C Zinoni

Growth and characterization of single quantum dots emitting at 1300 nm

We have optimized the molecular-beam epitaxy growth conditions of self-organized InAs∕GaAs quantum dots (QDs) to achieve a low density of dots emitting at 1300 nm at low temperature. We used an ultralow InAs growth rate, lower than 0.002ML∕s, to reduce the density to 2dots∕μm2 and an InGaAs capping layer to achieve longer emission wavelength. Microphotoluminescence spectroscopy at low-temperature reveals emission lines characteristic of exciton-biexciton behavior. We also study the temperature dependence of the photoluminescence, showing clear single QD emission up to 90 K. With these results, InAs∕GaAs QDs appear as a very promising system for future applications of single photon sources in fiber-based quantum cryptography.

Time-resolved and antibunching experiments on single quantum dots at 1300 nm

We present time integrated and time-resolved photoluminescence (PL) measurements on a single InAs∕GaAs quantum dot (QD), embedded in a planar microcavity, emitting in the 1300nm telecom band. The results of both measurements clearly identify the exciton and biexciton transitions from a single QD. By optimizing the extraction efficiency of the QD PL into the single mode fibers and carefully tuning two InGaAs avalanche photodiodes, we were able to measure the second order correlation function with integration times comparable to those made with silicon based technology. These measurements demonstrate that our single QDs are efficient sources of triggered single photons for quantum key distribution in the O band.

Single-photon experiments at telecommunication wavelengths using nanowire superconducting detectors

The authors report fiber-coupled superconducting single-photon detectors with specifications that exceed those of avalanche photodiodes, operating at telecommunication wavelength, in sensitivity, temporal resolution, and repetition frequency. The improved performance is demonstrated by measuring the intensity correlation function g(2)(τ) of single-photon states at 1300nm produced by single semiconductor quantum dots.

Single-Photon Detection System for Quantum Optics Applications

We describe the design and characterization of a fiber-coupled double-channel single-photon detection system based on superconducting single-photon detectors (SSPD), and its application for quantum optics experiments on semiconductor nanostructures. When operated at 2-K temperature, the system shows 10% quantum efficiency at 1.3-¿m wavelength with dark count rate below 10 counts per second and timing resolution <100 ps. The short recovery time and absence of afterpulsing leads to counting frequencies as high as 40 MHz. Moreover, the low dark count rate allows operation in continuous mode (without gating). These characteristics are very attractive-as compared to InGaAs avalanche photodiodes-for quantum optics experiments at telecommunication wavelengths. We demonstrate the use of the system in time-correlated fluorescence spectroscopy of quantum wells and in the measurement of the intensity correlation function of light emitted by semiconductor quantum dots at 1300 nm.

Controlling the charge environment of single quantum dots in a photonic-crystal cavity

We demonstrate that the presence of charges around a semiconductor quantum dot (QD) strongly affects its optical properties and produces nonresonant coupling to the modes of a microcavity. We show that, besides (multi)exciton lines, a QD generates a spectrally broad emission which efficiently couples to cavity modes. Its temporal dynamics shows that it is related to the Coulomb interaction between the QD (multi)excitons and carriers in the adjacent wetting layer. This mechanism is suppressed by the application of an electric field, making the QD closer to an ideal two-level system.

Enhanced spontaneous emission rate from single InAs quantum dots in a photonic crystal nanocavity at telecom wavelengths

The authors demonstrate coupling at 1.3μm between single InAs quantum dots (QDs) and a mode of a two dimensional photonic crystal (PhC) defect cavity with a quality factor of 15 000. By spectrally tuning the cavity mode, they induce coupling with excitonic lines. They perform a time integrated and time-resolved photoluminescence and measure an eightfold increase in the spontaneous emission rate inducing a coupling efficiency of 96%. These measurements indicate the potential of single QDs in PhC cavities as efficient single-photon emitters for fiber-based quantum information processing applications.

Three-dimensional wavelength-scale confinement in quantum dot microcavity light-emitting diodes

We introduce a microcavity light-emitting diode (LED) structure that uses submicrometer oxide aperture and a quantum dot active region to achieve strong three-dimensional confinement of both the carrier distribution and the optical field. Light–current curves show optical emission for devices as small as 400nm in diameter. Spectroscopy on electrically pumped LEDs, with apertures ranging from 2.5 down to 0.7μm, show several spectral lines corresponding to cavity modes. A strong blueshift of the resonant modes for smaller apertures demonstrates the role of the oxide aperture in confining laterally the optical wave in a volume comparable to (λ∕n)3. Due to the high quality factors and low mode volumes, the devices could be good candidates for the demonstration of the Purcell effect under electrical pumping.

Structural and optical properties of low-density and In-rich InAs/GaAs quantum dots

Self-assembled InAs∕GaAs quantum dots have been grown at very low InAs growth rate in order to form sparse and large quantum dots (QDs) emitting in the near infrared (1300–1400nm), for application as single-photon sources. The structural and optical properties of these QDs as a function of the growth rate were systematically investigated. The QDs grown at the lowest rate (∼10−3ML∕s) present a very low dot density (∼2×108dots∕cm2), high In content, and good size homogeneity. Photoluminescence and time-resolved photoluminescence measurements performed at different powers and temperatures provide information on their luminescence efficiency, and on the recombination processes occurring in the low-density QDs as compared to higher densities.

Modeling the temperature characteristics of InAs/GaAs quantum dot lasers

A systematic investigation of the temperature characteristics of quantum dot lasers emitting at 1.3 μm is reported. The temperature dependence of carrier lifetime, radiative efficiency, threshold current, differential efficiency, and gain is measured, and compared to the theoretical results based on a rate equation model. The model accurately reproduces all experimental laser characteristics above room temperature. The degradation of laser characteristics with increasing temperature is clearly shown to be associated to the thermal escape of holes from the confined energy levels of the dots toward the wetting layer and the nonradiative recombination therein.

Telecom-wavelength single-photon sources for quantum communications

This paper describes the progress towards the realization of efficient single-photon sources based on semiconductor quantum dots (QDs), for application in quantum key distribution and, more generally, quantum communications. We describe the epitaxial growth of QD arrays with low areal density and emitting in the telecom wavelength range, the nanofabrication of single-QD structures and devices, and their optical and electro-optical characterization. The potential for integration with monolithic microcavities is also discussed.