Christian Gilfert

High gain 1.55 μm diode lasers based on InAs quantum dot like active regions

InP diode lasers with InAs quantum dot (QD) like active regions emitting at 1.55 μm have been fabricated. The QDs were grown in an As2 mode, which reduces the degree of elongation of the nanospecies yielding nearly circular shapes. Lasers with four to six dot layers show low absorption αi<10 cm−1 and high modal gain Γg0 of 10 cm−1 per QD layer (QDL) and above. The high gain values are compatible with an inhomogeneous linewidth that is much narrower than in quantum dash material, which is the common nanoscale gain material in the InP system.

Influence of the As2/As4 growth modes on the formation of quantum dot-like InAs islands grown on InAlGaAs/InP (100)

The formation process of InAs quantum dashes and quantum dots (QDs) grown on quaternary InAlGaAs surfaces lattice-matched to n-type InP(100) are investigated. A clear trend of the InAs to form dashes or dots depending on the species of supplied arsenic could be demonstrated. Using As4, elongated quantum dashes can be observed. Changing the growth mode to As2 molecules enables a shape transition from dashes to dome-shaped QDs. The dot ensembles exhibit improved photoluminescence (PL) intensity and linewidth over their elongated counterparts. With this basic concept, low temperature PL linewidths as low as 23 meV have been achieved.

High Speed 1.55 μm InAs/InGaAlAs/InP Quantum Dot Lasers

We report static and dynamic characteristics of InAs/InP quantum dot (QD) lasers emitting near 1.55 μm. The gain section was optimized for a high speed operation using a unique spatially resolved model. The measured modulation capability dependence on structural parameters (barrier width and the number of QD layers) is consistent with the model predictions. Short cavity lasers with a modal gain of more than 10 cm-1 per dot layer exhibit a small signal modulation bandwidth above 9 GHz and large signal modulation at up to 22 Gb/s with an on/off ratio of 3 dB.

High-Speed Low-Noise InAs/InAlGaAs/InP 1.55-

We present the static and dynamic properties of InAs quantum-dot (QD) lasers emitting near 1.55 μm. The used laser material comprises four QD layers and exhibits a high modal gain of about 40 cm-1. The 340-μ.m-long lasers show a room temperature threshold current of 38 mA and a maximum output power of 16 mW. The small signal modulation response is highly damped and carrier transport limited with a moderate 3-dB bandwidth of 5 GHz. This is accompanied by a flat relative intensity noise spectrum at a low level of -150 dBc/Hz. Neverthe- less, the laser exhibits record large signal modulation capabilities for a 1.5-μ.m QD laser: 15 Gb/s with a 4-dB on/off ratio.

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High-power 1060-nm InGaAs-(Al)GaAs quantum-dot (QD) laser material was developed with an integrated InGaAs quantum film acting as a tunnel injector for electrons. In comparison to a QD laser without tunnel-injection design, the new type of lasers exhibit a strongly improved temperature stability of the threshold current and internal quantum efficiency.

Quantum-Dot Lasers

Low temperature carrier dynamics in the InGaAs/GaAs quantum dot-based tunnel injection structure is studied by the time resolved photoluminescence experiment. We observed strongly modified photoluminescence kinetics between tunnel injection and reference quantum dot structures. Slowing down of the photoluminescence rise time in the tunnel injection system under weak and moderate excitation powers, we attributed to a fingerprint of a feeding process of quantum dot states with nonresonant carriers tunneling from the quantum well reservoir. We propose a simple three-level rate equation model to explain qualitatively the observed photoluminescence temporal behavior. Its result shows a good agreement with our experimental data.

High-Power Tunnel-Injection 1060-nm InGaAs–(Al)GaAs Quantum-Dot Lasers

A 920 nm InGaAs/(Al)GaAs high-power quantum-dot laser material was developed with optimized geometric parameters, i.e., dot size, dot density and size distribution, and an appropriate laser design to allow an internal temperature compensation of the emission wavelength by a tailored spectral gain profile. The laser structure was grown by solid source molecular-beam epitaxy and consisted in a separate confinement heterostructure design with a large optical cavity. Due to the cavity length dependence of the total cavity losses, the operation point is shifted and reached a very low temperature coefficient of 0.081 nm K−1 for a cavity length of 2.6 mm.

Time-resolved photoluminescence spectroscopy of an InGaAs/GaAs quantum well-quantum dots tunnel injection structure

InGaAs/(Al)GaAs quantum dot lasers emitting at 1100?nm are developed with a relatively low In content of 28% used for dot formation and an appropriate laser design to allow for high power emission. In comparison with an InGaAs QD laser with a similar design but a higher In content of 60% the newly developed lasers exhibit an improved temperature stability of the threshold current and internal quantum efficiency.

GaInAs/(Al)GaAs quantum-dot lasers with high wavelength stability

The lateral carrier diffusion process is investigated in coupled InGaAs/GaAs quantum dot-quantum well (QD-QW) structures by means of spatially resolved photoluminescence spectroscopy at low temperature. Under non-resonant photo-excitation above the GaAs bandgap, the lateral carrier transport reflected in the distorted electron-hole pair emission profiles is found to be mainly governed by high energy carriers created within the 3D density of states of GaAs. In contrast, for the case of resonant excitation tuned to the QW-like ground state of the QD-QW system, the emission profiles remain unaffected by the excess kinetic energy of carriers and local phonon heating within the pump spot. The lateral diffusion lengths are determined and present certain dependency on the coupling strength between QW and QDs. While for a strongly coupled structure the diffusion length is found to be around 0.8 μm and monotonically increases up to 1.4 μm with the excitation power density, in weakly coupled structures, it is deter...

1100?nm InGaAs/(Al)GaAs quantum dot lasers for high-power applications

With high modal gain InAs/AlGaInAs/InP quantum dot laser material short cavity ridge waveguide lasers were fabricated with cavity lengths down to 275 μm. These devices show new record values in direct digital signal modulation at 22 GBit/s. In addition also strong improvement are expected from this laser material in the static properties, in particular on the linewidth due to the suppression of the linewidth enhancement factor. First distributed feedback lasers on similar quantum dot laser material were processed and preliminary linewidth measurements indicate a significant linewidth reduction. Quantitative investigations are under way and will be presented at the conference.