Single Quantum Dot Emission Research Articles

Spectroscopy of single InAs quantum dots

Ensembles of InAs quantum dots with a very low density (∼106 cm−2) are grown by molecular beam epitaxy, which allows the spectral characteristics of emission of single quantum dots to be studied by the method of cryogenic microphotoluminescence. With increasing quantum dot size, the splitting of exciton states is demonstrated to increase steadily to ∼102 µeV. In the exciton energy range of 1.3–1.4 eV, the magnitude of this splitting is comparable with the natural width of the exciton lines. This result is important for the development of emitters of entangled photon pairs based on InAs quantum dots.

Acoustically regulated carrier injection into a single optically active quantum dot

We study the carrier injection into a single InGaAs/GaAs quantum dot regulated by a radio frequency surface acoustic wave. We find that the time of laser excitation during the acoustic cycle programs both the emission intensities and time of formation of neutral $(X^0)$ and negatively charged $(X^-)$ excitons. We identify underlying, characteristic formation pathways of both few-particle states in the time-domain experiments and show that both exciton species can be formed either with the optical pump or at later times by injection of single electrons and holes "surfing" the acoustic wave. All experimental observations are in excellent agreement with calculated electron and hole trajectories in the plane of the two-dimensional wetting layer which is dynamically modulated by the acoustically induced piezoelectric potentials. Taken together, our findings provide insight on both the onset of acousto-electric transport of electrons and holes and their conversion into the optical domain after regulated injection into a single quantum dot emitter.

Broadband enhancement and inhibition of single quantum dot emission in plasmonic nano-cavities operating at telecommunications wavelengths

A plasmonic microcavity providing broadband control of spontaneous emission for large and sparse semiconductor quantum dots emitting at telecommunications wavelengths is proposed. By designing and fabricating such a cavity, we demonstrate a broadband Purcell effect with spontaneous emission enhancement over a broad spectral range of Δλ≃20 nm with a 3.9-fold maximum enhancement, as well as inhibition over Δλ≃100 nm around 1.3 μm. The broadband feature relaxes the constraint on spectral matching between the dot emission and the cavity mode, favourable for implementing efficient non-classical light sources or nanoscale lasers.

Widely tunable, efficient on-chip single photon sources at telecommunication wavelengths

We demonstrate tunable on-chip single photon sources using the Stark tuning of single quantum dot (QD) excitonic transitions in short photonic crystal waveguides (PhC WGs). The emission of single QDs can be tuned in real-time by 9 nm with an applied bias voltage less than 2V. Due to a reshaped density of optical modes in the PhC WG, a large coupling efficiency \beta>65% to the waveguide mode is maintained across a wavelength range of 5 nm. When the QD is resonant with the Fabry-Perot mode of the PhC WG, a strong enhancement of spontaneous emission is observed leading to a maximum coupling efficiency \beta=88%. These results represent an important step towards the scalable integration of single photon sources in quantum photonic integrated circuits.

Silver Embedded Nanomesas as Enhanced Single Quantum Dot Emitters in the Telecommunication C Band

We use high-density InAs quantum dots, which were grown by molecular beam epitaxy on InP(311)B substrates, as photon sources in the telecommunication C band at approximately 1.55 µm. To select a small numbers of dots, we fabricate sub-micrometer sized mesas by electron beam lithography and reactive ion etching. The benefit of using high-density quantum dot samples is that at least one optically active quantum dot can be expected in every single mesa. We show that the etching rate and resulting mesa shape of the In0.53Al0.22Ga0.25As epitaxial layer can be varied with the chamber pressure during the etching process. Furthermore, under constant pressure and with increasing etching time, the sequential etching of the epitaxial layer and the underneath substrate leads to a significant modification in the mesa shape, too. We demonstrate that the isolation of a small number of quantum dots within one mesa results in the appearance of single quantum dot emission with a narrow line width and minimal spectral overlap between different emission lines. We moreover present significant enhancement of the luminescence collected from single dots in silver-embedded nanomesas when compared with as-etched mesas.

Controlling quantum dot emission by integration of semiconductor nanomembranes onto piezoelectric actuators

AbstractThis paper reviews the recent advances obtained by integrating semiconductor epitaxial films with embedded self‐assembled quantum dots (QDs) on top of single‐crystal piezoelectric substrates made of lead magnesium niobate–lead titanate (PMN‐PT). This combination allows us to study in detail the effects produced by variable strains (up to about ± 0.2%) on the excitonic emission of single QDs and to add a powerful “tuning knob” to QDs. Biaxial stress can be used to reversibly shift the emission wavelength of QDs in a spectral range wider than 10 meV and to modify the relative binding energies of excitonic species. Anisotropic stress has instead a strong influence on the fine structure splitting of neutral excitons. Finally, we present experimental results on the effect of biaxial strain on the optical modes of microring optical resonators and show a simple approach enabling the compensation of piezo‐creep via a closed‐loop system.magnified image Schematic illustration of a QD membrane integrated on top of a PMN‐PT substrate. Stress provided by the piezoelectric substrate allows broad range tuning of the emission properties of the overlying dots.

Optically Gated Resonant Emission of Single Quantum Dots

We report on the resonant emission in coherently driven single semiconductor quantum dots. We demonstrate that an ultraweak nonresonant laser acts as an optical gate for the quantum dot resonant response. We show that the gate laser suppresses Coulomb blockade at the origin of a resonant emission quenching, and that the optically gated quantum dots systematically behave as ideal two-level systems in both regimes of coherent and incoherent resonant emission.

Photoluminescence from In0.5Ga0.5As/GaP quantum dots coupled to photonic crystal cavities

We demonstrate room temperature visible wavelength photoluminescence from In0.5Ga0.5As quantum dots embedded in a GaP membrane. Time-resolved above band photoluminescence measurements of quantum dot emission show a biexpontential decay with lifetimes of ~200 ps. We fabricate photonic crystal cavities which provide enhanced outcoupling of quantum dot emission, allowing the observation of narrow lines indicative of single quantum dot emission. This materials system is compatible with monolithic integration on Si, and is promising for high efficiency detection of single quantum dot emission as well as optoelectronic devices emitting at visible wavelengths.

A circular dielectric grating for vertical extraction of single quantum dot emission

We demonstrate a nanostructure composed of partially etched annular trenches in a suspended GaAs membrane, designed for efficient and moderately broadband (≈5 nm) emission extraction from single InAs quantum dots. Simulations indicate that a dipole embedded in the nanostructure center radiates upward into free space with a nearly Gaussian far field, allowing a collection efficiency >80% with a high numerical aperture (NA = 0.7) optic and with ≈12× Purcell radiative rate enhancement. Fabricated devices exhibit a ≈10% photon collection efficiency with a NA = 0.42 objective, a 20× improvement over quantum dots in unpatterned GaAs. A fourfold exciton lifetime reduction indicates moderate Purcell enhancement.

Single quantum dot emission at telecom wavelengths from metamorphic InAs/InGaAs nanostructures grown on GaAs substrates

We report on the growth by molecular beam epitaxy and the study by atomic force microscopy and photoluminescence of low density metamorphic InAs/InGaAs quantum dots. subcritical InAs coverages allow to obtain 108 cm−2 dot density and metamorphic InxGa1−xAs (x=0.15,0.30) confining layers result in emission wavelengths at 1.3 μm. We discuss optimal growth parameters and demonstrate single quantum dot emission up to 1350 nm at low temperatures, by distinguishing the main exciton complexes in these nanostructures. Reported results indicate that metamorphic quantum dots could be valuable candidates as single photon sources for long wavelength telecom windows.

Theoretical study of photon emission from single quantum dot emitter coupled to surface plasmons

Motivated by the recent pioneering advances on nanoscale plasmonics and also nanophotonics technology based on the surface plasmons (SPs), in this work, we give a master equation model in the Lindblad form and investigate the quantum optical properties of single quantum dot (QD) emitter coupled to the SPs of a metallic nanowire. Our main results demonstrate the QD luminescence results of photon emission show three distinctive regimes depending on the distance between QD and metallic nanowire, which elucidates a crossover passing from being metallic dissipative for much smaller emitter-nanowire distances to surface plasmon (SP) emission for larger separations at the vicinity of plasmonic metallic nanowire. Besides, our results also indicate that, for both the resonant case and the detuning case, through measuring QD emitter luminescence spectra and second-order correlation functions, the information about the QD emitter coupling to the SPs of the dissipative metallic nanowire can be extracted. This theoretical study will serve as an introduction to understanding the nanoplasmonic imaging spectroscopy and pave a new way to realize the quantum information devices.

Single InGaAs Quantum Dot Coupling to the Plasmon Resonance of a Metal Nanocrystal.

We report the observation of coupling of single InGaAs quantum dots with the surface plasmon resonance of a metal nanocrystal, which leads to clear enhancement of the photoluminescence in the spectral region of the surface plasmon resonance of the metal structures. Sharp emission lines, typical for single quantum dot emission, are observed, whereas for reference samples, only weak continuous background emission is visible. The composite metal–semiconductor structure is prepared by molecular beam epitaxy utilizing the principle of strain-driven adatom migration for the positioning of the metal nanocrystals with respect to the quantum dots without use of any additional processing steps.

Giant Stark effect in the emission of single semiconductor quantum dots

We study the quantum-confined Stark effect in single InAs/GaAs quantum dots embedded within a AlGaAs/GaAs/AlGaAs quantum well. By significantly increasing the barrier height we can observe emission from a dot at electric fields of 500 kV cm−1, leading to Stark shifts of up to 25 meV. Our results suggest this technique may enable future applications that require self-assembled dots with transitions at the same energy.

Single-dot Spectroscopy of GaAs Quantum Dots Fabricated by Filling of Self-assembled Nanoholes.

We study the optical emission of single GaAs quantum dots (QDs). The QDs are fabricated by filling of nanoholes in AlGaAs and AlAs which are generated in a self-assembled fashion by local droplet etching with Al droplets. Using suitable process parameters, we create either uniform QDs in partially filled deep holes or QDs with very broad size distribution in completely filled shallow holes. Micro photoluminescence measurements of single QDs of both types establish sharp excitonic peaks. We measure a fine-structure splitting in the range of 22–40μeV and no dependence on QD size. Furthermore, we find a decrease in exciton–biexciton splitting with increasing QD size.

Correlations between Subsequent Blinking Events in Single Quantum Dots

We explain the long-range correlations found by Stefani and his co-workers between blinking times of single colloidal quantum dot emission. Our explanation is based on the multiple recombination center model we recently suggested. The model produces positive correlations between subsequent on--on and off--off times and negative on--off correlations, as observed in the experiment. We also reproduce qualitatively the dependence of correlations between subsequent on--on, on-off, and off--off times on the number of switching events separating them.

Optical properties of InGaN quantum dots in monolithic pillar microcavities

The integration of InGaN quantum dots into GaN-based monolithic microcavities grown by metal-organic vapor-phase epitaxy is demonstrated. Microphotoluminescence spectra reveal distinct spectrally sharp emission lines around 2.73 eV, which can be attributed to the emission of single InGaN quantum dots. The samples are structured into airpost pillar microcavities. The longitudinal and transversal mode spectra of these cavities are in good agreement with theoretical calculations based on a vectorial transfer-matrix method. Quality factors up to Q=280 have been achieved.

Emission polarization control in semiconductor quantum dots coupled to a photonic crystal microcavity

We study the optical emission of single semiconductor quantum dots weakly coupled to a photonic-crystal micro-cavity. The linearly polarized emission of a selected quantum dot changes continuously its polarization angle, from nearly perpendicular to the cavity mode polarization at large detuning, to parallel at zero detuning, and reversing sign for negative detuning. The linear polarization rotation is qualitatively interpreted in terms of the detuning dependent mixing of the quantum dot and cavity states. The present result is relevant to achieve continuous control of the linear polarization in single photon emitters.

Emission properties and photon statistics of a single quantum dot laser

A theoretical description for a single quantum-dot emitter in a microcavity is developed.We analyze for increasing steady-state pump rate the transition from the strong-coupling regime with photon antibunching to the weak-coupling regime with coherent emission. It is demonstrated how Coulomb interaction of excited carriers and excitation-induced dephasing can strongly modify the emission properties. Our theoretical investigations are based on a direct solution of the Liouville-von Neumann equation for the coupled carrier-photon system. We include multiple carrier excitations in the quantum dot, their Coulomb interaction, as well as excitation-induced dephasing and screening. Similarities and differences to atomic systems are discussed and results in the regime of recent experiments are interpreted.

Slow-light-enhanced single quantum dot emission in a unidirectional photonic crystal waveguide

We report the observation of a Purcell enhancement of the in-plane spontaneous emission rates of InAs self-assembled quantum dots coupled to a mode of a unidirectional photonic crystal waveguide fabricated in GaAs(001). Three-dimensional finite-difference time-domain simulations predict the existence of high quality-factor modes due to the slow light resonances of the waveguide. These modes have been observed experimentally with microphotoluminescence and produce enhanced in-plane emission when resonant with a quantum dot.

Photon emission of single quantum dot driven by continuous external field studied by using the generating function

We studied the properties of the X- and Y- polarized photon emission from the single quantum dot system driven by external cw laser using the generating function approach developed recently. The results demonstrate that the X- and Y- polarized photon show sine or cosine behavior with the field direction in the weak field region. They show, however, stronger nonlinear behavior in the strong field region.