## Optical properties of In(Ga)As/GaAs quantum dot-based devices emitting at 1.3[mu]m

Self-assembled quantum-dots (QDs) represent a distributed ensemble of zero dimensional structures with a near-singular density of states. Implemented as the active medium in a diode laser their unique properties lead to improved and often novel characteristics as compared to bulk or quantum-well (QW) devices. In particular In(Ga)As QD lasers on GaAs substrates are of largest interest as the spectral range of their emission wavelength reaches further into the infrared than that of QW lasers of the same material system. The emission wavelength of strained InGaAs QW lasers is limited to about 1150nm, whereas in In(Ga)As QD lasers an emission wavelength beyond 1.3µm is feasible. In the recent past ultralow threshold current densities combined with decreased temperature sensitivity, high modulation bandwidth and low chirp were reported on these 1.3µm QD lasers - as predicted many years ago. The differential efficiency of these devices is to date lower than expected. There are, however, difficulties associated with the random shape and size distribution of the QDs, and a strongly reduced density of states. The latter facilitates a low transparency current and consequently a low threshold current. But it is also accompanied by a smaller modal gain than it is usual in a QW laser, which has severe consequences for both the technical feasibility and the functionality of such a device. Particularly in applications demanding laser structures with larger total loss - e.g. for short cavity lengths and without high reflection coatings at the facets - the modal gain that can be obtained from a single layer of QDs is insufficient. One major goal of this thesis has been to realize In(Ga)As QD lasers emitting at 1.3µm. To achieve this, various QD growth structures have been analyzed in photoluminescence experiments, in order to optimize the QD properties with respect to emission wavelength and material gain. In addition comparative studies with other material systems emitting at 1.3µm have been conducted, e.g. GaInNAs QWs. In this context the major advantages and drawbacks of QDs appeared very clearly - the much better efficiency and the much lower density of states. Further on the QD efficiency has been tested in various light emitting diode (LED) structures, in order to optimize the design of the laser structures. During the characterization of the first fabricated broad-area QD lasers a novel and intriguing behavior became apparent: under certain conditions the devices operate at two well separated wavelengths, corresponding to a ground-state and an excited-state transition. In order to understand this behavior a comprehensive numerical model based on a rate equation system has been developed. The effect of simultaneous two-state lasing has then been attributed to incomplete clamping of the excited-state population above ground-state lasing threshold. The exact behavior of the energy level populations during lasing operation have been studied further in spontaneous emission spectra detected from a µm-sized hole on the top contact of the devices. In a very fruitful collaboration with Opto+, Alcatel CIT (Marcoussis, France) the dynamical properties of single transverse-mode lasers based on the same device structure have been investigated. Also here the QD lasers showed a different behavior as compared to QW devices: (1) The linewidth enhancement factor increases dramatically above threshold. (2) The modulation performance is degraded when the second lasing line appears. (3) Indications for the absence of relaxation oscillations on the ground state transition have been found. To a certain extent the experimentally found behavior could be explained in the framework of the rate equation model. For example the increase of the linewidth enhancement factor is well explained by an increasingly asymmetric gain spectrum due to the increase of excited state population above threshold. Finally quantum dot lasers including an intracavity quantum dot absorber section have been fabricated and characterized. In such a device the total loss can be manipulated by changing the bias on the absorber section. The basic idea was to achieve an efficient wavelength switch between the lasing lines of the ground state energy transition and the excited state energy transition, by electrically manipulating the total loss. This effect could not be demonstrated. The characteristics of this device could be qualitatively described in a traveling-wave rate equation model. This model also predicts the occurrence of self-sustained pulses in this device - a behavior that could not be investigated experimentally during this thesis.

Deveaud-Plédran, Benoît
Year:
2004
Publisher:
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis3060-5

Note: The status of this file is: EPFL only
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