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.     

Quantum dot laser with 75 nm broad spectrum of emission

We report on a quantum dot laser having an emission spectrum as broad as 74.9 nm at 25 degrees C in the 1.2-1.28 wavelength interval with a total pulsed output power of 750 mW in single lateral mode regime and the average spectral power density of >10 mW/nm. A significant overlap and approximate equalization of the ground-state and the excited-state emission bands in the laser's spectrum is achieved by means of intentional inhomogeneous broadening of the quantum dot energy levels.     

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.     

Correlated photons from multi-carrier complexes in GaAs quantum dots

We have studied micro-photoluminescence spectra of a self-assembled single GaAs quantum dot under 8 K. With strong pulsed excitation, the micro-photoluminescence spectrum shows bright emission lines originated from an exciton, a positively charged exciton, and a biexciton, together with weak lower energy emissions reflecting multi-excitonic structures with more carriers. We have identified the origins of these weak emission lines, and showed the existence of charged biexciton states, through single photon correlation measurements and excitation power dependence of the photoluminescence intensity. In addition, investigating the radiative recombination process of the charged biexciton, we have determined the electron–hole exchange energy in the GaAs quantum dot.     

Lasing spectra of 1.55 μm InAs/InP quantum dot lasers: theoretical analysis and comparison with the experiments

In this paper, a theoretical model is used to investigate the lasing spectrum properties of InAs/InP (113)B quantum dot (QD) lasers emitting at 1.55 μm. The numerical model used is based on a multi-population rate equation (MPRE) analysis. It takes into account the effect of the competition between the inhomogeneous broadening (due to the QD size dispersion) and the homogenous broadening as well as a nonlinear gain variation associated to a multimode laser emission. The double laser emission and the temperature dependence of lasing spectra of self-assembled InAs/InP quantum dot lasers is studied both experimentally and theoretically.     

PHOTOLUMINESCENCE STUDY OF InAs/GaAs SELF-ORGANIZED QUANTUM DOTS WITH BIMODAL SIZE DISTRIBUTION

We have investigated the temperature dependence of the photoluminescence (PL) spectrum of self-organized InAs/GaAs quantum dots. A distinctive double-peak feature of the PL spectra from quantum dots has been observed, and a bimodal distribution of dot sizes has also been confirmed by scanning tunneling microscopy image for uncapped sample. The power-dependent PL study demonstrates that the distinctive PL emission peaks are associated with the ground-state emission of islands in different size branches. The temperature-dependent PL study shows that the PL quenching temperature for different dot families is different. Due to lacking of the couple between quantum dots, an unusual temperature dependence of the linewidth and peak energy of the dot ensemble photoluminescence has not been observed. In addition, we have tuned the emission wavelength of InAs QDs to 1.3 μm at room temperature.     

微腔中CdSe量子点荧光增强效应

文章主要研究了CdSe量子点微腔结构,微腔结构包括上下分布式布拉格反射镜(DBR),中间的有源层为溶解在聚甲基丙烯酸甲酯(PMMA)中的CdSe胶体量子点.采用传递矩阵法模拟微腔的反射光谱,对实验测试曲线进行较好的拟合.通过测试微腔结构的光致荧光(PL)光谱,其半峰宽(FWHM)由未加入微腔的CdSe量子点样品的27.9 nm,减小到微腔结构的7.5 nm,在微腔中的量子点,由于腔模式的出现,其发光谱的品质因数增加了3.6倍,达到了荧光增强的效果. 关键词： CdSe量子点 微腔效应 荧光增强     

Signature of symmetry of laterally coupled quantum dots in far-infrared spectrum

We study the effect of structure asymmetry on the energy spectrum and the far-infrared spectrum (FIR) of a lateral coupled quantum dot. The calculated spectrum shows that the parity break of coupled quantum dot results in more coherent superpositions in the low-lying states and exhibits unique anti-crossing in the two-electron FIR spectrum modulated by a magnetic field. We also find that the Coulomb correlation effect can make the FIR spectrum of coupled quantum dot without strict parity deviate greatly from Kohn theorem, which is just contrary to the symmetric case. Our results therefore suggest that FIR spectrum may be used to determine the symmetry of coupled quantum dot and to evaluate the degree of Coulomb interaction.     

Nonequilibrium properties of an atomic quantum dot coupled to a Bose-Einstein condensate

We study nonequilibrium properties of an atomic quantum dot (AQD) coupled to a Bose-Einstein condensate (BEC) within Keldysh-Green’s function formalism when the AQD level is varied harmonically in time. Nonequilibrium features in the AQD energy absorption spectrum are the side peaks that develop as an effect of photon absorption and emission. We show that atoms can be efficiently transferred from the BEC into the AQD for the parameter regime of current experiments with cold atoms.     

Electron states in diluted magnetic semiconductors

One of the remarkable properties of the II–VI diluted magnetic semiconductor (DMS) quantum dot (QD) is the giant Zeeman splitting of the carrier states under application of a magnetic field. This splitting reveals strong exchange interaction between the magnetic ion moment and electronic spins in the QD. A theoretical study of the electron spectrum and of its relaxation to the ground state via the emission of a longitudinal optical (LO) phonon, in a CdSe/ZnMnSe self-assembled quantum dot, is proposed in this work. Numerical calculations showed that the strength of this interaction increases as a function of the magnetic field to become more than 30 meV and allows some level crossings. We have also shown that the electron is more localized in this DMS QD and its relaxation to the ground state via the emission of one LO phonon is allowed.     

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.     

Spontaneous emission from a quantum dot embedded in a bi-sphere microcavity

The spontaneous emission from a quantum dot embedded in a bi-sphere microcavity is analyzed theoretically. By taking full account of the photon degree of freedom, we derive theoretically the constant of coupling between the quantum dot and the cavity as well as the radiation damping constants, which are used in the conventional cavity QED approach. In addition, we show that the power spectrum of the spontaneous emission at a local observation point is strongly affected by the photon Green function.     

Single-mode spontaneous emission from a single quantum dot in a three-dimensional microcavity

The spontaneous emission from an isolated semiconductor quantum dot state has been coupled with high efficiency to a single, polarization-degenerate cavity mode. The InAs quantum dot is epitaxially formed and embedded in a planar epitaxial microcavity, which is processed into a post of submicron diameter. The single quantum dot spontaneous emission lifetime is reduced from the noncavity value of 1.3 ns to 280 ps, resulting in a single-mode spontaneous emission coupling efficiency of 78%.     

Spin-flip effects in a parallel-coupled double quantum dot molecule

We investigate theoretically the electronic transport through a parallel-coupled double quantum dot (DQD) molecule attached to metallic electrodes, in which the spin-flip scattering on each quantum dot is considered. Special attention is paid to the effects of the intradot spin-flip processes on the linear conductance by using the equation of motion approach for Green’s functions. When a weak spin-flip scattering on each quantum dot is present, the single Fano peak splits into two Fano peaks, and the Breit–Wigner resonance may be suppressed slightly. When the spin-flip scattering strength on each quantum dot becomes strong, the linear conductance spectrum consists of two Breit–Wigner peaks and two Fano peaks due to the quantum interference effects. The positions and shapes of these resonant peaks can be controlled by using the magnetic flux through the quantum device.     

Electronic properties of A<Subscript>2</Subscript>B<Subscript>6</Subscript> quantum dots incorporated into Langmuir–Blodgett films

An experimental and theoretical study is performed for features of the electronic spectrum of CdSe/CdS/ZnS quantum dots incorporated into Langmuir–Blodgett films. Analysis of the investigated samples assesses the state of the first three levels of the electron spectrum for a quantum object. Good qualitative and quantitative agreement is achieved between the experimental results and theoretical estimates. It is shown that the mechanism of the observed field emission current through a quantum dot is described satisfactorily by the Morgulis–Stratton theory under the considered experimental conditions.     

Kinetics of thermalized luminescence of a single quantum dot at room temperature

We have developed a theory of transient secondary emission of a single quantum dot from the lowest energy states of electron-hole pairs. We consider a process in which laser pulses excite a certain highenergy state of electron-hole pairs of a quantum dot at room temperature, with the electronic subsystem then relaxing to low-energy states and photons being emitted. Therefore, the investigated secondary emission process is thermalized luminescence. For definiteness, the developed model takes into account two states of electron-hole pairs that contribute to the luminescence. We have analyzed the dependence of the secondary emission signal on the energy gap between these states, the value of which is determined by the quantum dot size. In terms of the Pauli master kinetic equation, an analytical expression for the time dependent signal of the thermalized luminescence has been obtained. We show that, as the spectral width of the exciting laser pulse tends to zero, this expression yields the signal of stationary luminescence.     

Gate adjustable coherent three and four level mixing in a vertical quantum dot molecule

We study level mixing in the single-particle energy spectrum of one of the constituent quantum dots in a vertical double quantum dot by performing magneto-resonant-tunneling spectroscopy. The device used in this study differs from previous vertical double quantum dot devices in that the single side gate is now split into four separate gates. Because of the presence of natural perturbations caused by anharmonicity and anisotropy, applying different combinations of voltages to these gates allows us to alter the effective potential landscape of the two dots and hence influence the level mixing. We present here preliminary results from one three level crossing and one four level crossing high up in the energy spectrum of one of the probed quantum dots, and demonstrate that we are able to change significantly the energy dispersions with magnetic field in the vicinity of the crossing regions.     

Photon antibunching from a single quantum-dot-microcavity system in the strong coupling regime

We observe antibunching in the photons emitted from a strongly coupled single quantum dot and pillar microcavity in resonance. When the quantum dot was spectrally detuned from the cavity mode, the cavity emission remained antibunched, and also anticorrelated from the quantum dot emission. Resonant pumping of the selected quantum dot via an excited state enabled these observations by eliminating the background emitters that are usually coupled to the cavity. This device demonstrates an on-demand single-photon source operating in the strong coupling regime, with a Purcell factor of 61+/-7 and quantum efficiency of 97%.     

Accuracy of circular polarization as a measure of spin polarization in quantum dot qubits

A quantum dot spin light emitting diode provides a test of carrier spin injection into a qubit and a means for analyzing carrier spin injection and local spin polarization. Even with 100% spin-polarized carriers the emitted light may be only partially circularly polarized due to the geometry of the dot. We have calculated carrier polarization-dependent optical matrix elements for InAs/GaAs self-assembled quantum dots (SAQDs) for electron and hole spin injection into a range of quantum dot sizes and shapes, and for arbitrary emission directions. Calculations for typical SAQD geometries with emission along [110] show light that is only 5% circularly polarized for spin states that are 100% polarized along [110]. Measuring along the growth direction gives near unity conversion of spin to photon polarization and is the least sensitive to uncertainties in SAQD geometry.     

Spectroscopy on single columns of vertically aligned InAs quantum dots

We have investigated the carrier relaxation dynamics in single columns of tenfold stacked vertically aligned InAs quantum dots by micro-photoluminescence measurement. The excitation spectrum in the stacked dots is much different from that in the single dot characterized by the existence of a zero-absorption region and sharp multiple phonon emission lines. We have observed a broad continuum absorption far below the wetting layer band edge in the spectrum of the single columns although we have confirmed the existence of a zero-absorption region in the same sample with reduced number of dot layers to almost single, realized by surface etching. The broad absorption feature suggests the existence of additional carrier relaxation channels through non-resonant tunneling between the dots.