A Andrea Fiore

Simultaneous two-state lasing in quantum-dot lasers

We demonstrate simultaneous lasing at two well-separated wavelengths in self-assembled InAs quantum-dot lasers, via ground-state (GS) and excited-state (ES) transitions. This effect is reproducible and strongly depends on the cavity length. By a master-equation model, we attribute it to incomplete clamping of the ES population at the GS threshold.

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.

Tuning InAs/GaAs quantum dot properties under Stranski-Krastanov growth mode for 1.3 μm applications

In this paper, we present a systematic study of the effect of growth parameters on the structural and optical properties of InAs quantum dot (QD) grown under Stranski-Krastanov mode by molecular beam epitaxy. The dot density is significantly reduced from 1.9x10(10) to 0.6x10(10) cm(-2) as the growth rate decreases from 0.075 to 0.019 ML/s, while the island size becomes larger. Correspondingly, the emission wavelength shifts to the longer side. By increasing the indium fraction in the InGaAs capping layer, the emission wavelength increases further. At indium fraction of 0.3, a ground state transition wavelength as long as 1.4 mum with the excited state transition wavelength of around 1.3 mum has been achieved in our dots. The optical properties of QDs with a ground state transition wavelength of 1.3 mum but with different growth techniques were compared. The QDs grown with higher rate and embedded by InGaAs have a higher intensity saturation level from excitation dependent photoluminescence measurements and a smaller intensity decrease from temperature dependent measurements. Finally, single mirror light emitting diodes with a QD embedded in InGaAs have been fabricated. The quantum efficiency at room temperature is 1.3%, corresponding to a radiative efficiency of 21.5%

Differential Gain and Gain Compression in Quantum-Dot Lasers

The dynamics of optical gain in semiconductor quantum dots (QDs) is investigated. Simple analytical expressions are derived, which directly connect the laser dynamical response to capture and intradot relaxation rates. The effect of hole spreading in the valence band and spectral hole burning in the QD ensemble is also quantitatively assessed. The analysis shows that intradot relaxation constitutes the main limitation in the dynamics and points to possible routes towards the improvement of QD lasers

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.

Two-state switching and dynamics in quantum dot two-section lasers

The electrical control of the lasing wavelength in two-section quantum dot lasers is investigated. By changing the optical loss in the absorber section, the control of the ground-state (GS) and excited-state (ES) lasing thresholds and output powers is achieved. Additionally, a complex self-pulsation dynamics with simultaneous oscillations of the GS and ES intensities is observed. The experimental results are well explained in the framework of a rate equation model.

Comparison of radiative properties of InAs quantum dots and GaInNAs quantum wells emitting around 1.3 μm

The emission properties of self-assembled InAs quantum dots (QDs) and lattice-matched GaInNAs quantum wells (QWs) emitting around 1.3u2009μm were investigated by temperature-dependent and time-resolved photoluminescence (PL). The QDs have much higher PL efficiency at low excitation, but saturate faster as the excitation is increased, due to the lower density of states. Lifetime measurements show that nonradiative recombination plays a more important role in the GaInNAs QW than in QDs.

Matrix effects on the structural and optical properties of InAs quantum dots

InAs quantum dots (QDs) have been grown by molecular-beam epitaxy on different InGaAs or GaAs surface layers to investigate the effect of the matrix on the structural and optical properties of the QDs. The density of QDs directly grown on GaAs is 1.1×1010u200acm−2, and increases to 2.3×1010u200acm−2 for dots grown on a 1 nm InGaAs layer. Single-mirror light-emitting-diode (SMLED) structures with InAs QDs capped by InGaAs and grown on GaAs and InGaAs layers were fabricated to compare the electroluminescence efficiency between the two structures. The maximum external quantum efficiency for QDs on a GaAs structure is 1.1% while that for QDs on InGaAs is 1.3%. The corresponding radiative efficiency could be deduced to be 17.5% for QDs on GaAs and 21.5% for QDs on InGaAs, respectively.

Scaling quantum-dot light-emitting diodes to submicrometer sizes

We introduce a device structure and a fabrication technique that allow the realization of efficient light-emitting diodes (LEDs) with dimensions of the active area in the ≈100u2009nm range. Using optical lithography, selective oxidation, and an active region consisting of InAs quantum dots (QDs), we fabricated LEDs with light–current–voltage characteristics which scale well with nominal device area down to 600 nm diam at room temperature. The scaling behavior provides evidence for strong carrier confinement in the QDs and shows the potential for the realization of high-efficiency single-photon LEDs operating at room temperature.