Cette thèse s’est concentrée sur l’étude des lasers à blocage de modes destinés à la fenêtre de télécommunication de 1,55 µm et à base de bâtonnets quantiques dont la croissance a été réalisée sur des substrats d’InP. Un des principaux objectifs de ce travail de thèse a consisté à apporter de nouveaux éléments permettant de mieux appréhender les aspects physiques sous-tendant le mécanisme du ML dans ces dispositifs. En effet, une meilleure compréhension du comportement intrinsèque de ces dispositifs et la façon dont ils sont affectés par des facteurs externes, qui pourraient être présents dans les conditions réelles d’utilisation, est cruciale pour leur intégration dans des technologies futures. Un deuxième objectif majeur de ce travail a consisté à démontrer le potentiel d’exploitation de ces dispositifs dans différents scenarii d’applications / This PhD thesis focuses on the study of mode locked laser diodes based on novel optimized InAs Quantum Dash structures grown on InP substrates. It covers several important modelling aspects, the clean room processing of single and two section shallow ridge waveguide lasers, characterization of the fabricated devices and the evaluation of their performance in different application scenarios. Systematic characterization experiments and subsequent analyses have allowed to gain a much deeper comprehension of the physical mechanisms related to the mode locking regime in these devices, thus far not completely understood. This has allowed to better control most of the main physical phenomena limiting device performance, resulting in first demonstrations of record peak power, sub-picosecond pulse, low radio frequency linewidth and high repetition frequency mode locked lasers grown on InP substrates, opening the way to a vast number of potential applications in the 1.55 µm telecommunication window

Over the last decade, rapid growth of broadband services necessitated research aimed at increasing transmission capacity in fiber-optic communication systems. Wavelength division multiplexing (WDM) technology has been widely used in fiber-optic systems to fully utilize fiber transmission bandwidth. Among optical amplifiers for WDM transmission, semiconductor optical amplifier (SOA) is a promising candidate, thanks to its broad bandwidth, compact size, and low cost. In transmission systems using SOAs, due to their large noise figures, high signal launching powers are required to ensure reasonable optical signal-to-noise ratio of the received signals. Hence the SOAs are operated in the saturation region and the signals will suffer from SOA impairments including self-gain modulation, self-phase modulation, and inter channel crosstalk effects such as cross-gain modulation, cross-phase modulation, and four-wave mixing in WDM. One possibility to circumvent these nonlinear impairments is to use constant-intensity modulation format in the 1310 nm window where dispersion is also negligible. In this dissertation, differential phase-shift keying (DPSK) WDM transmission in the 1310 nm window using SOAs was first considered to increase the capacity of existing telecommunication network. A WDM transmission of 4 x 10 Gbit/s DPSK signals over 540 km standard single mode fiber (SSMF) using cascaded SOAs was demonstrated in a recirculating loop. In order to increase the transmission reach of such WDM systems, those SOA impairments must be compensated. To do so, an accurate model for quantum-dot (QD) SOA must be established. In this dissertation, the QD-SOA was modeled with the assumption of overall charge neutrality. Static gain was calculated. Optical modulation response and nonlinear phase noise were studied semi-analytically based on small-signal analysis. The quantitative studies show that an ultrafast gain recovery time of ~0.1 ps can be achieved when QD-SOAs are under high current injection, which leads to high saturation output power. However more nonlinear phase noise is induced when the QD-SOAs are used in the transmission systems operating at 10 Gbit/s or 40 Gbit/s. Electronic post-compensation for SOA impairments using coherent detection and digital signal processing (DSP) was investigated next in this dissertation. An on-off keying transmission over 100 km SSMF using three SOAs at 1.3 [micrometer] were demonstrated experimentally with direct detection and SOA impairment compensation. The data pattern effect of the signal was compensated effectively. Both optimum launching power and Q-factor were improved by 8 dB. For advanced modulation formats involving phase modulation or in transmission windows with large dispersion, coherent detection must be used and fiber impairments in WDM systems need to be compensated as well. The proposed fiber impairment compensation is based on digital backward propagation. The corresponding DSP implementation was described and the required calculations as well as system latency were derived. Finally joint SOA and fiber impairment compensations were experimentally demonstrated for an amplitude-phase-shift keying transmission.

Advantage of the single QD active layer is its potential for very low threshold current density, which in turn can produce low internal optical loss. The low threshold current density and low internal loss thus enable a significant increase in laser diode cavity length. Because of the importance of the threshold current density in heatsinking, future technology of broad-area monolithic laser diodes can be implemented. The dissertation describes the development and the unique characteristics of single QD active layer laser with long cavity. The data are presented on single layer QD laser diodes that reach threshold current densities values of 11.7 A/cm2 in a p-up mounted 2 cm long cavity and as low as 10 A/cm2, with CW output power of 2 W in a p-down mounted 1.6 cm long cavity. The 8.8 A/cm2 in a p-down mounted 2 cm long cavity is reported. To our knowledge the value 8.8 A/cm2 is the lowest threshold current density ever reported for a room temperature laser diode. These single layer QD laser diodes reach an internal loss of ~0.25 cm-1, which is also the lowest ever reported for a room temperature laser diode. These unique characteristics of single layer QD and laser diode size are potentially promising for the monolithic mode-locked laser because of relatively high peak power with a low repetition rate that is on the order of a few GHz, which can be the novel device for external clocking in the optical interconnect applications. In this dissertation, the stable optical pulse train in a 40 µm wide stripe with a repetition rate of 3.75 GHz with 1.1 cm cavity length through the passive mode-locked onto the monolithic two-section device fabricated from this single layer QD laser is observed.

No description available.

<p>Semiconductor laser diode has been a popular research topic for longer than half a century and plays a crucial role in optical communication systems. The work in this thesis focuses on the development of the semiconductor laser diode with rapid-evolving nanotechnologies: by incorporating specific semiconductor or metal structures in the nanometer scale into the laser cavity, several key advantages are achieved.</p> <p>One category of the nano-materials is semiconductor quantum dots (QD). QD laser is a promising product by providing three-dimensional confinement to the injected electrons and holes. However, in order to realize the single-longitudinal-mode operation, which is critical to optical communications in purpose of reducing the dispersion and partition noise, the Fabry-Perot (FP) QD laser still needs further development to suppress the gain-broadening effects; otherwise the mode-selective structure must be adopted, such as the distributed feedback (DFB) cavity. In this thesis, the QD FP laser and QD DFB laser are both researched by advanced modelling techniques and the work is summarized as follows.</p> <p>1) For the QD FP laser, a comprehensive rate-equation model has been applied for simulation, with the emphasis on describing the interplay of inhomogeneous and homogeneous gain-broadening effects. According to the laser-behaviour simulations, it is found that for each given inhomogeneous broadening, the optimum homogeneous broadening can be obtained for the single longitudinal-mode selectivity. Based on the optimal gain-broadening parameters, the single-mode QD FP laser is designed and analysed. The quantitative conditions for the performance feasibility are examined with respect to the gain-broadening parameters.</p> <p>2) A one-dimensional (1D) standing wave model is developed for the QD DFB laser. This model can provide more information for the laser operation and better describe the dynamic behaviour compared with the rate-equation model. Based on it, the statistic operation and output spectrum of a typical QD DFB laser are simulated; and then the dynamic properties of the laser are analysed.</p> <p>The other category is the metal nano-structure, including the metal nano-particle and the metal nano-strip Bragg grating. The related work is summarized as follows.</p> <p>1) The optical properties of a single metal nano-particle with different size, composition and shape are researched by Mie theory, with respect to the localized surface plasmon polariton (LSPP) effect. It shows that both the resonance wavelength and Q-factor can be tuned in a large scale by proper methods.</p> <p>2) A novel metal nano-strip distributed Bragg grating (DBR) laser is proposed and investigated theoretically. Firstly the metal nano-strip Bragg grating is simulated by the couple-mode theory and the mode-matching method. It shows that the coupling constant and reflection spectrum can be tuned to meet different requirements when varying the grating parameters. Then for the designed metal-grating DBR laser, the rate-equation simulation results show that it works under the single-mode operation for a broad range of the design parameters.<br /> <strong></strong></p> / Doctor of Philosophy (PhD)

Kvantsammanflätning har varit ett populärt ämne bland fysiker i snart 100 år då det tydligt belyser hur annorlunda kvantmekanikens värld är jämfört med den klassiska verklighet vi lever i. Med tiden har kvantsammanflätning blivit mer och mer välförstått och teknologier ämnade att utnyttja det har de senaste årtionden kommit allt närmare till industriell använding. Kvantdatorer är fortfarande i forskningsstadiet men idag excisterar det en kvantdator som kan lösa vissa problem betydligt mycket snabbare än en klassisk dator. På grund av algorithmer som Shors faktoriseringsalgoritm och Grovers sökalgoritm så riskerar dagens krypteringsprotokoll för kommunikation att bli otillräckliga. Som svar på detta har en fysikalisk icke-hackbar krypterings metodik tagits fram i form av QKD. Det baseras på att generara krypteringsnycklar från slumptal och att dessa distribueras tack vare kvantsammanflätning. För att lyckas med detta så krävs generering av sammanflätade kvanttillstånd, kvantbitar, samt singel-fotonsdetektorer. I den här masteruppsatsen har en kvantprick karaktäriserats och används för att generera sammanflätade kvantbitar i QNP-gruppens lab på KTH samt för att skicka enstaka fotoner via Stockholms fibernät till Ericsson i Kista där de detekteras av singel foton detectorer. Multifoton sannolikheten har uppmäts till 0.049 för exciton fotoner samt 0.169 för biexciton fotoner i labbet medan ett värde på 0.176 har uppmäts för exciton fotoner detekterade hos Ericsson, vilket är betydligt lägre än singel emission gränsen 0.5 (dvs foton källan sänder ut singel fotoner). Synkronisering av data är avgörande för att få QKD att fungera varpå en post process-tidssynkroniserings metod baserad på biexciton-exciton kaskad-sönderfall har implementerats i lab. / Quantum entanglement has been a popular topic amongst physicists for almost 100 years as it clearly illuminates the extreme difference between the quantum mechanical world and our classical reality. Over time, the quantum physical property of entanglement became more and more well understood and technologies utilizing entanglement are coming closer to reach industry. Quantum computers are still in the research stage but there already exists a quantum computer capable of solving tailored problems significantly faster than a classical computer. Due to algorithms like Shor’s factorization algorithm and Grover’s search algorithm the current cryptography schemes used to ensure secure communication risk rendering obsolete. A response to this was the invention of the theoretically un­hackable Quantum key Distribution (QKD) scheme, based on generating and distributing random cryptography keys by using quantum entanglement. To achieve this, the generation of entangled photons, or qubits, as well as detection of single photons is required. In this thesis a Quantum Dot (QD) is characterized and used to generate quantum entangled states in the Quantum Nano Photonics (QNP)­group lab at KTH as well as sending single photons via the metropolitan fiber network in Stockholm to Ericsson in Kista, where they are detected using single photon detectors. A multi­photon emission probability of 0.049 was measured for the exciton emission and 0.169 for the biexciton emission in the KTH lab as well as a probability of 0.176 was measured for the exciton photons sent to Kista which is significantly lower than the single emitter limit of 0.5 (i.e. the source is emitting pure single photons). Synchronization of data is of high importance in order to implement a working QKD scheme, therefore a post process temporal synchronization method based on the biexciton­exciton cascaded decay is implemented in the lab.

The quantum confined Stark effect (QCSE) and ultrafast absorption dynamics near the bandedge have been investigated in p-i-n waveguides comprising quantum confined heterostructures grown on GaAs substrates, for emission at 1.3um. The materials are; isolated InAs/InGaAs dot-in-a-well (DWELL) quantum dots (QD), bilayer InAs quantum dots and GaInNAs multiple quantum wells (MQW). The focus was to investigate these dynamics in a planar waveguide geometry, for the purpose of large scale integration in optical systems. Initial measurements of the QCSE using photocurrent measurements showed a small shift for isolated QDs whilst a significant shift of 40nm (at 1340nm) was demonstrated for bilayer dots, comparable to that of GaInNAs MWQ (30nm at 1300nm). These are comparable to InP based quaternary multiple quantum wells used in modulator devices. With the use of a broadband continuum source the isolated quantum dots exhibit both a small QCSE (15nm at 1280nm) and minimal broadening which is desirable for saturable absorbers used in monolithic modelocked semiconductor lasers (MMSL). A robust experimental set-up was developed for characterising waveguide modulators whilst the electroabsorption and electro-refraction was calculated (dn=1.5x10⠻³) using the Kramers-Kronig dispersion relation. Pump probe measurements were performed at room temperature using 250fs pulses from an optical parametric oscillator (OPO) on the three waveguide samples. For the isolated QDs ultrafast absorption recovery was recorded from 62ps (0V) to 700fs (-10V and the shortest times shown to be due to tunneling. Additionally we have shown good agreement of the temperature dependence of these dots and the pulse width durations from a modelocked semiconductor laser using the same material. Bilayer QDs are shown to exhibit ultrafast absorption recovery from 119ps (0V) to 5ps (-10V) offering potential for applications as modelocking elements. The GaInNAs multiple quantum wells show absorption recovery of 55ps (0V), however under applied reverse bias they exhibit long lived field screening transients. These results are explained qualitatively by the spatial separation of electrons and holes at heterobarrier interfaces.

Colloidal semiconductor quantum dots (QDs) have gained substantial interest as adjustable, bright and spectrally tunable fluorophores in the past decades. Besides their in-depth analyses in the scientific community, first industrial applications as color conversion and color enrichment materials were implemented. However, stability and processability are essential for their successful use in these and further applications. Methods to embed QDs into oxides or polymers can only partially solve this challenge. Recently, our group introduced the embedding of QDs into ionic salts, which holds several advantages in comparison to polymer or oxide-based counterparts. Both gas permeability and environmental-related degradation processes are negligible, making these composites an almost perfect choice of material. To evaluate this new class of QD-salt mixed crystals, a thorough understanding of the formation procedure and the final composites is needed. The present work is focused on embedding both aqueous-based and oil-based metal-chalcogenide QDs into several ionic salts and the investigations of their optical and chemical properties upon incorporation into the mixed crystals. QDs with well-known, reproducible and high-quality synthetic protocols are chosen as emissive species. CdTe QDs were incorporated into NaCl as host matrix by using the straightforward "classical" method. The resulting mixed crystals of various shapes and beautiful colors preserve the strong luminescence of the incorporated QDs. Besides NaCl, also borax and other salts are used as host matrices. Mercaptopropionic acid stabilized CdTe QDs can easily be co-crystallized with NaCl, while thioglycolic acid as stabilizing agent results in only weakly emitting powder-like mixed crystals. This challenge was overcome by adjusting the pH, the amount of free stabilizer and the type of salt used, demonstrating the reproducible incorporation of highest-quality CdTe QDs capped with thioglycolic acid into NaCl and KCl salt crystals. A disadvantage of the "classical" mixed crystallization procedure was its long duration which prevents a straightforward transfer of the protocol to less stable QD colloids, e.g., initially oil-based, ligand exchanged QDs. To address this challenge, the "Liquid-liquid-diffusion-assisted-crystallization" (LLDC) method is introduced. By applying the LLDC, a substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process opens the field of incorporating ligand-exchanged Cd-free QDs into NaCl matrices. To overcome the need for a ligand exchange, the LLDC can also be extended towards a two-step approach. In this modified version, the seed-mediated LLDC provides for the first time the ability to incorporate oil-based QDs directly into ionic matrices without a prior phase transfer. The ionic salts appear to be very tight matrices, ensuring the protection of the QDs from the environment. As one of the main results, these matrices provide extraordinary high photo- and chemical stability. It is further demonstrated with absolute measurements of photoluminescence quantum yields (PL-QYs), that the PL-QYs of aqueous CdTe QDs can be considerably increased upon incorporation into a salt matrix by applying the "classical" crystallization procedure. The achievable PL enhancement factors depend strongly on the PL-QYs of the parent QDs and can be described by the change of the dielectric surrounding as well as the passivation of the QD surface. Studies on CdSe/ZnS in NaCl and CdTe in borax showed a crystal-induced PL-QY increase below the values expected for the respective change of the refractive index, supporting the derived hypothesis of surface defect curing by a CdClx formation as one main factor for PL-QY enhancement. The mixed crystals developed in this work show a high suitability as color conversion materials regarding both their stability and spectral tunability. First proof-of-concept devices provide promising results. However, a combination of the highest figures of merit at the same time is intended. This ambitious goal is reached by implementing a model-experimental feedback approach which ensures the desired high optical performance of the used emitters throughout all intermediate steps. Based on the approach, a white LED combining an incandescent-like warm white with an exceptional high color rendering index and a luminous efficacy of radiation is prepared. It is the first time that a combination of this highly related figures of merit could be reached using QD-based color converters. Furthermore, the idea of embedding QDs into ionic matrices gained considerable interest in the scientific community, resulting in various publications of other research groups based on the results presented here. In summary, the present work provides a profound understanding how this new class of QD-salt mixed crystal composites can be efficiently prepared. Applying the different crystallization methods and by changing the matrix material, mixed crystals emitting from blue to the near infrared region of the electromagnetic spectrum can be fabricated using both Cd-containing and Cd-free QDs. The resulting composites show extraordinary optical properties, combining the QDs spectral tunability with the rigid and tight ionic matrix of the salt. Finally, their utilization as a color conversion material resulted in a high-quality white LED that, for the first time, combines an incandescent-like hue with outstanding optical efficacy and color rendering properties. Besides that, the mixed crystals offer huge potential in other high-quality applications which apply photonic and optoelectronic components.

The design, construction and characterisation of a novel, arrayed, open-access optical microcavity is described. Included in this thesis are the precise fabrication details, making use of the focused ion beam. A technique for analysing and optimising the microcavities constructed, making use of an atomic force microscope is also included. Results from the optical characterisation of the fabricated microcavities are presented, including quality factors of around 104, and fitnesses of around 400. The optical analysis then progressed onto coupling colloidal semiconductor nanocrystals to the microcavity modes. This yielded room temperature Purcell enhancements, single particle sensing, and also allowed for the characterisation of a second iteration of cavities. This improved set was shown to achieve fitnesses in excess of 1800 and quality factors with a lower limit of 15000. The optical identification of single NV centres in nanodiamond is discussed, along with the development of an optical apparatus to couple them to microcavities at cryogenic temperatures. Finally several results from finite difference time domain simulations will be presented, showing ultimate mode volumes of less than 0.5 cubic wavelengths are possible for this approach.

Ce mémoire porte sur les mécanismes de relaxation et de fuite des excitons dans des systèmes de boîtes quantiques(BQs) d’InAs/InP. Les systèmes sont composés d’un sub- strat volumique d’InP, appelé matrice (M), d’un puits quantique d’InAs, nommé couche de mouillage (CM), et des familles de BQs d’InAs. La distinction entre les familles est faite par le nombre de monocouche d’épaisseur des boîtes qui sont beaucoup plus larges que hautes. Une revue de littérature retrace les principaux mécanismes de relaxation et de fuite des excitons dans les systèmes. Ensuite, différents modèles portant sur la fuite thermique des excitons des BQs sont comparés. Les types de caractérisations déjà produites et les spécifications des croissances des échantillons sont présentés. L’approche adoptée pour ce mémoire a été de caractériser temporellement la dynamique des BQs avec des mesures d’absorbtion transitoire et de photoluminescence résolue en temps (PLRT) par addition de fréquences. L’expérience d’absorption transitoire n’a pas fait ressortir de résultats très probants, mais elle est expliquée en détails. Les mesures de PLRT ont permis de suivre en température le temps de vie effectif des excitons dans des familles de BQs. Ensuite, avec un modèle de bilan détaillé, qui a été bien explicité, il a été possible d’identifier le rôle de la M et de la CM dans la relaxation et la fuite des excitons dans les BQs. Les ajustements montrent plus précisément que la fuite de porteurs dans les BQs se fait sous la forme de paires d’électrons-trous corrélées. / This thesis focuses on the mechanisms of relaxation and leakage of excitons in systems of quantum dots (QDs) InAs / InP. The systems are composed of a substrate of InP volume, called matrix (M), of a quantum well of InAs, named wetting layer (CM), and of QD families of InAs. The distinction between the families can be explained by the number of monolayer-thick boxes that are wider than high. A literature review highlights the main relaxation mechanisms and leakage of excitons in systems. Then, different models on the thermal leakage of the QD excitons are compared.Then, a presentation of the different types of characterizations already and of the specifications on the samples growths. The approach used for this thesis is to temporarily characterize the dynamic of the QDs with transient absorption and upconversion. The transient absorption experiment’s results are not very convincing, but are minutely explained. PLRT measures were used to follow in temperature the excitons effective lifetime in the QDs families. Then, with a detailed balance model, which has been well explained, it was possible to identify the role of theMand CM in relaxation and leakage of excitons in QDs. As shown by the adjustement, the escape of carriers in the QDs is made in a correlated electron-hole pairs form.