Properties Of Single Quantum Dots Research Articles

Bright and Polarized Fiber In-Line Single-Photon Source Based on Plasmon-Enhanced Emission into Nanofiber Guided Modes

We demonstrate a bright and polarized fiber in-line single-photon source based on plasmon-enhanced emission of colloidal single quantum dots into an optical nanofiber. We show that emission properties of single quantum dots can be strongly enhanced in the presence of single gold nanorods leading to a bright and strongly polarized single-photon emission. The single photons are efficiently coupled to guided modes of the nanofiber and eventually to a single-mode optical fiber. The brightness (fiber-coupled photon count rate) of the single-photon source is estimated to be $12.2\ifmmode\pm\else\textpm\fi{}0.6$ MHz, with high single-photon purity [${g}^{2}(0)=0.20\ifmmode\pm\else\textpm\fi{}0.04$] and degree of polarization as high as 94--97 %. The polarized and fiber-coupled single photons can be implemented for potential applications in quantum photonics.

Photoluminescence Enhancement, Blinking Suppression, and Improved Biexciton Quantum Yield of Single Quantum Dots in Zero Mode Waveguides.

The capability of quantum dots to generate both single and multiexcitons can be harnessed for a wide variety of applications, including those that require high optical gain. Here, we use time-correlated photoluminescence (PL) spectroscopy to demonstrate that the isolation of single CdSeTe/ZnS core-shell, nanocrystal quantum dots (QDs) in Zero Mode Waveguides (ZMWs) leads to a significant modification in PL intensity, blinking dynamics, and biexciton behavior. QDs in aluminum ZMWs (AlZMWs) exhibited a 15-fold increase in biexciton emission, indicating a preferential enhancement of the biexciton radiative decay rate as compared to the single exciton rate. The increase in biexciton behavior was accompanied by a decrease in blinking events due to a shortening in the dark state residence time. These results indicate that plasmon mediated enhanced decay rates of QDs in AlZMWs lead to substantial changes in the photophysical properties of single quantum dots, including an increase in biexciton behavior.

Three-Electrode Device for Applying Two-Dimensional Vector Electric Fields to Single InAs Quantum Dots

InAs/GaAs quantum dots (QDs) and quantum dot molecules (QDMs) are self-assembled semiconductor nanostructures that can trap a single electron or hole with well-defined spin projections. QDs and QDMs have excellent optical properties and have long been of interest for incorporation into quantum optoelectronic devices ranging from single photon sources to multi-bit quantum computers. The properties of single QDs, or carriers confined within those QDs, can be tuned by external electric fields, which provides an important tool for the development of scalable and tunable devices. Deterministic charging of a QD with a single electron or hole is an important tool for quantum devices and is well-established under the application of growth-direction electric fields in a diode structure. Here, we report a new charging mechanism for a single QD in a 3-electrode device that does not contain a vertical diode and can be used to control electric field profiles in two dimensions. We fabricate the device with E-beam lithography and characterize photoluminescence from single QDs under different bias configurations. Using a combination of experimental data and COMSOL band structure calculations, we explain how the charging originates in the electrical-injection of holes induced by lateral electric fields. We discuss the applications for this device and the potential for full 2-D electric field control of a single QD and QDM.

Single-Photon Emission from InAs/AlGaAs Quantum Dots

The results of investigation of the radiative characteristics of heterostructures with InAs/AlGaAs quantum dots (QDs) grown by molecular beam epitaxy have been presented. The properties of single QDs were determined by spectroscopy of micro-photoluminescence in cylindrical mesa-structures with a diameter of 200–1000 nm or columnar microresonators with distributed Bragg mirrors. The single-photon nature of the radiation is confirmed by measurements and analysis of the second-order correlation function g2(τ) in a wide spectral range from 630 to 730 nm.

Однофотонное излучение квантовых точек InAs/AlGaAs

AbstractThe results of investigation of the radiative characteristics of heterostructures with InAs/AlGaAs quantum dots (QDs) grown by molecular beam epitaxy have been presented. The properties of single QDs were determined by spectroscopy of micro-photoluminescence in cylindrical mesa-structures with a diameter of 200–1000 nm or columnar microresonators with distributed Bragg mirrors. The single-photon nature of the radiation is confirmed by measurements and analysis of the second-order correlation function g ^2(τ) in a wide spectral range from 630 to 730 nm.

Large optical Stark shifts in single quantum dots coupled to core-shell GaAs/AlGaAs nanowires.

Nanowire quantum dots (NW-QDs) can be used for future compact and efficient optoelectronic devices. Many efforts have been made to control the QD states by inserting the QDs in doped structures and applying an electric field in a nanowire system. In this paper, we use down-conversion and up-conversion photoluminescence excitations to explore the optical and electronic properties of single quantum dots in GaAs/AlGaAs core-shell nanowires. We investigate a large optical Stark shift in this system as a new method to tune the QD states. When the tunable laser lies within the spectral bandwidth of ZB/WZ GaAs (780 nm-860 nm), we observe an extremely large optical Stark shift of 1.3 nm (0.5 nm) with increasing excitation power at a resonant wavelength of 800 nm (840 nm) in GaAs states. The ability to in situ control the energy states of self-catalyzed NW-QDs should open a new way for quantum light sources and nonlinear optics in a nanowire system.

Photo Properties and Applications of Single Quantum Dots

As novel fluorescent nanomaterials , quantum dots ( QDs ) have played important roles in many fields , such as chemical analysis , biology sensor , molecular imaging , ow ing to their excellent optical properties. Studies on the properties of single quantum dots can help to find new experimental phenomenology w hich can ' t be found in the ensemble-approach , provide a mentality to improve the properties of quantum dots , contribute to a better application of quantum dots in various fields. In this review , identifications criterion of single quantum dots , photo -properties ( such as fluorescence enhancement , bleaching , blinking , bluing , etc. ) of single quantum dots , and quantum dots application ( such as single quantum dots tracking , single quantum dots biosensor , super- localization technology ) at single nanoparticle levels are commented. The challenges and existing problems of application of single quantum dots are summarized. In the future , synthesis in quantum dots should simultaneously satisfy many excellent photo -properties , including small size , high quantum yield , non-blinking , lager blue shift range , no -biotoxicty . M eanw hile , plasmonic quantum dots that not only exhibit strong fluorescence , but also become excellent probes for surface plasmon scattering , are another significant research field.

The influence of continuous vs. pulsed laser excitation on single quantum dot photophysics.

The impact of pulsed versus continuous wave (cw) laser excitation on the photophysical properties of single quantum dots (QDs) has been investigated in an experiment in which all macroscopic variables are identical except the nature of laser excitation. Pulsed excitation exaggerates the effects of photobleaching, results in a lower probability of long ON fluorescence blinking events, and leads to shorter fluorescence lifetimes with respect to cw excitation at the same wavelength and average intensity. Spectral wandering, biexciton quantum yields, and power law exponents that describe fluorescence blinking are largely insensitive to the nature of laser excitation. These results explicitly illustrate important similarities and differences in fluorescence dynamics between pulsed and cw excitation, enabling more meaningful comparisons between literature reports and aiding in the design of new experiments to mitigate possible influences of high photon flux on QDs.

Single spins in self-assembled quantum dots

Self-assembled quantum dots have excellent photonic properties. For instance, a single quantum dot is a high-brightness, narrow-linewidth source of single photons. Furthermore, the environment of a single quantum dot can be tailored relatively easily using semiconductor heterostructure and post-growth processing techniques, enabling electrical control of the quantum dot charge and control over the photonic modes with which the quantum dot interacts. A single electron or hole trapped inside a quantum dot has spintronics applications. Although the spin dephasing is rather rapid, a single spin can be manipulated using optical techniques on subnanosecond timescales. Optical experiments are also providing new insights into old issues, such as the central spin problem. This Review provides a snapshot of this active field, with some indications for the future. It covers the basic materials and optical properties of single quantum dots, techniques for initializing, manipulating and reading out single spin qubits, and the mechanisms that limit the electron-spin and hole-spin coherence.

Quantitative analysis of quantum dot dynamics and emission spectra in cavity quantum electrodynamics

We present detuning-dependent spectral and decay-rate measurements to study the difference between the spectral and dynamical properties of single quantum dots embedded in micropillar and photonic crystal cavities. For the micropillar cavity, the dynamics is well described by the dissipative Jaynes–Cummings model, whereas systematic deviations are observed for the emission spectra. The discrepancy for the spectra is attributed to the coupling of other exciton lines to the cavity and interference of different propagation paths toward the detector of the fields emitted by the quantum dot. In contrast, quantitative information about the system can readily be extracted from the dynamical measurements. In the case of photonic crystal cavities, we observe an anti-crossing in the spectra when detuning a single quantum dot through resonance, which is the spectral signature of a strong coupling. However, time-resolved measurements reveal that the actual coupling strength is significantly smaller than anticipated from the spectral measurements and that the quantum dot is rather weakly coupled to the cavity. We suggest that the observed Rabi splitting is due to cavity feeding by other quantum dots and/or multi-exciton complexes giving rise to collective emission effects.

Optical polarization properties of a nanowire quantum dot probed along perpendicular orientations

We report on the optical properties of single quantum dots in nanowires probed along orthogonal directions. We address the same quantum dot from either the nanowire side or along the nanowire axis via reflection on a micro-prism. The collected photoluminescence intensity from nanowires lying on a substrate is improved 3-fold using the prism as compared to usual collection from the top. More importantly, we circumvent the polarizing effect of the nanowire and access the intrinsic polarization properties of the quantum emitter. Our technique is compatible with the design of complex nanowire devices for the development of quantum opto-electronics.

Suppression of quantum dot blinking in DTT-doped polymer films.

In this report we evaluate the emission properties of single quantum dots embedded in a thin, thiol containing polymer film. We report the suppression of quantum dot blinking leading to a continuous photon flux from both organic and water soluble quantum dots and demonstrate their application as robust fluorescent point sources for ultrahigh resolution localization. In addition, we apply the polymer coating to cell samples immunostained with antibody conjugated QDs and show that fluorescence intensity from the polymer embedded cells shows no sign of degradation after 67 hours of continuous excitation. The reported thin polymer film coating may prove advantageous for immuno-cyto/histo-chemistry as well as for the fabrication of quantum dot containing devices requiring a reliable and stable photon source (including a single photon source) or stable charge characteristics while maintaining intimate contact between the quantum dot and the surrounding matrix.

Influence of the Dark Exciton State on the Optical and Quantum Optical Properties of Single Quantum Dots

The dark exciton state strongly affects the optical and quantum optical properties of flat InP/GaInP quantum dots. The exciton intensity drops sharply compared to the biexciton with rising pulsed laser excitation power while the opposite is true with temperature. Also, the decay rate is faster for the exciton than the biexciton and the dark-to-bright state spin flip is enhanced with temperature. Furthermore, long-lived dark state related memory effects are observed in second-order cross-correlation measurements between the exciton and biexciton and have been simulated using a rate-equation model.

Probing the dynamic fluorescence properties of single water-soluble quantum dots

The influence of surface modifications, including carboxyl-, amine-, and mercaptoundecanoicacid-functionalized chemistries, on the dynamic fluorescence properties of single quantum dots (QDs) are probed using a combined confocal fluorescence microscopy and spectroscopy system. Water-soluble QDs with carboxyl- or amine-functionalized surfaces prepared through a linker arm show significant fluorescence intermittency and appear statistically identical. In contrast, water-soluble QDs functionalized via mercaptoundecanoicacid, an electron-donating thiol group, demonstrate suppressed fluorescence intermittency suggesting the photostability of water-soluble QDs is strongly dependant upon the method of surface functionalization. In addition, the long-term photostability of the mercaptoundecanoicacid-functionalized QD demonstrates an excitation-time-dependent blue-shift in the fluorescence emission spectra. Our measurement and analysis techniques promise a wealth of insight in elucidating the photophysics of bio-conjugated QDs and assessing and designing bio-conjugated QDs for biomedical applications requiring long-term photostabililty.

Optical properties of single InGaN quantum dots up to 150 K

AbstractWe present photoluminescence measurements on single InGaN quantum dots grown by metalorganic vapor phase epitaxy. The spatially and spectrally resolved luminescence properties of the single quantum dots were measured using low‐temperature micro‐photoluminescence spectroscopy. The observed sharp emission lines of the quantum dots were characterized by excitation density dependent measurements. The photoluminescence of individual quantum dots can easily be detected for temperatures up to 150 K. In addition, temporal fluctuations of the emission peak positions of single quantum dots are attributed to spectral diffusion processes. Indications for recombination processes from excited states are observed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Emission characteristics of quantum dots in planar microcavities

The emission properties of single quantum dots in planar microcavities are studied experimentally and theoretically. Fivefold enhanced spontaneous emission outside the microcavity is found for dots in resonance with the cavity mode, relative to detuned dots. Using high-power excitation we obtain the in-plane cavity dispersion. Near-field images of the emission show spatial distributions of several micrometers for resonant dots, which decrease in size with the detuning from resonance. These experimental findings are explained using a simple and intuitive model.

Influence of an in-plane electric field on exciton fine structure in InAs-GaAs self-assembled quantum dots

The influence of an in-plane electric field on the optical properties of single quantum dots is investigated. On a sample containing a plane of InAs∕GaAs dots, micrometer-size electro-optical structures were produced in order to apply an external electric field in the dot plane. A large decrease of the anisotropic exchange splitting, correlated with the in-plane Stark shift, is observed.

Photon mapping of quantum dots using a scanning tunneling microscope

Scanning tunneling microscopy (STM) and scanning tunneling luminescence (STL) have been used to investigate the geometric and optical properties of individual self-assembled InP quantum dots overgrown with a thin layer of GaInP. STL spectra and monochromatic photon maps were used to correlate the surface topography with the optical properties of single quantum dots. We find a spatial resolution of about 10 nm in the photon maps. Theoretical emission spectra were calculated by six-band k⋅p theory using a realistic shape of the dot as well as of the cap layer. The calculated emission spectrum of a single dot is in good agreement with the experimental findings.