Physics - Doctoral Theses

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    Toward single-growth monolithically integrated electro-absorption modulated lasers
    (University College Cork, 2023) Mulcahy, Jack; Peters, Frank H.; Corbett, Brian; Science Foundation Ireland; Rockley Photonics
    Every year the demand for bandwidth is growing exponentially due to the emergence of data-intensive services such as high-definition video streaming, cloud-based computing, and machine-to-machine communication. This rapid expansion is primarily driven by the extensive deployment of fibre-based optical communication networks. Consequently, there is an increasing need for photonic components to meet the requirements of these networks, which are expanding both in geographical coverage and terminal density. To satisfy this demand, the photonics industry must enhance its production capabilities and adopt more efficient fabrication processes. A crucial aspect of streamlining fabrication involves eliminating slow and costly processes. In photonics fabrication, epitaxial regrowth and advanced lithography steps are typically time-consuming and expensive, making them prime targets for process optimisation. Moreover, the integrated electronics approach provides valuable insights by enabling the monolithic integration of multiple photonic components fabricated simultaneously. This integration technique allows for the creation of highly complex circuits while reducing overall fabrication complexity. This research focuses on a key component at the heart of photonic circuits: the tunable single-mode laser. The aim is to contribute to the development of components that can be fabricated without the need for regrowth or advanced lithography. Additionally, the study emphasises the importance of monolithic integration, specifically with electro-absorption modulators (EAMs). By integrating EAMs with tunable lasers, the resulting devices can offer enhanced functionality and performance, leading to more efficient and compact photonic systems. The issue at hand, however is the varied epitaxial requirements of lasers and EAMs, which provides a noted barrier to a monolithic, regrowth-free integration process. This thesis aims to advance the development of single-growth monolithically integrated externally modulated lasers (EMLs) based on electro-absorption modulators (EAMs). The design of quantum well structures is explored, revealing the significance of introducing an imbalance in the position of the quantum wells to optimise the transit times of carriers in EAMs, thus maximising the bandwidth. Simulation studies on epitaxial structures led to the identification of an optimal material that balances the performance of lasers and EAMs, providing an ideal platform for EML fabrication. Different laser designs are investigated, including slotted Fabry P\'erot lasers and snails, with a focus on achieving a redshifted single-mode laser. Simulation models are developed to predict laser reflectivity and spectral output, which were verified through fabrication and testing. The optimal laser design for integrated EMLs was determined through critical evaluation with a laser being produced with $>$ \SI{40}{dB} SMSR and a tuning range of \SI{60}{nm}. A high-speed process for fabricating EAMs is developed, featuring optimised lithographic mask layers for the isolation of contact pads and metal bridges to reduce parasitic capacitance. The resulting EAMs exhibited a predicted bandwidth of approximately \SI{80}{GHz}. Drawing upon the knowledge gained from laser and EAM simulation, fabrication, and characterisation, a new high-speed process for EMLs is devised. The o-band lasers and EAMs were designed based on optimal principles determined in previous chapters. The fabricated single-mode lasers were successfully matched to simulated models. Further analysis identified potential avenues for improving future EML fabrication yields. In summary, this thesis provides valuable insights and tools for the creation of single-growth monolithically integrated electro-absorption modulated lasers. The journey spans from material design to device outputs, with the aim of enabling readers to replicate and enhance the development of EMLs.
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    Evaluation and properties of site-controlled pyramidal quantum dots for quantum information processing
    (University College Cork, 2023) Ranjbar Jahromi, Iman; Pelucchi, Emanuele; Townsend, Paul
    Semiconductor quantum dots (QDs) are a promising platform for optical quantum information processing (QIP) due to their unique optoelectronic properties, such as their discrete energy levels and strong light-matter interactions. These properties allow for the manipulation and control of the optical and spin states of individual QDs, making them suitable for applications such as quantum cryptography,quantum computing, and quantum simulation.One of the key advantages of QDs for QIP is, aside of being embedded in asemiconductor matrix, their small size, which allows for the confinement of carriers within a few nanometers. This results in a discrete energy spectrum and a strong confinement of the carrier wavefunctions, which leads to a strong light-matter interaction. This strong interaction allows for the manipulation of the optical and spin states of individual QDs, enabling the implementation of quantum gates and other quantum operations.In particular, site-controlled pyramidal QDs have emerged as a promising candidate for optical quantum information processing, due to their unique structuraland optical properties. Pyramidal QDs are formed by patterning a semiconductor material into a pyramid shape recesses, which results in a confinement of the electronic and optical states in the inverted pyramid apex. This confinement allows for a high level of control over the electronic and optical properties of the dot, such as the energy level spacing and the optical transition dipole moment. Furthermore,pyramidal QDs (intrinsically) could exhibit a high degree of symmetry, which makes them well suited for implementing various quantum operations.One of the key advantages of pyramidal QDs is their ability to achieve high optical quality, which is crucial for implementing high-fidelity quantum operations. In addition, pyramidal QDs can be integrated with other semiconductor technologies, such as waveguides and electro-optic modulators, which allows for the implementation of more complex quantum operations. This integration can be done using various techniques such as epitaxial growth, nano-fabrication and transfer printing or other heterogeneous integration techniques.Overall, site-controlled pyramidal quantum dots have emerged as a promising candidate for optical quantum information processing, due to their unique structuraland optical properties, which allow for high-fidelity quantum operations and the integration with other semiconductor technologies. This research field is still in the early stage and a lot of work is currently ongoing to improve the performance and scalability of pyramidal QD devices.In this work, we evaluate and optimize some features of this specific type of QDs for optical quantum information processing. One of the main prominent factor/feature, called fine structure splitting (FSS), which is detrimental to the quality of entanglement (essential to quantum information processing), is widely studied and different strategies are discussed to minimize its pernicious effect in the GaAs QD family. Resonant excitation of anti-binding InGaAs QDs, bindingGaAs QDs and charge distribution of GaAs QDs sandwiched between super-lattice structure are other topics covered in this work.
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    Theory of radiative and nonradiative recombination processes in nitride-based heterostructures
    (University College Cork, 2023) McMahon, Joshua M.; Schulz, Stefan; Science Foundation Ireland; Sustainable Energy Authority of Ireland
    Most modern blue-violet short wavelength visible light-emitting diodes (LEDs) incorporate group III-nitride (III-N) semiconductor quantum wells (QWs), and ultraviolet (UV) LEDs could be developed using similar materials and heterostructures. Relative to filament and fluorescent bulb technologies, modern blue-violet short wavelength emitting III-N QW-based devices are significantly more efficient. However, they suffer from efficiency ``droop'' effects, the fundamental causes of which are under debate. For example, the efficiency of III-N LEDs drops with increasing drive current, and thus increased carrier density in the well, an effect known as ``current droop''. Furthermore, even at fixed drive current, when the temperature of the device rises, there is a corresponding drop in device efficiency, the so-called ``thermal droop''. On top of current and temperature dependent droop, III-N QW-based light emitting devices also suffer significant reductions in efficiency at longer (green to red) and shorter (deep UV) wavelengths. The reduction in efficiency as III-N devices are engineered to emit at longer wavelengths is a major contributor to the so-called ``green gap'' phenomenon, wherein neither III-N nor other III-V heterostructure-based devices (such as those based on phosphides or arsenides emitting in the red to infrared spectral range) can be designed to emit efficiently in the green to yellow spectral range. Input from theory is important for understanding the fundamental origins of these droop phenomena, as well as to guide device design to improve the efficiency of not only blue-violet visible wavelength emitters that suffer from droop, but also devices emitting at the shorter and longer wavelengths mentioned above. However, theoretical results obtained for other semiconductor structures (such as those based on II-VI and other III-V heterostructures) cannot be carried over to III-N systems due to significant differences in the fundamental material properties. For instance, alloy disorder causes strong carrier localisation in III-N semiconductor alloys, which can significantly alter the electronic and optical properties of III-N heterostructures. Although experimental data indicates the importance of such effects, only very recently have they been accounted for in theoretical studies, as they present a significant modelling challenge. The aim of this thesis is to address this challenge using atomistic modelling. Auger recombination has been discussed in the literature as an important nonradiative process in c-plane InGaN/GaN QWs in which an electron recombines with a hole, but instead of a photon being emitted (as in radiative recombination) another carrier is excited. If the rate of Auger recombination grows faster than the rate of radiative recombination as temperature or carrier density increases, overall, there will be a negative impact on device performance. For instance, there is much evidence that Auger recombination plays a significant role in carrier density dependent droop, but the exact nature of the Auger process underlying this droop phenomenon is still under debate, and several Auger recombination mechanisms have been suggested, including a defect-assisted process as well as alloy disorder enhanced Auger recombination. Auger recombination has also been explored as a possible cause of thermal droop but there is still much debate over its relevance to this droop phenomenon. Despite experimentally established links between Auger recombination and these droop phenomena, on the theoretical side the impact of alloy disorder on the Auger recombination process in III-N QW systems is widely unexplored. Our theoretical framework is based on a nearest neighbour sp3 tight-binding model, which takes input from a valence force field model to determine the equilibrium lattice positions of the alloy disordered QW structures studied. On top of this, local variations in strain and polarisation fields are accounted for in the framework, along with polarisation field screening effects at high carrier densities in the well. The tight-binding energies and wave functions are then used to calculate the radiative and Auger recombination rates. For InGaN/GaN QW systems, our predicted values for radiative and Auger recombination rates lie within the wide range of experimentally reported values, and confirm that the coefficient of Auger recombination is not as small as one may expect from the expected dependence of Auger coefficient on band gap. To study the thermal droop, we have evaluated the radiative and Auger recombination rates at a fixed carrier density but as a function of temperature. Our results reflect the unconventional but experimentally observed increase of the radiative recombination rate with increasing temperature. When focusing on the competition between radiative and alloy-enhanced Auger effects, neglecting, e.g., defect-related processes such as Shockley-Read-Hall recombination, our results indicate an improvement of device performance with increasing temperature, in contrast to experimentally determined efficiency data. Thus, we expect that alloy-enhanced Auger recombination, intrinsic to InGaN-based QWs, is not responsible for thermal droop. As a result, efficiency improvement strategies that target, for instance, factors extrinsic to the well, such as reducing defect densities, should be considered. Turning to the carrier density dependent droop (i.e. the current droop), the competition between radiative and Auger rates as a function of carrier density (but at fixed temperatures) was determined. In InGaN/GaN QW systems, already at low carrier densities our model predicts Auger recombination rates large enough to be considered significant contributors to this droop phenomenon (based on expectations from the literature). When investigating the carrier density dependence of the recombination rates, we find that the Auger rate grows faster than the radiative rate as carrier density increases in the c-plane InGaN/GaN QWs studied here. Working within a commonly used model of efficiency in LEDs, we compared the theoretically determined carrier density dependent efficiency data to experimental results from collaborators at the Universities of Manchester and Cambridge for samples with varying defect densities. We found a good agreement between theory and experiment at carrier densities where this droop phenomenon is observed; the temperature is kept constant in the experiments and calculations. Furthermore, the drop in efficiency with increasing carrier density was essentially independent of sample, and thus independent of defect density. Overall, these results indicate that defect-assisted Auger processes may be of secondary importance, and alloy-enhanced Auger recombination can be sufficient to explain the current droop. Moreover, despite the large Auger coefficients, the green emitting samples studied in our theory experiment comparison were found to have relatively high internal quantum efficiencies, suggesting that Auger recombination may not be limiting the performance of longer wavelength visible light-emitting devices. Lastly, UV LEDs based on AlGaN QW systems have attracted significant attention in recent years for the potential development of more efficient and environmentally friendly UV emitters. Despite this, and despite the experimental observation of both thermal and current droop in AlGaN-based QWs, Auger recombination and carrier localisation have also been widely unexplored in these systems, particularly from a theoretical perspective. Thus, we applied our theoretical framework to determine the radiative and Auger recombination rates at a fixed carrier density but as a function of temperature in c-plane AlGaN/AlN QWs, thus providing insight into the thermal droop of such emitters. We found Auger recombination rates on the same order of magnitude as those determined for the InGaN/GaN QWs studied above. Based on our results, we expect that Auger recombination is not the driving cause of thermal droop in AlGaN/AlN QWs. However, based on expectations from the literature, the values calculated here suggest that Auger recombination could be a significant contributor to current droop in AlGaN QWs. Overall, our calculations show that alloy-enhanced Auger recombination is indeed strongly contributing to nonradiative recombination processes in III-N heterostructures. However, the thermal droop does not seem to be driven by this intrinsic nonradiative process. Instead, other factors, such as defect-assisted Auger recombination or carrier injection deficiencies, may be responsible. Thus, device optimisation strategies that target these extrinsic effects may still improve efficiency. With regard to the current density dependent droop, our calculations indicate that alloy-enhanced Auger recombination plays a significant role. Efforts to mitigate the impact of alloy-enhanced Auger recombination on the internal quantum efficiency could consider targeting larger active regions to reduce current density, or improved current spreading through the multiple QWs employed in LEDs. Although alloy-enhanced Auger recombination may present an intrinsic roadblock to reducing current droop, our results suggest that the reduction in efficiency of longer wavelength III-N devices emitting in the visible range (i.e. green) is not driven by this Auger process. Thus, device performance optimisation may still be achievable by targeting extrinsic factors such as carrier injection efficiency and homogeneity.
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    Monolithically integrated, high coherence frequency comb generation through on-chip gain switching
    (University College Cork, 2023) McCarthy, John T.; Peters, Frank H.; Corbett, Brian; Kelleher, Bryan; Science Foundation Ireland
    As the number of internet users continuous to increase, methods of developing new forms of communication networks are being widely considered. Optical frequency comb sources have the potential to reduce or eliminate the spectrally inefficient guard bands that are currently used to prevent cross-talk between adjacent channels. With their common laser source and fixed phase relation, frequency combs can offer to not only to replace the hundreds of lasers being used to generate hundreds of channels, but also reduce the present day separation between channels. This thesis demonstrates on-chip frequency combs that are generated through gain switching. Simulation analysis is carried out to investigate the effects of experimental parameters on the quality of gain switched combs and extensive experimental analysis is carried out to examine the experimental conditions required to enhance the quality of these combs. Typically a two laser design is used where a gain switched Fabry–Pérot laser is phase locked to a single mode laser creating a primary-secondary configuration. Different coupling techniques were investigated and developed, with stable combs being generated as a result of bidirectional coupling, and greater comb enhancement being demonstrated using mutually coupled techniques. Utilising both the combs generated through mutual coupling and the knowledge of on-chip, stable, bidirectionally coupled combs, the conditions required to generate an enhanced comb with additional comb lines are developed. Methods of on-chip comb line filtering are demonstrated for the purpose of future de-multiplexing systems. Finally, a new method of frequency comb generation through on-chip gain switching, without the previously required additional optical injection, is analysed and developed. The versatile design and ease of integration of these comb sources shows great promise for future generation designs and optical networks.
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    Upconverting nanoparticles: pushing theory and technology towards biomedical applications
    (University College Cork, 2023) Souza Matias, Jean; Andersson-Engels, Stefan; Melgar Villeda, Silvia; Science Foundation Ireland
    Biophotonics faces a significant challenge in developing non-invasive imaging and interrogation techniques with high spatial resolution and penetration depth for precise diagnosis and treatment. These techniques rely on non-ionising radiation and non-toxic contrast agents possessing excellent photo and chemical stabilities. Among the various non-ionising wavelengths, the near-infrared (NIR) optical window offers the highest tissue penetration due to its minimal scattering and absorption in living tissues. Upconverting nanoparticles (UCNPs) possess the desirable properties of absorbing and emitting NIR light, making them ideal contrast agents for biomedical applications. UCNPs have shown promise in deep tissue imaging, optogenetics, photodynamic therapy, temperature sensing, drug delivery, and super-resolution microscopy. However, the efficiency of upconversion (UC) in UCNPs, as quantified by the quantum yield (QY), remains a significant challenge, particularly at low excitation power densities (PDs) where the non-linearity of upconversion luminescence (UCL) dominates. Furthermore, the lack of commercially available devices and standardised protocols that account for the crucial parameters affecting PD-dependent QY further complicates accurate characterisation of these materials. To address these issues, this thesis presents the design and construction of a comprehensive, broad-band, multi-variable QY characterisation system. This opto-electronically engineered setup enables simultaneous measurement of absorption and luminescence at two selected wavelengths. In addition, the fully automated system incorporates capabilities for characterising excitation beam profiles, scattering, and the emission spectrum of luminescent compounds in aqueous solutions. Accurate characterisation of the excitation beam profile is of particular importance due to its influence on the PD-dependent QY behaviour of UCNPs. Varying beam profiles lead to distinct QY values, necessitating beam profile compensation to derive an intrinsic QY property of the material independent of measurement configuration. However, achieving this compensation requires a comprehensive understanding of the mechanisms governing each UC emission wavelength, which has not yet been extensively studied across a wide range of PDs. Consequently, this thesis also includes a detailed theoretical study based on the rates of populating and depopulating the electronic energy states involved in UC processes. The investigation successfully quantifies the transition points at which the UCL transitions from non-linear behaviour of different orders to linearity. The theoretical model is validated using experimental UCL data acquired from two distinct UCNP compounds at various wavelengths. These combined theoretical and experimental studies are of utmost importance for the accurate characterisation and engineering of optimal UCNPs, representing a crucial advancement in the development of high-resolution, deep-penetration biomedical techniques and devices.