injection layer for flexible quantum-dot light-emitting diodes

A low-temperature solution-processable inorganic vanadium oxide (V₂O₅) hole injection layer (HIL) was synthesized for flexible quantum-dot light-emitting diodes (QLEDs). Efficient hole injection characteristics were observed in the hole-only devices; furthermore, the process temperature of V₂O₅ was as low as 30 °C. We investigated the source of the efficient hole injection behavior using ultraviolet and x-ray photoelectron spectroscopy. The density of gap states was found to increase in accordance with process temperature reduction. Therefore, QLEDs with low-temperature solution-processable V₂O₅ HILs were fabricated on a glass substrate, which showed excellent characteristics. The maximum luminance and luminous efficiency of the device were 56,717 Cd/m² and 4.03 Cd/A, respectively. Due to the low-temperature process of the V₂O₅ HIL, it was also possible to fabricate QLEDs on a flexible plastic substrate without mechanical or thermal deformation of the substrate. Our results suggest that the low-temperature V₂O₅ inorganic HIL is a feasible alternative to organic HILs for flexible QLEDs.