Investigation into the coupling of quantum dots to photonic crystal nanocavities at telecommunication wavelengths

Laurent Balet
EPFL, 2009
Thèse EPFL

Résumé

Recently, the emission of single photons with emission wavelength in the 1.3 µm telecommunication window was demonstrated for InAs quantum dots. This makes them strong candidates for applications such as quantum cryptography, and in a longer term, quantum computing. However, efficient extraction of the spontaneous emission from semiconductors still represents a major challenge due to total internal reflection at the semiconductor/air interface. In particular, single photon sources based on quantum dots are plagued by low extraction efficiency and poor coupling to single-mode fibers, typically on the order of 10-3 ~ 10-4, which prevents their application to quantum communication. To seek a solution to this problem, this thesis work explores the integration of quantum dots, with emission at 1.3 µm, in photonic crystal microcavities. Photons emitted in a mode of the cavity are funneled out of the semiconductor, and thus bypass the total internal reflection. In addition, the modified density of electromagnetic states in the cavity affects the emission lifetime of a weakly coupled emitter: in resonance, we assist to an increase of the emission rate, known as the Purcell effect, that would allow faster data transmission. Photonic crystal microcavities conveniently address this objective as they provide modes with the required small volumes and high quality factors. They also allow the engineering of the farfield pattern of the cavity modes, and thus of the collection efficiency. In the following pages, after brie y reviewing single photon emitters, the Purcell effect, and photonic crystal cavities, we present our results on the coupling of quantum dots to photonic crystal cavities. We report on the different strategies we used to control the tuning between the cavity mode and the quantum dot emission frequency. We also show our efforts in improving the collection of coupled photons by engineering the shape of the microcavity. Finally, we present our time-resolved measurements demonstrating the Purcell effect under optical and electrical operation.

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https://infoscience.epfl.ch/record/135603?ln=fr

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Laurent Balet
EPFL, 2009
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/135603?ln=fr

À propos de ce résultat

Concepts associés (28)

Concepts associés

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Publications associées (35)

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Growth and characterization of single quantum dot devices for fiber-based quantum communications

Blandine Alloing

Single photon sources have recently attracted significant interest for their potential role in the future applications of quantum cryptography, and in the longer term, quantum computing. Among the different types of single photon emitters, semiconductor quantum dots (QDs) are good candidates as they combine discrete electronic transitions with the possibility of electrical excitation. Self-assembled InAs QDs on GaAs substrate are by far the most studied system as they can easily be grown into a microcavity structure. Embedding these into a microcavity is important as it increases the extraction efficiency, directionality and speeds up the photon emission. Until now, most studies have concentrated on QDs emitting in the λ

EPFL2006

Chargement

Realization and study of InAs/GaAs quantum dot superluminescent diodes emitting at 1.3µm

Marco Rossetti

A quantum dot is a semiconductor nanostructure that confines the motion of conduction band electrons and valence band holes in all three spatial directions, thus creating fully discrete energy levels. The confinement in the InAs/GaAs material system is generated by the bandgap difference for the two materials (∼ 0.4 eV for InAs, and ∼ 1.4 eV for GaAs) which provides a minimum of potential energy for both electrons and holes inside the InAs nanoparticle. The appearance of new nanoscale effects modifies the electro-optical properties of such a system which makes possible the improvement of existing device performance or even fascinating novel device applications. For the creation of coherent structures with very high purity and high density, as it is required for application in light emitting devices, self-assembling is a very important pathway. The quantum dot creation mostly relies on the relaxation of the strain accumulated during the growth of lattice mismatched materials, which happens in a non-equlibrium condition. This results in a statistical distribution of all the parameters characterizing the InAs nanocrystals as size, composition and strain, whose direct manifestation is the inhomogeneously broadened light emission deriving from quantum dot ensembles. At the dawning of the quantum dot development the broadening was considered as a limiting factor for application in semiconductor lasers. Today, it is considered favorably for all the applications where a broad gain spectrum is required as optical amplifiers, monolithic and external-cavity tunable lasers, and superluminescent diodes. The objective of this thesis is the application of InAs/GaAs self-assembled quantum dots to the active region of high power and broad-band superluminescent diodes emitting in the 1.3 μm wavelength region. Superluminescent diodes are edge-emitting semiconductor light sources based on the amplification of spontaneous emission along a waveguide where optical gain is achieved through current injection. They combine the high power and brightness of laser diodes with the low coherence of LEDs. The latter is a fundamental feature for the application in optical coherence tomography medical imaging, where the image resolution is related to the spectral bandwidth of the light source used. The achievement of bandwidths larger than 100 nm are demonstrated in this work through the optimization of molecular beam epitaxy growth conditions. This may be done optimizing the statistical size-dispersion of the dot ensemble, or through the use of different dot layers emitting at slightly different wavelengths. The large bandwidth emission is obtained also due to the peculiar energy structure of this material, for which a multi-state emission appears under particular injection conditions. As a term of comparison, best commercial single-chip superluminescent diodes for the imaging application at 1.3 μm show bandwidths around 60 nm. Moreover, the discrete nature of the energy levels in InAs quantum dots together with the implementation of a GaAs-based technology can be beneficial for the device temperature stability (the competing technology, based on InP, suffers from strong thermal degradation). The analysis and understanding of the device temperature characteristics will be detailed in the last chapter of the manuscript. Even though standard devices show an important temperature dependence, a better stability may be achieved through the use of p-doped active regions. In the last few years quantum dot devices have shown outstanding properties if compared to the competing quantum well technology. Low threshold currents, high temperature stability and low chirp in lasers, large bandwidths and low gain saturation in amplifiers have been demonstrated. However, in spite of the large interest among the scientific community, many of the microscopic processes at the origin of the electro-optical characteristics of this material are not completely understood yet. The physics of the quantum dot active material, will be modeled in this work through the use of systems of rate equations with different complexity depending on the system they are applied to. We will provide a mean-field rate equation model allowing to get insights about carrier dynamics in QD lasers. Also, we will develop two different traveling-wave rate equation models, one for the modeling of the L-I characteristics and the other for the modeling of the spectral characteristics of quantum dot superluminescent diodes. Considering the carrier and photon density distributions across the device cavity is essential in superluminescent diodes, where the high single pass gain results in large non-homogeneities of photon and carrier distributions. The work is motivated by an industrial cooperation with EXALOS AG, a leading company in the development, manufacturing, and sales of broadband superluminescent diodes.

EPFL2007

Chargement

Quantum dots (QDs) are the result of quantum confinement in the three spatial directions, as such they exhibit remarkable properties, of which the most important is perhaps the absence of a continuum of states. The allowed energetic levels are discrete, similarly to the energy dispersion in single atoms. As a result, for an individual QD, single photon emission can be achieved. In particular, GaN QDs display an enhanced thermal robustness in comparison to conventional III-V QDs and remain bright even at room temperature. Furthermore, as opposed to other thermally stable QD systems such as colloidal QDs, GaN QDs are epitaxially grown and hence can be easily integrated into photonic devices. However, III-nitride technology is comparatively less mature than traditional III-V QDs and several challenges remain regarding material quality, particularly in terms of structural defects, point defects, and doping of high Al content AlGaN. With the aim of integrating GaN QDs into photonic nanostructures such as waveguides and cavities based on membrane photonic crystals (PhC), the thesis is oriented towards the development of GaN QDs on thin AlN layers on Si(111). III-nitrides enjoy a high etching selectivity with Si, which allows the fabrication of membrane PhC. The thesis begins by developing high quality AlN on Si(111) and describing the growth of AlN by NH3-molecular beam epitaxy (MBE) at temperatures above 1000 °C. At the limit between layer by layer and step flow growth, very smooth AlN can be achieved even for thin layers on Si(111). These layers, together with the knowledge acquired in the growth of AlN serve as a stepping stone for the growth of GaN QDs. GaN QDs have been produced by NH3-MBE since the first demonstration in 1999, by a modified Stranski-Krastanov (m-SK) growth that relies on a growth interruption. Here we develop the growth of GaN on AlN so that a spontaneous transition into a SK growth occurs. This is analogous to the seminal InAs/GaAs system and as such, the same principles for QD size and density control hold. III-nitrides are often grown heteroepitaxially and the resulting layers have a large threading dislocations density (TDD) (10^9 - 10^10 1/cm²). Dislocations offer nucleation centers due to a strain dipole that limits the control over the size and density of the QDs. Then by employing, AlN single crystal substrates with TDD in the order of 10^3 1/cm² this limitation is overcome. In this manner, we demonstrate size and density control by varying the amount of deposited GaN and the diffusion length. QD densities ranging from 10^8 to 10^11 1/cm² are achieved. Finally, an approach capable of yielding low QD densities even on highly dislocated substrates based on ripening and evaporation is presented. The optical properties of GaN/AlN QDs were investigated. They display high internal quantum efficiencies at 300 K in the order of 40-60 %. The control over the QD size and density is demonstrated by PL measurements. Single QDs are inquired thanks to the possibility of low QD densities. A sample consisting of GaN QDs in AlN on Si(111) was studied in depth. These QDs exhibit multiexcitonic complexes and remain bright even when increasing the temperature to 300 K. At this temperature, single photon emission is revealed by second-order autocorrelation measurements g(2) (¿) with g(2) (0) = 0.17 ± 0.03. Finally, GaN QDs are demonstrated to be bright at room temperature with absolute counts in the order of 10^6 1/s.

EPFL2019

Chargement

EPFL2019

Realization and study of InAs/GaAs quantum dot superluminescent diodes emitting at 1.3µm

Marco Rossetti

EPFL2007