D. Birkedal

Binding of quasi two-dimensional biexcitons

Summary form only given. In this presentation we report on a determination of the biexciton binding energies in GaAs-AlGaAs quantum wells of different widths and the results of a theoretical calculation of the ratio of the biexciton binding energy to that of the exciton. We determine the binding energies using spectrally resolved transient four-wave mixing (TFWM) and photoluminescence (PL).

Ultrafast gain dynamics in quantum-dot amplifiers: theoretical analysis and experimental investigations

Ultrafast gain dynamics in an optical amplifier with an active layer of self-organized quantum dots (QDs) emitting near 1.3 /spl mu/m is characterized experimentally in a pump-probe experiment and modeled theoretically on the basis of QD Maxwell-Bloch equations. Experiment and theory are in good agreement and show ultrafast subpicoseconds gain recovery followed by a slower 5 ps recovery. This behavior is found to be mainly caused by longitudinal optical phonon scattering and strongly dependents on electronic structure and confinement energy of the dots. A low amplitude-phase coupling (/spl alpha/ factor) is theoretically predicted and demonstrated in the experiments. The fundamental analysis reveals the underlying physical processes and indicates limitations to QD-based devices.

Submonolayer InGaAs/GaAs quantum-dot lasers with high modal gain and zero-linewidth enhancement factor

The gain spectra of a submonolayer (SML) InGaAs∕GaAs quantum dot (QD) laser working at 30°C were measured using the Hakki–Paoli method. It is found that the maximum modal gain of QD ground states is as high as 44cm−1 and no gain saturation occurs below the threshold at the lasing wavelength of 964.1nm. When the injection current is about 0.98 times the threshold, the gain spectrum becomes symmetric with respect to the lasing wavelength, and zero-linewidth enhancement factor is observed. These properties are attributed to the high density and the high uniformity of SML QDs in our laser diode.

Structure and optical anisotropy of vertically correlated submonolayer InAs/GaAs quantum dots

A vertically correlated submonolayer (VCSML) InAs/GaAs quantum-dot (QD) heterostructure was studied using transmission electron microscopy, high-resolution x-ray diffraction (HRXRD) and polarization-dependent photoluminescence. The HRXRD (004) rocking curve was simulated using the Tagaki–Taupin equations. Excellent agreement between the experimental curve and the simulation is achieved assuming that indium-rich VCSML QDs are embedded in a quantum well (QW) with lower indium content and an observed QD coverage of 10%. In the VCSML QDs, the vertical lattice mismatch of the InAs monolayer with respect to GaAs is around 1.4%, while the lattice mismatch in the QW is negligible. The photoluminescence is transverse magnetic—polarized in the edge geometry.

Structural and electrooptical characteristics of quantum dots emitting at 1.3 /spl mu/m on gallium arsenide

We present a comprehensive study of the structural and emission properties of self-assembled InAs quantum dots emitting at 1.3 /spl mu/m. The dots are grown by molecular beam epitaxy on gallium arsenide substrates. Room-temperature emission at 1.3 /spl mu/m is obtained by embedding the dots in an InGaAs layer. Depending on the growth structure, dot densities of 1-6/spl times/10/sup 10/ cm/sup -2/ are obtained. High dot densities are associated with large inhomogeneous broadenings, while narrow photoluminescence (PL) linewidths are obtained in low-density samples. From time-resolved PL experiments, a long carrier lifetime of /spl ap/1.8 ns is measured at room temperature, which confirms the excellent structural quality. A fast PL rise (/spl tau//sub rise/=10/spl plusmn/2 ps) is observed at all temperatures, indicating the potential for high-speed modulation. High-efficiency light-emitting diodes (LEDs) based on these dots are demonstrated, with external quantum efficiency of 1% at room temperature. This corresponds to an estimated 13% radiative efficiency. Electroluminescence spectra under high injection allow us to determine the transition energies of excited states in the dots and bidimensional states in the adjacent InGaAs quantum well.

Heterodyne technique for measuring the amplitude and phase transfer functions of an optical modulator

We propose a technique based on heterodyne detection for accurately and simultaneously measuring the amplitude and phase transfer functions of an optical modulator. The technique is used to characterize an InGaAsP multiple quantum-well electroabsorption modulator. From the measurements we derive the small-signal /spl alpha/-parameter and the time-dependent chirp for different operation conditions.

InGaAs/GaAs?quantum-dot?quantum-well heterostructure formed by submonolayer deposition

Discrete emission lines from self-assembled InGaAs quantum dots (QDs) grown in the submonolayer (SML) deposition mode have been observed in micro-photoluminescence (PL) spectra at 10 K. For the first time, the SML-grown InGaAs/GaAs QD heterostructure is verified to be a quantum-dot–quantum-well (QDQW) structure, by using high power PL and selective PL with excitation energies below the band gap of the GaAs barriers and temperature dependent PL. As the temperature is increased from 10 to 300 K, a narrowing of the full width at half-maximum at intermediate temperatures and a sigmoidal behaviour of the peak energy of PL band of the SML QD ensemble are observed and attributed to thermally activated carrier transfer between QDs via QW states.

Properties of InGaAs quantum dot saturable absorbers in monolithic mode-locked lasers

Saturable absorbers properties are characterised in monolithic mode-locked InGaAs quantum dot lasers. We analyse the impact of weak quantum confined Stark effect, fast absorber recovery time and low absorber saturation power on the mode-locking performance.

Exciton-exciton collisions and conversion of interwell excitons in GaAs/AlGaAs superlattices

The ratio of the densities of intra-and interwell excitons in a symmetric system of coupled quantum wells — a superlattice based on a GaAs/AlGaAs heterostructure — is investigated over a wide range of optical excitation power densities. Conversion of interwell excitons into intrawell excitons as a result of exciton-exciton collisions is observed at high exciton densities. Direct evidence for such a conversion mechanism is the square-root dependence of the interwell exciton density on the optical excitation level. The decrease in the lifetime of interwell excitons with increasing excitation density, as measured directly by time-resolved spectroscopy methods, confirms the explanation proposed for the effect.

Ultrafast nonlinear optics in GaAs/AlGaAs quantum wells

By degenerate four-wave mixing experiments in a two-beam geometry, we have investigated the initial coherence and dephasing of quasi two-dimensional excitions and biexcitons in GaAs multiple quantum wells. The dephasing has contributions from phonon scattering, interface-roughness scattering and exciton-exciton scattering. Inhomogeneous broadening and generation of coherent wavepackets play a significant role in the coherent exciton dynamics. The incoherent exciton dynamics, diffusion and recombination, is studied by three-beam transient grating experiments. A significant difference in the interface roughness scattering of coherent and incoherent (thermalized) excitons is observed.