Single InP Quantum Dots Research Articles

The effects of discrete and gradient mid-shell structures on the photoluminescence of single InP quantum dots.

We investigated the dependence of the spectral diffusion and blinking behaviors of indium phosphide (InP) based core/shell/shell quantum dots (QDs) on their mid-shell compositions. We synthesized two types of core/shell/shell QDs having different mid-shell structures by controlling the shell thickness, the total sizes, and the selenium to sulfur ratios. The QDs with a discrete mid-shell (DS-QDs) exhibited a higher photoluminescence (PL) quantum yield (QY) and a narrower PL linewidth than the QDs with a gradient mid-shell (GS-QDs). By analyzing X-ray diffraction (XRD) patterns, and Raman spectra, we found that GS-QDs showed a larger lattice mismatch between the core and the shell than DS-QDs. Also, the spectral diffusion, PL blinking, Auger ionization efficiencies, and the lifetime blinking behavior on single QDs revealed that DS-QDs were nearly unaffected by the defect traps.

Site-Controlled Growth of Single InP QDs

The MOVPE growth of InP/GaInP quantum dots (QDs) on GaAs substrate “defects” formed by a focused beam of Ga+ ions is studied. It is shown that ordered arrays of QDs with a density of 0.25 (μm)−2 can be obtained in the InP/GaInP system. It is demonstrated that effective luminescence can be obtained by using two QD sheets separated by a GaAs/GaInP buffer layer.

Lasing of whispering‐gallery modes in GaInP waveguide micro‐discs and rings with InP quantum dots

AbstractWhispering‐gallery‐mode (WGM) GaInP micro‐disc (MD) and micro‐ring (MR) resonators (diameters D=1‐3 mm, quality factors Q up to 5×103) with embedded InP quantum dots (size/density ∼100 nm/∼109cm‐2, emission range 720‐770 nm) have been fabricated using a wet oxidation technique. Ultralow (sub‐nanoWatt) lasing thresholds for a single InP quantum dot have been demonstrated for these resonators at 15 K under optical pumping. Observed thresholds are two orders of magnitude lower than for single InAs QD lasing (M. Nomura et al., Opt. Express 17, 15975 (2009) [1]). (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrically pumped single-photon emission in the visible spectral range up to 80 K

We present an electrically pumped single-photon emitter in the visible spectral range, working up to 80 K, realized using a self-assembled single InP quantum dot. We confirm that the electroluminescense is emitted from a single quantum dot by performing second-order autocorrelation measurements and show that the deviation from perfect single-photon emission is entirely related to detector limitations and background signal. Emission from both neutral and charged exciton complexes was observed with their relative intensites depending on the injection current and temperature.

Interferometric spectroscopy for excitons in InP single quantum dots

Dynamical dephasing processes of an exciton and a charged exciton in single InP quantum dots were studied by using interferometric spectroscopy. Interferometric spectroscopy enabled us to observe with high sensitivity the dephasing of exciton or other exciton complexes in single quantum dots. In order to observe the dephasing of the exciton or exciton complexes, emitted single-photons generated from single InP quantum dots were detected through the Michelson interferometer. The contrast of the interferometric signal due to the exciton and the charged exciton shows non-exponential decay under band-to-band excitation for the GaInP matrix. The band-to-band excitation generates carriers trapped in the matrix and the trapped carriers modulate the energy of the quantum dots because of the quantum-confined Stark effect. Therefore the non-exponential decays are caused by energy fluctuation due to the trap carriers in the long timescale.

Deutsch-Jozsa Algorithm Using Triggered Single Photons from a Single Quantum Dot

We demonstrate a two-qubit Deutsch-Jozsa algorithm with single photons from a single InP quantum dot. The qubits are implemented via the spatial mode and the polarization of a single photon. Our photon source is operated both under continuous and pulsed excitation, the latter allowing deterministic quantum logic by generating photons on demand with a strong suppression of two-photon events. The computation reached a success probability of up to 79%. We also exploit the concept of decoherence-free subspaces that helps to make our experimental setup robust against sources of phase noise.

Magneto‐optics from type‐II single quantum dots

We investigated single InP quantum dots embedded in GaAs using micro-photoluminescence as a function of the excitation intensity. InP/GaAs dots exhibit a type-II band alignment, which leads to a spatial separation of the carriers. The effect of a magnetic field on these type-II quantum dots were also investigated through micro-photoluminescence measurements. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Generating visible single photons on demand with single InP quantum dots

We present photon correlation measurements performed on a device based on single InP quantum dots. The device consists of a 400 nm thick membrane containing a low density of quantum dots on a metal mirror. Measurements done under continuous excitation reveal a very pronounced antibunching dip while measurements done under pulsed excitation enable the generation of single photons on demand at the optimum wavelength for silicon-based single-photon detectors.

Optical Properties of InP Self-Assembled Quantum Dots Studied by Imaging and Single Dot Spectroscopy

Optical properties of single InP quantum dots (QDs) were investigated by means of micro-photoluminescence (µ-PL) spectroscopy. The temperature dependence of the integrated µ-PL intensity revealed that each QD has different thermal activation energy ranging from 5 to 40 meV. The PL intensities of some QDs recovered when the temperature was raised beyond a certain value. This phenomenon was explained by competition between radiative and nonradiative processes. In this work, the mechanism of fluorescence intermittency was also investigated. It was found that the switching is triggered by a photo-induced process and that the confined excitons in the excited states play an important role. From the observation of the µ-PL spectra in the external electric field and their temperature dependence, we concluded that the large intensity change is caused by a local electric field generated by a carrier trapped at a localized center. These results indicate that the optical properties of the zero-dimensional system are strongly influenced by the local environment surrounding the QDs.

Electron accumulation in single InP quantum dots observed by photoluminescence

Single quantum-dot spectroscopy has revealed characteristic but so far unexplained differences in the optical spectra from different quantum-dot systems. We propose a size-dependent accumulation of carriers as the dominant mechanism behind these differences. We support our hypothesis with photoluminescence spectroscopy on single InP/GaInP quantum dots positioned below a transparent Schottky gate. We show that without external bias, the dots are filled with 15--20 electrons. By applying a reverse bias, we are able to reduce the electron accumulation while monitoring the evolution of the emission spectrum. We find that the emission peaks disappear one by one until, at a sufficiently low number of electrons in the dot, the remaining broad peaks are replaced by numerous very sharp peaks.

Fluorescence Intermittency in Single InP Quantum Dots

Fluorescence “blinking” kinetics of isolated colloidal InP quantum dots (QDs) are investigated via confocal single molecule microscopy. Analysis of fluorescence trajectories reveals inverse power law behavior (∝ 1/τm) in on/off time probability densities with mon ≈ 2.0(2) and moff ≈ 1.5(1). Such an inverse power law in both on/off times is inconsistent with a static distribution of electron/hole trapping sites and highlights the role of fluctuations in the QD nanoenvironment.

Magnetic field effects in InP self-assembled quantum dots

Optical properties of InP self-assembled quantum dots (SADs) were investigated in the presence of a magnetic field up to 10 T. By using a μm–sized InP/GaInP mesa, we succeeded in observing sharp photoluminescence (PL) lines from a single InP quantum dot. With the increase of the magnetic field, the diamagnetic shift and the Zeeman splitting were clearly observed in the Faraday configuration. The diamagnetic coefficient and the effective g-value of the confined excitons show fluctuations reflecting dispersion in the size and the shape of the quantum dots. The estimated g-value of the InP quantum dot is about half of that of the bulk InP because of the heavy-hole light-hole band mixing.