Optical properties of individual manganese-doped quantum dots

The magnetic state of a single magnetic ion (Mn²⁺) embedded in an individual quantum dot is optically probed using micro-spectroscopy. The fine structure of a confined exciton in the exchange field of a single Mn²⁺ ion (S=) is analyzed in detail. The exciton–Mn²⁺ exchange interaction shifts the energy of the exciton depending on the Mn²⁺ spin component and six emission lines are observed at zero magnetic field. The emission spectra of individual quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton–Mn²⁺ exchange interaction (dependent on the Mn position) and the anisotropic part of the electron–hole exchange interaction (related to the asymmetry of the quantum dot).     

Optical modelling of quantum dots

The analogy between quantum mechanics and electromagnetism is used to design an optical waveguide with the same transmission and traversal time as a quantum dot. Two different quantum dot geometries are considered for two typical applications: ultrafast devices and computing.     

Optical properties of InAs quantum dots in a Si matrix

We report on the optical properties of nanoscale InAs quantum dots in a Si matrix. At a growth temperature of 400°C, the deposition of 7 ML InAs leads to the formation of coherent islands with dimensions in the 2–4 nm range with a high sheet density. Samples with such InAs quantum dots show a luminescence band in the 1.3 μm region for temperatures up to 170 K. The PL shows a pronounced blue shift with increasing excitation density and decays with a time constant of 440 ns. The optical properties suggest an indirect type II transition for the InAs/Si quantum dots. The electronic structure of InAs/Si QDs is discussed in view of available band offset information.     

Photocurrent spectroscopy of InAs/GaAs self-assembled quantum dots: observation of a permanent dipole moment

The nature of the confined electronic states in InAs/GaAs self-assembled quantum dots is studied using photocurrent spectroscopy measured as a function of applied electric field. A field asymmetry of the quantum confined Stark effect is observed, consistent with the dots possessing a permanent dipole moment. The sign of this dipole indicates that for zero field the hole wave function lies above that of the electron, in disagreement with the predictions of all recent calculations. Comparison with a theoretical model demonstrates that the experimentally determined alignment of the electron and hole can only be explained if the dots contain a non-zero and non-uniform Ga content.     

Micro-photoluminescence spectroscopy of hierarchically self-assembled quantum dots

Novel, self-assembled quantum dot (QD) structures suitable for single-dot optical spectroscopy are fabricated by combining III–V molecular beam epitaxy and in situ, atomic layer precise etching. Several growth and etching steps are used to produce lateral InAs/GaAs QD bimolecules and unstrained GaAs/AlGaAs QDs with low surface density . Micro-photoluminescence is used to investigate the ensemble and single-QD properties of GaAs QDs. Single-QD spectra show resolution-limited sharp lines at low excitation and broad “shell-structures” at high excitation intensity.     

Quantum confined Stark effect in single self-assembled GaN/AlN quantum dots

We have experimentally and theoretically investigated quantum confined Stark effect in hexagonal self-assembled GaN/AlN quantum dots. We have observed a blueshift of up to 100 meV for vertical electric field applied against the built-in electric field while we have observed a redshift for the electric field along the built-in field. The experimental result is compared with a charge self-consistent effective mass calculation, taking into account strain, piezoelectric charge, and pyroelectric charge. The tunability of the emission energy and the exciton binding energy is discussed.     

Evidence of coupling between InAs self-assembled quantum dots in thin GaAs buffer layer

We have studied the optical properties of two layers of InAs self-assembled quantum dots (QDs). The QDs were separated by a GaAs barrier with thickness varied from 2.5 to 10 nm. All samples exhibited double peaks from low-temperature photoluminescence spectra. The energy difference between two peaks shows that the origin of the double peaks is different for each sample. In case of the thin barrier thickness, the double peaks are due to the coupling of the ground states of lower and upper dots. In the thick barrier case, the double peaks originate from the ground and excited states because the barrier is thick enough to separate the double QDs.     

Landau levels in a novel two-dimensional electron system interacting with charged quantum dots

Landau levels have been theoretically investigated in a two-dimensional electron gas near a quantum dot (QD) layer. By a diagrammatical method, we have formulated the self-energy for the Landau level and deduced its relation to the AC conductivity σ_loc(ω) in the QD layer. As an example, we have examined the density of states in the case where σ_loc(ω) is described by Aω^S(S=0.8). It is found that the Landau levels are broadened due to the interaction with the localized electrons in the QDs.     

Optical line-shape and the time-domain photon echo measurement in semiconductor quantum dots

It is shown theoretically that, because of the characteristic line-shape of the electronic spectral density, the experimental photon-echo signal, giving the dephasing of the electronic state of a quantum dot, should be modulated in time quasi-periodically. This suggestion is demonstrated numerically for the case of InAs quantum dots for several temperatures of the sample. A comparison with the recent experiments on CdSe nanocrystals is presented. The theoretical argumentation is based on the recent theoretical calculation of the homogeneous optical line-shape based on the multiple electron–LO–phonon scattering in quantum dots.     

Optical characterization of quantum dots entrained in microstructured optical fibers

We present a study of the basic optical properties of colloidal CdSe/ZnS core/shell quantum dots entrained in a microstructured optical fiber. Quantum dots suspended in heptane were pulled into the holes surrounding the solid core of a microstructured optical fiber of the holey fiber class via capillary action and are found to remain in the fiber. In this experiment, a laser coupled into the fiber photoexcited quantum dots along the length of the fiber. Quantum dot emission was observed to couple into the fiber core and propagate along the fiber. To investigate the use of such a system in fiber-based light generation or amplification, a second laser overlapping the low-energy portion of the quantum dot emission was simultaneously coupled into the fiber. We observed apparent amplification of this light when photoexciting the quantum dots well above their bandedge.     

量子点场效应单光子探测器二维电子气载流子浓度研究

设计了一种基于场效应晶体管的量子点场效应单光子探测器(quantum dot field effect transistor,QDFET),建立了二维电子气(two-dimensional electron gas,2DEG)的薛定谔方程和泊松方程,通过对薛定谔方程和泊松方程的自洽求解,对2DEG的载流子浓度进行了模拟。模拟结果显示,AlGaAs的Al组分、δ掺杂层的掺杂浓度以及隔离层的厚度对于2DEG的载流子浓度均有影响。为了使2DEG具有较高的载流子浓度,AlGaAs的Al组分应为0.2~0.4,δ掺杂浓度应为6~8×10~(13)/cm~2,隔离层厚度应在50nm以下。通过对2DEG的载流子浓度进行研究,可以掌握2DEG载流子浓度的影响因素,从而通过优化QDFET结构,可提高2DEG的载流子浓度。这对于高灵敏度QDFET的制备具有重要的意义和应用价值。     

Bandgap engineering of self-assembled InAs quantum dots with a thin AlAs barrier

We investigate the effects of a thin AlAs layer with different position and thickness on the optical properties of InAs quantum dots (QDs) by using transmission electron microscopy and photoluminescence (PL). The energy level shift of InAs QD samples is observed by introducing the thin AlAs layer without any significant loss of the QD qualities. The emission peak from InAs QDs directly grown on the 4 monolayer (ML) AlAs layer is blueshifted from that of reference sample by 219 meV with a little increase in FWHM from 42–47 meV for ground state. In contrast, InAs QDs grown under the 4 ML AlAs layer have PL peak a little redshifted to lower energy by 17 meV. This result is related to the interdiffusion of Al atom at the InAs QDs caused by the annealing effect during growing of InAs QDs on AlAs layer.     

Optical transitions in CdSe quantum dots: From discrete levels to broad gain spectra

The optical transition energies have been determined for the lowest electron-hole pair states of CdSe quantum dots embedded in glass. The data obtained by photoluminescence, differential absorption and photoluminescence excitation spectroscopy have been compared and a general size dependence could be established. Based on theoretical calculations, the dominant features in the spectra have been assigned to the different transitions from the ground state to the one-pair states. Within the lowest one pair transition, a fine structure due to exchange interaction has been observed. The change in the optical properties with increasing intensity has been studied and the two-pair states identified in luminescence. Spectrally broad optical gain has been found due to stimulated transitions involving both one- and two-pair states.     

Optical characterization of individual quantum dots

Optical characterization of single quantum dots (QDs) by means of micro-photoluminescence (μPL) will be reviewed. Both QDs formed in the Stranski–Krastanov mode as well as dots in the apex of pyramidal structures will be presented. For InGaAs/GaAs dots, several excitonic features with different charge states will be demonstrated. By varying the magnitude of an external electric or magnetic field and/or the temperature, it has been demonstrated that the transportation of carriers is affected and accordingly the charge state of a single QD can be tuned. In addition, we have shown that the charge state of the QD can be controlled also by pure optical means, i.e. by altering the photo excitation conditions. Based on the experience of the developed InAs/GaAs QD system, similar methods have been applied on the InGaN/GaN QD system.     

Polaron relaxation dynamics in InAs/GaAs self-assembled quantum dots

Polaron decay in n-type InAs quantum dots has been investigated using energy dependent, mid-infrared pump–probe spectroscopy. By studying samples with differing ground state to first excited state energy separations the relaxation time has been measured between 40 and 60 meV. The low-temperature decay time increases with increasing detuning between the pump energy and the optical phonon energy and is maximum (55 ps) at 56 meV. From the experimentally determined decay times we are able to extract a low-temperature optical phonon lifetime of 13 ps for InAs QDs. We find that the polaron decay time decreases by a factor of 2 at room temperature due to the reduction of the optical phonon lifetime.     

Absorption spectroscopy of single InAs self-assembled quantum dots

Excitonic transitions of single InAs self-assembled quantum dots were directly measured at 4.2 K in an optical transmission experiment. We use the Stark effect in order to tune the exciton energy of a single quantum dot into resonance with a narrow-band laser. With this method, sharp resonances in the transmission spectra are observed. The oscillator strengths as well as the homogeneous line widths of the single-dot optical transitions are obtained. A clear saturation in the absorption is observed at modest laser powers.     

Single-electron transistor spectroscopy of InGaAs self-assembled quantum dots

A single-electron transistor (SET) is used to detect tunneling of single electrons into individual InGaAs self-assembled quantum dots (QDs). By using an SET with a small island area and growing QDs with a low density we are able to distinguish and measure three QDs. The bias voltage at which resonant tunneling into the dots occurs can be shifted using a surface gate electrode. From the applied voltages at which we observe electrons tunneling, we are able to measure the electron addition energies of three QDs.     

Optical probe of InAs/GaAs self-assembled quantum dots grown using low growth rate and growth interruptions

We have investigated the optical properties of InAs/GaAs self-assembled quantum dots (QDs), grown at 500 °C using a low growth rate (0.014 ML/s), growth interruptions and a two-stage capping process. The samples exhibited large-size dots with densities in the range (3-4.5) × 10⁹ cm⁻². Macro-photoluminescence (macro-PL) measurements revealed the presence of five electronic sub-bands in the dots, with the ground state (GS) emission exhibiting a linewidth of ∼70 meV. Because of the dots large size and composition dispersions, associated with the growth method, it was possible to resolve single dots emissions using micro-PL (μ-PL) excitation in the barrier layers of the as-grown samples. The sharp PL lines were detected 60-140 meV above the GS peak energy. High-resolution resonant optical excitation of the dots PL evidenced that these fine lines originate from exciton complexes confined to the GS of individual dots. Non-resonant power dependence μ-PL spectroscopy results further confirmed the occurrence of both single exciton (X) and biexciton (XX) radiative recombinations. Finally, with increasing lattice temperature up to 95 K, PL emissions from most of these nanostructures suffered the usual thermal quenching, with activation energies (E_a) ranging between 12 and 41 meV. The relatively small values of E_a suggest that the growth technique implemented here favors the formation of defects centers in the vicinity of the QDs.     

Influence of a lateral electric field on the optical properties of InAs quantum dots

We have performed single dot photoluminescence and time-resolved ensemble photoluminescence measurements on InAs quantum dots embedded in a lateral in-plane p–i–n or n–i–n device, respectively, which makes the application of lateral electric fields, i.e. field direction perpendicular to the growth direction, feasible. Time-resolved measurements show an increase in the radiative lifetime of up to 30% with increasing field. We attribute this to the reduced overlap between the electron and hole wave functions. Single dot spectroscopy revealed a small red-shift of the emission energies of maximum 0.5 meV. This shift can be explained by the quantum confined Stark effect taking into account that the red-shift due to the band-tilting is partly compensated by a decrease in exciton binding energy.