Interface Fluctuation Quantum Dots Research Articles

Measurement of the Emission Lifetime of a GaN Interface Fluctuation Quantum Dot by Power Dependent Single Photon Dynamics

Photon autocorrelation measurements are used to investigate the power dependent single photon emission of recently reported interface fluctuation GaN quantum dots (QDs), which exhibit relatively narrow emission linewidths. The intrinsic exciton lifetime of such a dot is evaluated to be 2.0 ± 0.1 ns from its power dependent single photon dynamics. This result is comparable with typical SK GaN QDs that emit at similar energy, and provides further evidence that the relatively narrow emission linewidth in such structures results from a cleaner dot environment.

Nanosecond-scale spectral diffusion in the single photon emission of a GaN quantum dot

Autocorrelation measurements are used to reveal the spectral diffusion time scale in the single photon emission of a GaN interface fluctuation quantum dot. Typical characteristic diffusion times of such QDs are revealed to be of nanosecond order. The excitation power dependence of the diffusion rate is also investigated, whereby an increase in the diffusion rate with increasing excitation power is observed. This result provides information on experimental conditions that will be required for the generation of indistinguishable photons.

Visualization of space charge field effect on excitons in a GaAs quantum dot by near-field optical wavefunction mapping

Near-field photoluminescence (PL) imaging spectroscopy was used to investigate multi-exciton and charged-exciton states confined in a single GaAs interface fluctuation quantum dot. We determined the origin of peaks in the PL spectra by employing a wavefunction mapping technique. We observed distortion of the exciton wavefunction due to the electric field produced by an excess electron at a nearby confined state. Near-field wavefunction mapping was demonstrated to be a powerful tool for visualizing the local environment, which affects the emission properties of quantum dots.

Visualization of weak confinement potentials by near-field optical imaging spectroscopy of exciton and biexciton in a single quantum dot

Imaging spectroscopy of a single GaAs interface fluctuation quantum dot (IFQD) was performed using a near-field scanning optical microscope (NSOM) with a spatial resolution of 40nm. A difference in the emission profiles of an exciton and a biexciton was found for several IFQDs. By comparing with a numerical simulation based on the finite-difference time-domain method, this difference was attributed to the existence of a shallow potential dip in the IFQD. The NSOM wavefunction mapping for excitons and biexcitons as quasiparticles with different masses is a tool for investigating weak confinement potentials to detect local strain and disorder.

CONTROLLING SPIN COHERENCE WITH SEMICONDUCTOR NANOSTRUCTURES

We present two emerging opportunities for manipulating and communicating coherent spin states in semiconductors. First, we show that semiconductor microcavities offer unique means of controlling light-matter interactions in confined geometries, resulting in a wide range of applications in optical communications and inspiring proposals for quantum information processing and computational schemes. Studies of spin dynamics in microcavities — a new and promising research field — have revealed novel effects such as polarization beats, stimulated spin scattering, and giant Faraday rotation. Here, we study the electron spin dynamics in optically-pumped GaAs microdisk lasers with quantum wells and interface-fluctuation quantum dots in the active region. In particular, we examine how the electron spin dynamics are modified by the stimulated emission in the disks, and observe an enhancement of the spin coherence time when the optical excitation is in resonance with a high quality ( Q ~ 5000) lasing mode.1 This resonant enhancement, contrary to expectations from the observed trend in the carrier recombination time, is then manipulated by altering the cavity design and dimensions. In analogy to devices based on excitonic coherence, this ability to engineer coherent interactions between electron spins and photons may provide novel pathways towards spin dependent quantum optoelectronics. In a second example, the nitrogen-vacancy (N-V) center in diamond has garnered interest as a room-temperature solid-state system not only for exploring electronic and nuclear spin phenomena but also as a candidate for spin-based quantum information processing. Spin coherence times of up to 50 microseconds have been reported for ensembles of N-V centers and a two-qubit gate utilizing the electron spin of a N-V center and the nuclear spin of a nearby C-13 atom has been demonstrated. Here, we present experiments using angle-resolved magneto-photoluminescence microscopy to investigate anisotropic spin interactions of single N-V centers in diamond at room temperature.2 Negative peaks in the photoluminescence intensity are observed as a function of both magnetic field magnitude and angle, and can be explained by coherent spin precession and anisotropic relaxation at spin-level anticrossings. Additionally, precise field alignment with the symmetry axis of a single N-V center reveals the resonant magnetic dipolar coupling of a single "bright" electron spin of an N-V center to small numbers of "dark" spins of nitrogen defects in its immediate vicinity, which are otherwise undetected by photoluminescence. Most recently, we are exploring the possibility of utilizing this magnetic dipole coupling between bright and dark spins to couple two spatially separated single N-V center spins by means of intermediate nitrogen spins. Note from Publisher: This article contains the abstract only.

Enhancement of spin coherence using Q-factor engineering in semiconductor microdisc lasers

Semiconductor microcavities offer unique means of controlling light-matter interactions in confined geometries, resulting in a wide range of applications in optical communications and inspiring proposals for quantum information processing and computational schemes. Studies of spin dynamics in microcavities, a new and promising research field, have revealed effects such as polarization beats, stimulated spin scattering and giant Faraday rotation. Here, we study the electron spin dynamics in optically pumped GaAs microdisc lasers with quantum wells and interface-fluctuation quantum dots in the active region. In particular, we examine how the electron spin dynamics are modified by the stimulated emission in the discs, and observe an enhancement of the spin-coherence time when the optical excitation is in resonance with a high-quality (Q approximately 5,000) lasing mode. This resonant enhancement, contrary to expectations from the observed trend in the carrier-recombination time, is then manipulated by altering the cavity design and dimensions. In analogy with devices based on excitonic coherence, this ability to engineer coherent interactions between electron spins and photons may provide new pathways towards spin-dependent quantum optoelectronics.

Static and dynamic spectroscopy of(Al,Ga)As∕GaAsmicrodisk lasers with interface fluctuation quantum dots

We have studied the steady state and dynamic optical properties of semiconductor microdisk lasers whose active region contains interface fluctuation quantum dots in GaAs/(Ga,Al)As quantum wells. Steady-state measurements of the stimulated emission via whispering gallery modes yield a quality factor $Q \sim 5600$ and a coupling constant $\beta \sim 0.09$. The broad gain spectrum produces mode hopping between spectrally adjacent whispering gallery modes as a function of temperature and excitation power. Time- and energy-resolved photoluminescence measurements show that the emission rise and decay rates increase significantly with excitation power. Marked differences are observed between the radiative decay rates in processed and unprocessed samples.

Optically detected resonance spectroscopy of interface fluctuation quantum dots

We have performed optically detected resonance (ODR) spectroscopy on modulation-doped GaAs/AlGaAs quantum wells of different widths in which lateral fluctuations of the well width were purposely introduced by growth interruption at the interfaces. These monolayer fluctuations form quantum dots for which confinement and Coulomb correlation energies are comparable. By monitoring resonant changes of the dot ensemble photoluminescence induced by far-infrared (FIR) radiation in a magnetic field, we have observed cyclotron resonance (CR) of free electrons in the widest wells, as well as internal transitions of mobile and localized charged excitons. The latter, which are forbidden by magnetic translational invariance, have previously not been observed. For the narrower wells the effects of non-parabolicity and carrier localization on the CR and CR-like transitions have to be included for a proper interpretation of the measurements.

Measurement of relaxation between polarization eigenstates in single quantum dots

Low temperature relaxation of excitons between polarization eigenstates in single interface fluctuation quantum dots is studied using copolarized and cross-polarized transient differential transmission spectroscopy. The measured spin relaxation times are on the order of ∼100 ps. Such a spin relaxation time is longer than the reported times for thin quantum wells, but considerably shorter than the predicted times for interface fluctuation quantum dots.

Measurement of optical absorption by a single quantum dot exciton

We have measured the optical absorption due to an exciton confined to a single interface fluctuation quantum dot by directly measuring the transmission through both a submicron aperture and near-field microscopy. The measurements reveal large dipole moments approaching 100 D at 6 K. These results are compared to theoretical predictions based on a physical size estimated from photoluminescence excitation spectroscopy. The very large dipole moment makes these quantum dots attractive candidates for use in microcavities and quantum logic gates.

Anisotropic emission of interface fluctuation quantum dots

We calculate energy levels, dipole moments and radiative broadening of interface fluctuation quantum dots. For optically allowed states, the dipole moment grows proportionally to the lateral quantum dot radius while the radiative broadening saturates towards the quantum well radiative broadening for large lateral quantum dot radii. This is accompanied by a change in the angular emission pattern, concentrating emission in forward and backward direction. Optically forbidden states do not couple to light propagating in the growth direction yet they may have a considerable radiative broadening due to spontaneous emission in other directions.

Transient nonlinear spectroscopy of excitons and biexcitons in single quantum dots

Transient nonlinear spectroscopy is used to study excitons and biexcitons confined to single interface fluctuation quantum dots (QD's). Exciton lifetimes measured by this technique are found to be between 25 and 50 ps, depending on the QD studied. Biexciton binding energies and polarization selection rules agree with previous measurements from cw nonlinear spectroscopy, and biexciton lifetimes are found to be half of the exciton lifetime for the same quantum dot. The ability to perform transient nonlinear spectroscopy of excitons and biexcitons in single quantum dots is an important step towards the development of solid state quantum computing based on interacting excitons in quantum dots.

Relation between dipole moment and radiative lifetime in interface fluctuation quantum dots

A relation between dipole moment and radiative lifetime of quantum dots for arbitrary indices of refraction of the quantum dot and surrounding medium is derived. As limiting cases several formulas already in use are obtained and their correct use is discussed. We proceed to discuss the relation between dipole moment and radiative lifetime in interface fluctuation quantum dots (IFQD's) taking into account nonsphericity effects. This introduces a dependence on the light propagation direction. We calculate how to account for the nonsphericity of an interface fluctuation quantum dot depending on its diameter and well thickness and find that the lifetime of a typical IFQD with a 50 nm diameter is reduced by about 10% due to nonsphericity effects.