Дисертації з теми "Semiconductor light sources"

The research described in this thesis relates to the design, fabrication and testing of novel semiconductor-based light sources that have been designed for the generation of infra-red light. The thesis is formatted to account for two distinct components of my work, where the first part concerns sources producing coherent light by direct laser emission, notably, ultrashort-pulse quantum-dot lasers. These types of lasers continue to show considerable promise as efficient, compact sources of ultrashort pulses with durations of hundreds of femtoseconds, while giving rise to unique and interesting electronic properties such as low lasing thresholds through the quantum nature of their density of states. At the outset a study of the most relevant aspects of the lasing dynamics of an optically pumped quantum-dot laser is outlined. Pumping of the device with intense discrete optical pulses leads to output from multiple electronic states, each having a characteristic wavelength and temporal properties. I show that pulses produced by excited-state emission have shorter durations (24 ps) and arrive earlier in time than those due to transitions from the ground state, which themselves have durations of around 180 ps. Investigations are then made on two different mode-locked quantum-dot laser systems. One is an all-quantum-dot external-cavity laser that is mode locked using a quantum-dot SESAM device at a repetition frequency of 860 MHz with output power approaching 20 mW. This is followed by a study of a monolithic two-section quantum-dot laser that is mode locked stably in a wide temperature range of 20°C to 70°C. The excellent performance characteristics presented serve to demonstrate both the versatility of quantum-dot material as components in mode-locked laser systems and the temperature stability of such laser devices. The second part of the thesis relates to structures that are designed to take advantage of nonlinear frequency conversion in GaAs-based semiconductors. This material system possesses a nonlinear coefficient of ~170 pm/V and is transparent from around 0.9 μm through to 17 μm, making it attractive for the realisation of a new class of efficient, integrable, quasi-phase-matched, optical parametric oscillator devices. Initially, ion implantation is utilised as a vector to create a periodically-switched nonlinear ridge waveguided device. The observation is made that in the course of implantation the transmissive properties of the device are severely degraded. Unfortunately, the high losses incurred, which reached 250 dB/cm, could not be removed without also destroying the modulation in nonlinearity. During the course of this investigation, significant technological advances were made in the production of orientation-patterned GaAs structures. By recognising the elegance and potential of this new orientation-patterned (OP) methodology, a study of its implications and applicability in the context of my project is initiated.

Semiconductor quantum dots can be used as sources of entangled single photons, which constitute a crucial resource for optical quantum computing. We present theoretical research on entanglement verification and nuclear spin physics, leading to results that are relevant to both experimental work and the theory of quantum optics and mesoscopic quantum systems. Optical quantum computing requires large entangled photonic states, yet characterizing even few-photon states is a challenge in current experiments due to low photon detection efficiencies. We present a lower bound on a measure of computational usefulness of a potentially large quantum state that requires only measured values of three-photon correlations. Hence this bound provides a simple and applicable benchmarking method for quantum dot experiments. We then turn to the critical issue of the interaction between electron and nuclear spins in quantum dots. This interaction gives rise to decoherence that stands in the way of generating entangled photons as well as nuclear phenomena that might help to overcome this challenge. We formulate a quantum mechanical model of the nuclear spin system in a quantum dot driven by continuous-wave laser light. Based on the analytical steady state solution of this model we predict a novel nuclear spin effect, giving rise to nuclear spin polarization that counteracts the effect of an external magnetic field. Beyond the decoherence problem nuclear spins give rise to randomly time-varying transition energies. A quantum mechanical model of this noise as well as the effect of photon scattering is developed, leading to the insight that optical driving can continuously probe the electron transition energy and thereby prevent it from changing.

The isolation of qubits from decoherence is crucial to the prospect of building revolutionary quantum devices. This work is devoted to an optical study of the decoherence on spin qubits in self-assembled quantum dots. This thesis contributes towards a complete understanding of quantum decoherence, of which highlighted discoveries include bypassing the spectral diffusion in neutral quantum dot emission lines; observing for the first time the self-polarization phenomenon of nuclear spins, via the resonance-locking effect on a negatively charged quantum dot; and revealing the limiting factors on hole spin dephasing, by measuring polarization correlations on a positively charged quantum dot. Three studies were conducted using two different spectroscopy techniques. For the first study, the spectral diffusion of emission line due to random electrostatic fluctuations was revealed, by scanning a neutral quantum dot transition across the laser resonance. Exciting the quantum dot resonantly bypassed this problem, paving the way for an on-demand antibunched source that generates narrow-band photons. For the second study, evidences supporting the spontaneous self-polarization of nuclear spins were observed for the first time, since it was predicted nearly four decades ago by M. Dyankonov and V.I. Perel. The self-polarization phenomenon is a remarkable demonstration of dynamic nuclear spin polarization since it manifests without the ground state electron being spin-polarized. In the last study, factors limiting the hole spin lifetime was inferred from measuring polarization correlation of successively emitted photons from a positively charged quantum dot. Evidences support a strong dependence on the carrier repopulation rate and the single electron spin dephasing in the upper state, due to the Overhauser field. In combination with the observation of spontaneous nuclear polarization, this opens the possibility of an electron spin sensor, which can indirectly probe the nuclear field.

The development of compact and low-cost coherent sources in visible and infrared wavelength range can provide indispensible tools for a variety of scientific, technological and industrial applications. Great progress over the last years in material science, crystal growth and semiconductor material processing in combination with recent advances in some of the more traditional technologies, in particular nonlinear frequency conversion and parametric sources, have led to the realisation of a new generation of laser sources. Furthermore, the advent of a new generation of quasi-phase-matched, waveguided and semiconductor nonlinear materials together with novel semiconductor lasers have led to the development of new frequency conversion and parametric sources with previously unattainable performance capabilities. The research described in this thesis relates to the development and characterisation of novel semiconductor based laser sources tunable in the broad spectral ranges which are unattainable for conventional lasers due to a lack of suitable laser gain materials. In the first part of the thesis the subject matter is concerned with the direct emission from laser devices. In particular, a broadly tunable InGaAs/InP strained multi-quantum well external cavity diode laser, operating in the spectral range of 1494 nm – 1667 nm with a maximum CW output power in excess of 81 mW and side-mode suppression ratio higher than 50 dB is demonstrated. This represents the highest output power and side-mode suppression ratio ever to be generated in this spectral region. A record broadly tunable high-power external cavity InAs/GaAs quantum-dot diode laser with a tuning range of 202 nm (1122 nm - 1324 nm), a maximum output power of 480 mW and a side-mode suppression ratio greater than 45 dB, is also demonstrated. This represents a promising achievement for the development of a high-power fast swept tunable laser and compact nonlinear frequency generation schemes for the green-yellow-orange-red spectral range. The second part of the thesis relates to induced nonlinear processes, focusing on frequency doubling and optical parametric oscillation. In particular, an all-room-temperature CW second harmonic generation at 612.9 nm and 591.5 nm in periodically poled potassium titanyl phosphate waveguides pumped by a broadly-tunable quantum-dot external cavity diode laser with a conversion efficiency of 10.5% and 7.9%, respectively, is demonstrated. For the first time, a green-to-red tunable laser source with tunability of over 60 nm (567.7 nm – 629.1 nm) based on frequency doubling in a single periodically poled potassium titanyl phosphate waveguide pumped by a single broadly-tunable quantum dot laser is demonstrated. These results are an important step towards a compact tunable coherent visible light source, operating at room temperature. The possibility of nonlinear frequency conversion in orientation-patterned GaAs waveguides is also investigated. The technology of low-loss periodically poled GaAs waveguided crystals is developed and such crystals are fabricated. Second harmonic generation at 1621 nm in low-loss periodically poled GaAs waveguide is demonstrated. An optical parametric oscillator system used as the pump source for GaAs devices and based on the periodically poled 5 mol% MgO-doped Congruent Lithium Niobate crystal, generating light in the wavelength range between 1430 nm and 4157 nm, is presented. The obtained results show a great promise for realisation of efficient quasi-phase-matched optical parametric oscillator devices based on orientation-patterned GaAs waveguides, which enables the extending generated wavelength up to 16 µm.

This thesis describes a number of studies on organic semiconductors as laser gain media with the aim of simplifying the excitation scheme and exploring potential applications. A hybrid device taking the advantage of high power inorganic light emitting diodes (LEDs) and low threshold organic distributed feedback lasers is demonstrated to realize a LED pumped organic laser. When the drive current is higher than 152 A, a sharp peak is clearly observed in the laser output spectrum, implying the LED successfully pumps the polymer laser above threshold. This is the first time an incoherent LED has been used as the excitation source for an organic semiconductor laser. A strategy for further improving the performance of the hybrid device is explored with the use of a luminescent concentrator made of a dye doped SU8 film, to intensify the power density from the inorganic LED. The luminescent concentrator is capable of increasing the incident power density by a factor of 9 and reducing the lasing threshold density by 4.5 times. As a preliminary investigation towards mode-locked polymer lasers, the impact of a solid state saturable absorber on a solution based organic semiconductor laser is explored. The dye doped polystyrene thin film saturable absorber exhibits a saturation intensity of a few MW/cm². When it is placed into the laser cavity, a train of short pulses is generated and the underlying mechanism is discussed. Finally, the potential of using organic semiconductor lasers in the detection of nitro-aromatic explosive vapours is studied in distributed feedback polyfluorene lasers. A high sensing efficiency and fast response from the laser prove polyfluorene lasers can be used as disposal and low cost devices in explosive chemosensing.

In this work, resonant laser spectroscopy has been utilized in two major projects --resonance fluorescence measurements in solid-state quantum-confined nanostructures and laser-induced fluorescence measurements in gases. The first project focuses on studying resonant light-matter interactions in semiconductor quantum dots "artificial atoms" with potential applications in quantum information science. Of primary interest is the understanding of fundamental processes and how they are affected by the solid-state matrix. Unlike atoms, quantum dots are susceptible to a variety of environmental influences such as phonon scattering and spectral diffusion. These interactions alter the desired properties of the scattered light and hinder uses in certain single photon source applications. One application of current interest is the use of quantum dots in “quantum repeaters” for which two-photon interference is key. Motivated by such an application we have explored the limits imposed by environmental effects on two quantum dots in the same sample, the scattered light from which is being interfered. We find that both one-photon and two-photon interference, although substantial, are affected in a variety of ways, in particular by spectral diffusion. These observations are discussed and compared with a theoretical model. We further investigated correlations in pulsed resonance fluorescence, and found significant unexpected spectral and temporal deviations from those studied under continuous wave excitation. Under these conditions, the scattered light exhibits Rabi oscillations and photon anti-bunching, while maintaining a rich spectrum containing many spectral features. These observations are discussed and compared with a theoretical model. In the second project, the focus is on the investigation of the possibility of detecting N2+ ions in air using laser induced fluorescence, with potential applications in detection of fissile materials at a distance. A photon-counting analysis reveals that the fluorescence decay rate rapidly increases with increasing N2 pressure and thus limits the detection at elevated pressures, in particular at atmospheric pressure. We show that time-gated detection can be used to isolate N2+ fluorescence from delayed N2 emission. Based on the spontaneous Raman signal from N2 simultaneously observed with N2+ fluorescence, we could estimate a limit of detection in air of order 108-1010 cm3.

With the double pass (DP) technique it is possible to quantify the optical quality of a patient's eye by measuring its point spread function. Due to the low reflectivity of the retina a high-intensity, point-like illumination source is required. Usually laser diodes (LDs) are used, but the coherent laser light produces speckle, an interference phenomenon that deteriorates image quality and can make DP images difficult or impossible to interpret. A low-cost solution to reduce speckle in DP imaging is to include a vibrating mirror in the optical path of the system and average over the different speckle realizations. However, vibrating mechanical parts are undesired because they limit longevity and reliability of medical equipment. The goal of this thesis is to find an inexpensive non-mechanical solution for speckle reduction in DP imaging based on low-coherence semiconductor light sources. We compare an LD, a light-emitting diode (LED) and a superluminescent diode (SLED) in terms of speckle formation, cost and usability in DP systems. We find that the SLED is a good alternative to LD illumination, as the amount of speckle in the image is almost as low as that obtained with an LD and a vibrating mirror in the beam path. However, the SLED is not a low-cost solution. In order to identify a cost-efficient all-optical solution, we analyze the speckle generated by an LD as a function of its pump current. Our experiments suggest that driving the LD below the lasing threshold can be an inexpensive solution for speckle reduction. While undesired in many imaging applications, speckle can also contain useful information that is exploited, e.g., in blood flow analysis or for reconstruction of the object that generates the speckle pattern. We find that adjusting the pump current of an LD and the exposure time of the image acquisition system can be a simple and effective way to increase or reduce the amount of speckle by tailoring the coherence of light used for imaging. In particular, we identify conditions that allow to record images with similar average intensity, but with speckle contrast values as low as 0.16, or as high as 0.99.
Estudio experimental del speckle generado por fuentes de luz semiconductoras: aplicación en imágenes de doble paso Con la técnica de doble paso (DP) es posible cuantificar la calidad óptica del ojo de un paciente midiendo la función de dispersión de punto. Debido a la baja reflectividad de la retina, se requiere una fuente de luz puntual de alta intensidad. Una fuente de luz ampliamente utilizada en sistemas de DP son los diodos láser (LD). Los diodos láser son fuentes de luz coherentes que producen patrones de interferencia cuando se ilumina una superficie rugosa. Este fenómeno de interferencia, denominado speckle, deteriora la calidad de la imagen y puede hacer que las imágenes de DP sean difíciles o imposibles de interpretar. Una solución eficaz y de bajo coste para reducir el speckle en las imágenes de DP es incluir un espejo vibratorio en el camino óptico del sistema para promediar sobre las diferentes realizaciones de patrones de speckle. Sin embargo, las partes mecánicas vibratorias no son una solución óptima puesto que limitan la longevidad y fiabilidad de equipos médicos. El objetivo de esta tesis es encontrar una solución no mecánica de bajo coste para la reducción de speckle en imágenes de DP basada en fuentes de luz semiconductoras de baja coherencia. Comparamos un LD, un diodo emisor de luz (LED) y un diodo superluminiscente (SLED) en términos de formación de speckle, coste y usabilidad en sistemas de DP. Encontramos que el SLED es una buena alternativa a la iluminación con LD, ya que la cantidad de speckle en la imagen es prácticamente la misma que la obtenida con un LD y un espejo vibratorio. Sin embargo, el SLED no es una solución de bajo coste. Con el fin de identificar una solución totalmente óptica y económicamente rentable, analizamos los patrones de speckle generados por un LD en función de su corriente de bombeo. Nuestros experimentos sugieren que trabajar con corrientes por debajo del umbral del láser puede ser una solución económicamente viable para la reducción de speckle. En muchas aplicaciones de imaging se intenta evitar el speckle. De todas formas, el patrón de speckle puede contener información útil que se utiliza, por ejemplo, en el análisis del flujo sanguíneo o para la reconstrucción de objetos. Hemos observado que ajustar la corriente de bombeo de un LD (es decir, adaptar la coherencia de la luz utilizada para la toma de imágenes) puede ser una manera simple y efectiva de aumentar o reducir la cantidad de speckle. Para obtener imágenes con la misma intensidad hay que adaptar el tiempo de exposición del sistema de adquisición de imágenes. En particular, identificamos condiciones que permiten adquirir imágenes con una intensidad mediana similar, pero con valores de contraste de speckle que varían entre 0,16 y 0,99.
Estudi experimental de l’speckle generat per fonts de llum semiconductores: aplicació en imatges de doble pas Amb la tècnica de doble pas (DP) és possible quantificar la qualitat òptica de l’ull d’un pacient mesurant la funció de dispersió de punt. A causa de la baixa reflectivitat de la retina, es requereix una font de llum puntual d’alta intensitat. Una font de llum ampliament utilitzada en sistemes de DP són els díodes làser (LD). Els díodes làser són fonts de llum coherents que produeixen patrons d’interferència quan s’il·lumina una superficie rugosa. Aquest fenomen d’interferència, anomenat speckle, deteriora la qualitat de la imatge i pot fer que les imatges de DP siguin difícils o impossibles d’interpretar. Una solució eficaç i de baix cost per a reduir l’speckle en les imatges de DP és incloure un mirall vibratori en el camí òptic del sistema per tal de fer la mitjana sobre les diferents realitzacions de patrons d’speckle. No obstant, les parts mecàniques vibratòries no són una solució òptima ja que limiten la longevitat i fiabilitat d’equips mèdics. L’objectiu d’aquesta tesi és trobar una solució no mecànica de baix cost per a la reducció d’speckle en imatges de DP basada en fonts de llum semiconductores de baixa coherència. Comparem un LD, un díode emissor de llum (LED) i un díode superluminiscent (SLED) en termes de formació d’speckle, cost i usabilitat en sistemes de DP. Trobem que l’SLED és una bona alternativa a la il·luminació amb LD, ja que la reducció de la quantitat d’speckle en la imatge és pràcticament la mateixa que l’obtinguda amb un LD i un mirall vibratori. Tanmateix, l’SLED no és una solució de baix cost. Amb la finalitat d’identificar una solució totalment òptica i econòmicament rendible, analitzem els patrons d’speckle generats per un LD en funció del seu corrent de bombeig. Els nostres experiments suggereixen que treballar amb corrents per sota del llindar d’encesa del làser pot ser una solució econòmicament viable per a la reducció d’speckle. En moltes aplicacions d’imaging s’intenta evitar l’speckle. Tot i així, el patró d’speckle pot contenir informació útil que s’utilitza, per exemple, en l’anàlisi del flux sanguini o per a la reconstrucció d’objectes. Hem observat que ajustar el corrent de bombeig d’un LD (és a dir, adaptar la coherència de la llum utilitzada per a la presa d’imatges) pot ser una manera simple i efectiva d’augmentar o reduir la quantitat d’speckle. Per tal d’obtenir imatges amb la mateixa intensitat cal adaptar el temps d’exposició del sistema d’adquisició d’imatges. En particular, identifiquem condicions que permeten adquirir imatges amb una intensitat mitjana similar, però amb valors de contrast d’speckle que varien entre 0,16 i 0,99.

Les plasmons Tamm, ou modes Tamm optiques, sont des modes électromagnétiques présents à l'interface entre un miroir de Bragg (DBR) et une couche métallique. Ces modes présentent un fort intérêt pour la réalisation de nouvelles sources de lumière, notamment grâce à la partie métallique, qui peut d'une part fournir un contrôle et un confinement micrométrique à trois dimensions du mode optique, et d'autre part assurer l'injection d'un courant électrique dans la structure pour y exciter un milieu émetteur. De nombreuses sources de lumière pourraient être réalisées grâce à cette double fonction du métal, comme des sources polarisées intégrées, des générateurs de plasmons de surface ou encore des tableaux de laser adressables à grande échelle. Mon travail de doctorat a consisté à pousser les sources de lumière Tamm vers l'applicatif, en développant leur fonctionnement à température ambiante et en excitation électrique, par opposition aux démonstrations à température cryogénique et en pompage optique effectuées jusqu'alors. Ce développement a été effectué sur des structures semi-conductrices basées sur des alliages ternaires d'AlGaAs, mais est hautement transposable à d'autres familles de matériaux. La première partie de ce travail s'est concentrée sur l'obtention d'un effet laser à température ambiante. Grâce à une amélioration de la structure, consistant à insérer une couche de bas indice entre le DBR semi-conducteur et le métal, les pertes ohmiques dans ce dernier ont été réduites, ce qui a permis d'atteindre le régime laser à température ambiante. Le second volet de cette thèse concerne l'injection électrique des sources de lumière à mode Tamm. Partant d'un DBR dopé, deux procédés de micro-structuration en salle blanche ont été élaborés pour permettre cette injection. Le premier, inspiré de techniques usuelles de micro-fabrication, n'a pas fait ses preuves, en raison de la dégradation de la surface du DBR par certaines étapes classiques de structuration, et de la forte sensibilité du plasmon Tamm à la composition de surface du DBR. Nous avons donc développé une méthode de structuration alternative. Son originalité repose dans la protection permanente de la surface du DBR destinée au contact avec le métal. Cette nouvelle méthode a permis la fabrication des premières diodes électroluminescentes basées sur l'émission dans un mode Tamm. Leur caractérisation a montré la réussite de l'excitation du plasmon Tamm par l'injection électrique des émetteurs à puits quantiques, et prouve la possibilité d'utiliser un unique élément métallique pour confiner le mode optique et injecter les porteurs de charge. Ces résultats constituent une étape importante vers le développement d'une variété de sources de lumière intégrées utilisant les modes Tamm
Tamm plasmons, or optical Tamm states, are electromagnetic modes that exist at the interface between a Distributed Bragg Reflector (DBR) and a metallic layer. They are of high interest for the design of new light sources, thanks to the metallic part, which can provide 3D confinement and control of the optical mode but also electrical injection of the structure, in order to excite light emitters. Many light emitting devices could be realised using this dual function, such as integrated polarised light sources, surface plasmon generators or large-scale addressable laser arrays. This PhD work mainly consisted in pushing Tamm light emitting devices towards applicability, with the development of their room-temperature operation and electrical pumping, as opposed to previous demonstrations which were carried out under cryogenic temperature and optical pumping. Semiconducting heterostructures based on ternary alloys of AlGaAs were used for this development, but our results are highly transposable to other families of materials. The first part of this work focused on obtaining a laser effect at room temperature. By improving the structure with the insertion of a low-index layer between the semiconductor DBR and the metal, the ohmic losses in the metal were reduced, thus enabling lasing operation at room temperature. The second part of this work was about achieving the electrical injection of Tamm-based light sources. Starting from a doped DBR with quantum wells, we developed two processes, mostly based on cleanroom microfabrication techniques, to enable electrical injection. The first one, inspired by common microfabrication techniques, has not proved to be successful, due to the degradation of the DBR surface by some standard fabrication steps, and to the strong sensitivity of the Tamm plasmon to the surface composition of the DBR. Therefore, we developed a second method. Its originality lies in a permanent protection of the part of the DBR on which the metallic element will be deposited to form the Tamm mode and inject electrical current. This new method allowed the fabrication of the first light-emitting diodes based on Tamm mode emission. With electro-optical measurements, we demonstrated the excitation of the Tamm plasmon state through electrical pumping of the quantum wells, and proved the possibility to use a single metallic element to confine the optical mode and bring charge carriers into the structure. These results are an important step towards the development of new integrated light emitting devices using Tamm modes

GaAs₁₋xBix is an exciting new semiconductor material, which has been pro posed as a new material for infrared light emitting devices. Recent ad vancements in the growth of GaAs₁₋xBix films have made it possible to produce GaAs₁₋xBix light emitting diodes for the first time. Throughout this research we have grown, fabricated and characterized GaAs₁₋xBix light emitting diodes. Similarly structured InxGa₁₋xAs light emitting diodes were also produced and characterized for comparison to the GaAs₁₋xBix devices. Strong electroluminescence was obtained from GaAs₁₋xBix devices, showing two emission peaks, one corresponding to the GaAs₁₋xBix layer and the other to the GaAs cladding. Emission from InxGa₁₋x_As devices was about 100 times brighter than from GaAs₁₋xBix devices. Temperature dependent electroluminescence and photoluminescence measurements of a GaAs₁₋xBix light emitting diode were made and showed some unusual results. The wavelength of the peak in the electroluminescence from the GaAs₁₋xBix was independent of temperature in the range 100 K to 300 K while the GaAs peak shifted with temperature as expected. Photoluminescence measurements on the same structure show temperature dependence of the peak wavelength similar to the temperature dependence of GaAs.

We have characterized electroluminescent devices based on silicon rich oxide and/or silicon rich nitride. We have discussed the photoluminescence and structural characterization of the active layers and the electrical and electroluminescent characterization of full devices. We have noted that the electroluminescence can appear in the form of discrete points scattered across the active area of the devices, in the form of emission along the rim of the active area, or homogeneously distributed across the area. These different kinds of emission have been related to the optical and electrical properties of the devices. In the two former cases, the electroluminescence comes with high current densities,of the order of 1 A/cm2, and low efficiencies of the order of 10-8. On the other hand, the homogeneous emission comes with lower current densities, of the order of 0.01 A/cm2, and better efficiencies, in the range 10-7–10-5. We have concluded that the homogeneous emission is optimal in terms of efficiency. Furthermore, a simple model has been proposed to explain the appearance and occasional coexistence of the different kinds of emission. The effect of a nitride layer on top of the SRO has been explored, concluding that it helps in achieving a uniform conduction that favors the homogeneous emission in the active layer. The conductivity states of the active layer associated with the different kinds of emission have been related with its CV behavior. The results of the study show that the homogeneous emission corresponds to well behaved CV curves, whereas the emission through points does not. The injection mechanisms in PECVD and ion implanted samples have been studied, concluding that no single emission mechanism can account for the injection at all regimes in the studied range of electric fields. Fowler-Nordheim or trap assisted tunneling have been found to play a significant role in PECVD samples. In implanted samples, Fowler-Nordheim dominates at low fields, whereas Poole-Frenkel is more likely to be the dominant mechanism at higher fields. Comparison of the photoluminescence and electroluminescence spectra of bilayers SRO/SRN, allows us to conclude that each layer contributes a different band in the total emission, which results in a wider distribution of the energy across the visible spectrum. The comparison between the photoluminescence and electroluminescence has revealed massive differences in their spectra, which have been attributed to interference effects. A computer software based in the Crawford method for the study of the interference effects in multilayer stacks has been presented. The program has been used to quantitatively study the interference effects in the emission of our devices. We can conclude that the photoluminescence and electroluminescence spectra are the same despite their apparent difference. Our analysis has also made it apparent that a quantitative understanding of the interference effects in the system is important in order to draw valid conclusions regarding the origins of the luminescence. We have presented the design, fabrication and characterization of a CMOS compatible optical transceiver, and two main challenges in the integration of the emitter, waveguide and detector have been successfully overcome, namely achieving a reasonably flat and uniform silicon oxide trench and a good detector. In the end, the transceiver has not worked as expected, most likely due to a poor SRN emitter. More work is required in order to better control the fabrication process of the SRN layers. However, we believe the basic design to be valid, given the low electrical coupling detected between the emitter and the detector components of the transceiver.
Aquesta tesi presenta un estudi de les propietats òptiques de capes d'òxid de silici enriquit en silici (SRO) i nitrur de silici enriquit en silici (SRN) que han sofert un procés tèrmic d'alta temperatura. Aquest procés indueix la creació de nanoaglomerats de silici en la matriu dielèctrica. Aquestes nanoestructures de silici presenten una superior eficiència en l'emissió respecte al silici en bloc, i a més a més emeten en el visible en comptes de l'infraroig. Això és interessant per a l'obtenció de dispositius fotònics integrats basats en silici que poden ser fabricats monolíticament en un procés compatible amb la tecnologia CMOS que domina la indústria microelectrònica. A més a més, hem estudiat les propietats òptiques i elèctriques de dispositius metall-aïllant-semiconductor en les quals l'aïllant és una capa d'SRO o SRN amb nanoaglomerats de silici. N'hem mesurat paràmetres d'interès com ara l'eficiència de conversió d'energia elèctrica-òptica o la potència òptica, i n'hem estudiat els mecanismes d'injecció que hi tenen lloc. S'han identificat tres tipus diferents d'emissió: per punts, per la vora del dispositiu, i emissió homogènia, i hem determinat que l'emissió homogènia és la més adecuada pel que fa a l'eficiència dels dispositius. Hem desenvolupat un programa que permet el càlcul de les interferències òptiques que tenen lloc als sistemes multicapa que conformen els dispositius estudiats, i que distorsionen l'espectre observat respecte al que les capes realment emeten. L'habilitat de poder calcular aquests efectes ens permet, en molts casos, eliminar l'efecte de les interferències i determinar l'autèntic espectre d'emissió de les capes i per tant estar en millors condicions d'assignar l'emissió als mecanismes correctes. Finalment, hem proposat un prototip per a un transceptor en el qual l'emissor, la guia d'ones i el detector estan integrats monolíticament en un procés CMOS. Hem fabricat el dispositiu i l'hem caracteritzat. Tot i que no hem aconseguit acoblament òptic entre l'emissor i el detector, creiem que el disseny bàsic queda validat, ja que els principals obstacles en l'obtenció del dispositiu han sigut superats amb èxit.

In this thesis we examine the feasibility of developing a white light source capable of producing colors between 2500 and 7500 Kelvin on the black-body radiator spectrum by simply adjusting amperage to a blue and ultraviolet (UV) light emitting diode (LED). The purpose of a lighting source of this nature is to better replicate daylight inside a building at a given time of day. This study analyzes the proposed light source using a 385 nm UV LED, a 457 nm blue LED, a 479 nm blue LED, a 562 nm peak cerium doped yttrium aluminum garnet (YAG:Ce) phosphor, and a 647 nm peak selenium doped zinc sulfide (ZnS:Se) phosphor. Our approach to this study initially examined optical performance of yellow-emitting phosphor (YAG:Ce) positioned at specific distances above a blue LED using polydimethylsiloxane (PDMS) as a substrate. An understanding of how phosphor concentration within the PDMS, the thickness of the PDMS, and how substrate distance from the LED die affected light intensity and color values (determined quantitatively by utilizing the 1931 CIE 2° Standard Observer) enabled equations to be developed for various lens designs to efficiently produce white light using a 457 nm peak wavelength LED. The combination of two luminescent sources (457 nm LED and YAG:Ce) provided a linear trend on the 1931 CIE diagram which required a red illumination source to obtain Kelvin values from 2500 to 7500. Red-emitting phosphor (ZnS:Se), selected to compliment the system, was dispersed with YAG:Ce throughout PDMS where they were stimulated with a blue LED thereby enabling all desired Kelvin values with differing concentration lenses. Stimulating ZnS:Se with the addition of a UV LED did not provide the ability to change the color value of the set up to the degree required. Many other factors resulted in the decision to remove the UV LED contribution from the multi-Kelvin light source design. The final design incorporated a combination of ZnS:Se and YAG:Ce stimulated with a blue LED to obtain a 2500 Kelvin value. A separate blue LED provides the means to obtain 7500 Kelvin light and the other color values in between, with a linear approximation, by adjusting the amperages of both LEDs. In addition to investigating the feasibility of obtaining the Kelvin values from 2500 to 7500, this thesis also examined the problem of ZnS:Se’s inability to cure in PDMS and a method to create a lens shape to provide equal color values at all points above a phosphor converted LED source. ZnS:Se was found to be curable in PDMS if first coated with a low viscosity silicon oil prior to dispersion within PDMS. The lens configuration consists of phosphors equally distributed in PDMS and cured in the shape of a Gaussian distribution unique to multiple factors in LED-based white light design.

Зaвдяки poзвитку нoвiтнiх тeхнoлoгiй у гaлузi нaпiвпpoвiдникoвoї cвiтлoтeхнiки впepшe з'явилacь мoжливicть cтвopeння cвiтлoфopiв з викopиcтaнням cвiтлoдioдiв. Лaмпa ocвiтлeння мaє ККД 1,25-2,5 % i тepмiн eкcплуaтaцiї 1000 гoд., тoдi як cвiтлoдioд - 60-70 %, a тepмiн йoгo cлужби пepeвищує 500 000 гoд. Cвiтлoдioди дoзoляють oдepжувaти cвiтлoвi тoчки з нeoбхiдним cпeктpoм випpoмiнювaння, пpи цьoму вcя eлeктpoeнepгiя викopиcтoвуєтьcя тiльки нa oдepжaння випpoмiнювaння зaдaнoгo вузькoгo дiaпaзoну. В лaмпaх ocвiтлeння cпiввiднoшeння iнтeнcивнocтeй випpoмiнювaння чepвoнoї тa cиньoї чacтини cпeктpу є вeличинoю пocтiйнoю, тoдi як у cвiтлo дioднoму випpoмiнювaчi зa кoжну чacтину cпeктpу вiдпoвiдaє cвoя гpупa cвiт лoдioдiв, щo дaє мoжливicть oкpeмo кepувaти iнтeнcивнicтю випpoмiнювaння.
Метою роботи є обґрунтування доцільності застосування сигнальних систем дорожньої сигналізації на основі світлодіодів з врахуванням комплексу наукових та технологічних проблем. Проведений розрахунок економічної ефективності від заміни різних джерел світла у світлофорах при обслуговуванні їх протягом 10 років показав, що заміна ламп освітлення на еквівалентні за світловими параметрами світлодіоди зменшить загальні витрати на обслуговування обладнання в 3,6 рази, а при заміні галогенних ламп на світлодіоди – в 3,9 рази. Економія від впровадження світлодіодів замість ламп освітлення протягом 10 років становить 2171,46 грн., а термін окупності при цій заміні становить 0,32 року. Економія від заміни галогенних ламп на світлодіодні модулі із врахуванням прибутку від депозиту буде 2457,36 грн., а термін окупності при цій заміні становитиме 0,29 року.
The purpose of the work is to substantiate the feasibility of signaling systems of road signaling based on LEDs, taking into account a range of scientific and technological problems. . The calculation of economic efficiency from the replacement of different light sources in traffic lights during their maintenance for 10 years showed that the replacement of light bulbs with equivalent light parameters LEDs will reduce the total cost of maintenance of equipment by 3.6 times, and when replacing halogen lamps with LEDs - in 3.9 times. Savings from the introduction of LEDs instead of light bulbs for 10 years is 2171.46 UAH, and the payback period for this replacement is 0.32 years. Savings from the replacement of halogen lamps with LED modules, taking into account the profit from the deposit will be 2457.36 UAH, and the payback period for this replacement will be 0.29 years. .
ПEPEЛIК УМOВНИХ CКOPOЧEНЬ 6 ВCТУП 7 1 AНAЛIТИЧНИЙ POЗДIЛ 10 1.1 Aнaлiз pocту eфeктивнocтi piзних тeхнoлoгiй ocвiтлeння зa чac cвoгo poзвитку 10 1.2 Oцiнкa eфeктивнocтi зacтocувaння тeплoвих джepeл cвiтлa в cиcтeмaх дopoжньoї cигнaлiзaцiї 14 1.3 Пocтaнoвкa зaдaчi дo пpoeктувaння i oбґpунтувaння вибpaнoгo нaпpямку дocлiджeння 23 2 ПPOEКТНO-КOНCТPУКТOPCЬКИЙ POЗДIЛ 28 2.1 Eфeктивнicть викopиcтaння нaпiвпpoвiдникoвих джepeл cвiтлa в cиcтeмaх дopoжньoї cигнaлiзaцiї 28 2.2 Хapaктepиcтикa cвiтлoфopiв нa cвiтлoдioдaх 36 2.3 Aлгopитм poбoти cвiтлoдioднoгo cвiтлoфopa 38 2.4 Вибip нaдяcкpaвих cвiтлoдioдiв для cвiтлoфopнoї cвiтлoвoї aпepтуpи 40 2.5 Eлeктpичнi cхeми ввiмкнeння cвiтлoдioдiв 46 2.6 Виcнoвки дo poздiлу 2 48 3 POЗPAХУНКOВO-ДOCЛIДНИЦЬКИЙ POЗДIЛ 49 3.1 Poзpaхунoк пoнижуючoгo тpaнcфopмaтopa для cхeми живлeння 49 3.2 Poзpaхунoк eкoнoмiчнoї eфeктивнocтi вiд впpoвaджeння eнepгoeфeктивнoї cиcтeми дopoжньoї cигнaлiзaцiї 56 3.2.1 Poзpaхунoк пoтужнocтi, щo cпoживaєтьcя cвiтлoвoю тoчкoю 57 3.2.2 Poзpaхунoк eкoнoмiчнoгo eфeкту вiд зaмiни тeплoвих джepeл cвiтлa нa cвiтлoдioди 57 3.3 Виcнoвки дo poздiлу 3 61 4 БEЗПEКA ЖИТТЄДIЯЛЬНOCТI ТA OCНOВИ OХOPOНИ ПPAЦI 62 4.1 Зaхoди щoдo зaхиcту cиcтeми дopoжньoї cигнaлiзaцiї, щo пpoeктуєтьcя, вiд кopoткoгo зaмикaння i пepeнaвaнтaжeння 62 4.2 Пpaвилa тeхнiки бeзпeки пpи eкcплуaтaцiї eлeктpoуcтaнoвoк 64 4.3 Вимoги пoжeжнoї бeзпeки дo eлeктpooблaднaння 66 ЗAГAЛЬНI ВИCНOВКИ 68 ПEPEЛIК ПOCИЛAНЬ 70

Cette thèse présente la réalisation d'une source de photons uniques basée sur une boîte quantique InAs intégrée dans un fil photonique. Un fil photonique est un guide d'onde monomode constitué d'un matériau de fort indice de réfraction (GaAs dans notre cas). Pour un diamètre optimal voisin de 200 nm, pratiquement toute l'émission spontanée de l'émetteur (longueur d'onde dans le vide 950 nm) est dirigée vers le mode guidé fondamental. Le couplage des photons guidés à un objectif de microscope est ensuite optimisé en travaillant la géométrie des extrémités du fil. Ce dernier repose ainsi sur un miroir intégré et présente une extrémité supérieure en forme de taper. Cette approche non résonante combine de très bonnes performances à une grande tolérance sur la longueur d'onde de l'émetteur intégré. Cette thèse discute la physique des fils photoniques, la réalisation des structures en salle blanche et les résultats obtenus lors de la caractérisation optique. En particulier, nous avons réalisé une source combinant une efficacité record (0.72, état de l'art à 0.4) et une émission de photons uniques très pure. Nous discutons également le contrôle de la polarisation obtenu dans des fils de section elliptique
This thesis presents the realization of an efficient single-photon source based on an InAs quantum dot integrated in a photonic nanowire. A photonic nanowire is a monomode waveguide made of a high refractive index material (GaAs in our case). For an optimal wire diameter around 200 nm, nearly all the spontaneous emission of the embedded single-photon emitter (free space wavelength 950 nm) is funnelled into the fundamental guided mode. In addition, the outcoupling efficiency of the guided photon to a microscope objective can be brought close to one with a proper engineering of the wire ends. The source thus features an integrated bottom mirror and a smooth tapering of the wire upper end. High performances are maintained over a broad wavelength range, a key asset of this 1D photonic structure. This thesis presents the physics which governs these structures, their realization, and their characterization. Under pulsed optical pumping, we demonstrate a single-photon source with a record efficiency of 0.72, combined with highly pure single-photon emission. We also discuss the possibility to obtain polarization control, using wire with an elliptical section

Метою роботи є аналіз та дослідження теплових процесів при живленні світлодіодів імпульсним струмом та їх вплив на світлотехнічні і електротехнічні характеристики. Об’єктом дослідження є світлодіодні джерела світла. Предметом дослідження є особливості теплових процесів і генерація випромінювання джерел світла різної світності при роботі в імпульсному режимі. В роботі проведено аналіз впливу параметрів імпульсного струму на нагрів світлодіодної структури та зміну спектрів світлодіодних джерел світла. Досліджено динаміку нагріву та охолодження кристалу світлодіоду на основі значень теплового опору та теплоємності елементів конструкцій світлодіоду.
The purpose of the work is the analysis and study of thermal processes under the LED power pulsed current and their impact on lighting and electrical characteristics. The object of research are LED light sources. The subject of the research are features of thermal processes and the generation of radiation light sources of different luminosities at work in the pulsed mode. In this paper is analyzed the influence of parameters of pulsed current on heating of the LED structure and the change range of LED light sources. The dynamics of heating and cooling of the LED crystal based on the values of thermal resistance and heat capacity of the LED structural elements is researched.

The aim of the thesis is to introduce the basic photometric quantities and operating parameters of LED light sources, according to which the light sources are evaluated and compared. The thesis examines effects of high temperature and elevated stress on LED retrofits and a design of measurement methods to measure these influences. According to the proposed methods, nine samples of light sources were measured and compared to each other on a basis of the measurements. Comparison was made from the point of view of the energy as well as the quality of the produced light. The results were also compared to the parameters specified by the producers.

Optical coherence tomography (OCT) is an emerging medical imaging technology based on the coherent interference of light. Current use of OCT in clinical settings is limited by the lack of a suitable light source. This thesis describes the design of a new type of source for OCT, based on the GalnNAs semiconductor materials system. A semiconductor heterostructure consisting of several different quantum wells is discussed as a device for generating broadband (>100 nm) light in the near infrared (900-1500 nm). The use of temperature to control spectral shape and intensity is examined. Other aspects of device design are investigated, including models for quantum well emission and for the band gaps of dilute nitride semiconductors. Photoluminescence measurements are presented, providing a proof of principle demonstration of the source design. Emission centred at 1225 nm with a 195 nm bandwidth is achieved. The use of temperature to control inter-well carrier transfer is demonstrated and successfully modelled. Localization effects of nitrogen cluster states are shown to be greatly reduced in quantum well structures, as compared to bulk samples.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

M. Tech. Electrical Engineering.
Aims to develop directional light emitters, wave guiding of light along CMOS structures and coupling of light into secondary elements in CMOS integrated circuitry that can be modulated at a high speed up to (1Gb/sec PLUS). This could enable high density in connection in integrated circuit packages, helping to implement on-chip optical processing and to generate optical highway communication channels from and to the chip tp produce photonic signal processing using standard CMOS and bipolar-base integrated circuitry.

Organic semiconductors show promise to yield a novel class of bendable electronic devices, and much research efforts have focused on the optimization of these films for device performance. It is well known that the structure of organic films has a large influence over the electronic properties. In particular, the carrier mobility is often highly anisotropic, and domain boundaries have a detrimental effect on charge transport. Therefore the domain structure and lattice orientation are of particular interest. However, little is known about the domain structure of organic films, and techniques to study these properties have only begun to emerge in recent years. In this thesis, we apply two experimental techniques, Grazing-Incidence X-ray Diffraction (GIXD) and Lateral Force Microscopy (LFM), toward studying the lattice and domain structure of tetracene films grown on the silicon(001)-monohydride surface. We describe the necessary steps toward optimizing the sensitivity of these techniques to the domain structure. Results show that the crystalline tetracene films form a layered morphology in which the a-b plane lies parallel to the substrate surface. The film lattice structure is similar to bulk tetracene, and the lattice is confined to two orthogonal orientations, forming a partially-commensurate relationship with the substrate surface lattice along the film 'a' axis. LFM images reveal two types of polycrystalline domains. The first type ("major domains") are tens of microns in size, and are classified by their lattice orientation. They are subdivided into the second type ("sub-domains"), which range from 0.1 to 5um in size, and are argued to represent regions of uniform molecular tilt direction. The GIXD data show that the single-crystal domains which comprise these two larger domain types are anisotropic in size, being up to two times longer along the film 'b' axis than along 'a'. The single-crystal domains range from 0.05 to 0.2um in size, depending on lattice orientation and film thickness. The mathematical basis for these single-crystal domain size calculations is presented. The single-crystal domain sizes are thickness-dependent, and are two orders of magnitude smaller than a typical surface island observed in atomic-force microscopy (AFM) topographs. Substrate steps can also significantly influence the film structure by inducing boundaries in the single-crystal domains and sub-domains, but not in the major domains. This detailed knowledge of the domain structure of organic thin-films may assist in our understanding of the factors which affect charge transport in thin films, and may help to direct research efforts in optimizing the film structure for device performance.
Natural Sciences and Engineering Research Council (NSERC), Canadian Foundation for Innovation (CFI), Ontario Innovation Trust (OIT).