Physics - Doctoral Theses
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Item Fourier analysis of index-patterned Fabry-Pérot lasers(University College Cork, 2024) Boohan, Niall; O'Reilly, Eoin P.; Corbett, Brian; Science Foundation IrelandOptoelectronics has the potential to revolutionise mass-produced, consumer-level goods (health, LiDAR, and 6G internet) as it has done for high-tech, specialised applications in metrology, spectroscopy, and telecommunications. To achieve this, key components need to become more structurally robust and easier to manufacture. The index-patterned laser, using a perturbation-based grating within the cavity waveguide, shows great potential as a low-cost, single-frequency light source. However, further research and understanding of the dependence of the device lasing spectrum on the grating perturbations within the cavity is required. This thesis uses a first-order reflection approximation, to allow a Fourier transform-based analysis of grating patterns and cavity structures to improve single-mode selectivity and device yield. We demonstrate how placing the weighted average of the grating perturbation positions closer to the emitting facet can be used to select the lasing mode more strongly. The perturbations interact with the cavity facets to generate modal selection. However, it is not possible to control the facet-to-grating distance precisely enough to ensure a full yield of single-frequency lasers at predetermined wavelengths. We show that the phase difference between the laser light electric field at the perturbations and at the facet can be used to calculate the emission spectrum. This gives us a method for studying these facet position effects on the spectrum. In addition to this, we show how the change in the cavity resonances with length affects mode selection in the device. We analyse for the first time the effects of applying a high-reflectivity facet to the predominant reflecting facet of the cavity for mode selection. We also analyse the effects of higher-order reflections not included in our Fourier method, allowing us to assess its accuracy. The results are supported with a comparison against the experimental data. The grating pattern, position, and facet reflectivity can be used to create devices that have greater spectral purity with lower optical loss. Our ability to relate grating position and mode resonances in the cavity to the calculated device spectrum enables rapid analysis of grating designs for further potential yield improvements. Future modelling work could include applying our designed laser to two-dimensional models with a non-constant refractive index, as this will capture further important device effects not included in our Fourier model.Item Machine learning equalisation techniques for future passive optical networks(University College Cork, 2024) Murphy, Stephen; Townsend, Paul D.; Antony, Cleitus; Science Foundation IrelandPassive Optical Networks (PON) have become crucial in delivering reliable, high-capacity, and low-latency fiber internet connectivity, largely replacing legacy copper cabling. Research into next generation 100 Gbit/s systems is ongoing to support future bandwidth demands, and the adoption of advanced modulation formats, such as 4-level pulse amplitude modulation (PAM4), will be key to achieving this. However, optical amplification will be required to support PAM4 transmission due to the strict optical loss budgets imposed by legacy network infrastructure. Semiconductor optical amplifiers (SOAs) have been widely proposed, but introduce non-linear signal distortions which can greatly restrict PAM4 performance. Moreover, fiber dispersion impairments are also a concern given PAM4’s high linearity requirements. This thesis explores how neural network-based equalisation (NNE) can offer new solutions to these traditional physical limitations, and enable future 100 Gbit/s PAM4 PON systems. Through rigorous experimental analysis, this work identifies SOA-induced non-linear patterning as a major barrier to achieving the required 19.5 dB dynamic range requirement for upstream PON transmission. Gated recurrent neural networks are proposed to overcome this effect, and are shown to fully recover PAM4 signals for a range of bit rates up to 128 Gbit/s. The proposed NNE achieves > 28 dB dynamic range in a 100 Gbit/s experimental PAM4 PON scenario, where additional non-linear effects such as fiber dispersion and electrical receiver saturation are also present. Meanwhile, conventional linear equalisation techniques are shown to be insufficient to meet the PON dynamic range requirement. Furthermore, complexity analyses of the proposed NNE solutions are included throughout, and multi-symbol equalisation techniques are investigated to alleviate the challenges of feedback-based NNEs in future hardware implementations. The thesis’ most significant contribution is the invention of SkipNet, an original adaptive neural network algorithm developed by the author. It overcomes the significant obstacle of complex NNE training, and enables real-time, packet-by-packet adaptation to dynamic transmission conditions in burst-mode PON systems. SkipNet is shown to match the performance of conventionally trained NNE solutions, while using as little as 250 training symbols to mitigate non-linear SOA patterning and fiber dispersion. The introduction of SkipNet opens new pathways for designing future PON systems that are scalable, cost-effective, and capable of meeting the ever-growing demand for high-speed connectivity.Item Theory of tunneling and transport in emerging narrow-gap semiconductor alloys(University College Cork, 2024) Das, Sarita; O'Reilly, Eoin; Broderick, Christopher; Schulz, Stefan; Science Foundation IrelandThere is a high demand for narrow-band gap semiconductor alloys to support the development of advanced devices such as high-sensitivity photodiodes (PDs) and light sources (lasers and LEDs) for the application-rich wavelength range from 3 to 5 $\mu$m. These applications include environmental and industrial process monitoring, free-space optical communications, and imaging for aerospace and defence applications. Therefore, there is a need to access the electronic and optical properties of these alloys. For instance, the suppression of dark current is crucial to achieving high-sensitivity PDs to improve the detector signal-to-noise ratio. Leakage currents linked to band to band tunneling (BTBT) plays a substantial role in the dark current in PDs based on narrow-gap semiconductors. Determining the effectiveness of a device also involves looking into the electron transport properties of such alloys. Two types of emerging narrow-band gap alloys are investigated in this thesis, namely (i) highly mismatched dilute nitride and bismide alloys, and (ii) Ge$_{1-x}$Sn$_{x}$ alloys. We first theoretically analyse BTBT in highly-mismatched, narrow-gap dilute nitride and bismide alloys. Replacing As by N (or Bi) introduces localised impurity states into the conduction (valence) band, whose impact on the band structure can be described by a band-anticrossing (BAC) interaction between the localised impurity states and the host conduction (valence) band. For this class of semiconductors, the assumptions underpinning the widely-employed Kane model of BTBT in an applied electric field break down, due to strong band edge nonparabolicity resulting from the BAC interactions. Via numerical calculations based on the Wentzel-Kramers-Brillouin approximation, we demonstrate that BAC leads, at a fixed band gap, to a reduced (increased) BTBT current at low (high) applied electric field compared to that in a conventional InAs$_{1-y}$Sb$_{y}$ alloy. Our analysis reveals that BTBT in InN$_{x}$As$_{1-x}$ and InAs$_{1-z}$Bi$_{z}$ is governed by a field-dependent competition between the impact of N (Bi) incorporation on (i) the dispersion of the evanescent Bloch band linking the valence and conduction band edges, which dominates at low field strengths, and (ii) the conduction (valence) band dispersion, which dominates at high field strengths. The implications of our results for applications in long-wavelength avalanche photodiodes (APDs) and tunneling field-effect transistors (TFETs) are discussed. In addition to this study, we investigate the evolution of the electron transport properties of Ge$_{1-x}$Sn$_{x}$ alloys as a function of increasing Sn content, as the alloy goes from indirect-gap to direct-gap. Ge$_{1-x}$Sn$_{x}$ has attracted significant interest for applications in electronic and photonic devices as a (narrow-gap) direct-gap semiconductor compatible with existing CMOS processing. We use the Boltzmann transport equation to calculate as a function of Sn composition the evolution of the electron transport properties in an applied electric field. The intra and inter-valley alloy scattering rates are determined from tight-binding calculations. At extremely low electric field, we predict an increase of approximately two orders of magnitude in electron mobility in direct-gap alloys with higher Sn composition. However, our calculations predict that the electron mobility at these high Sn compositions decreases rapidly with increasing field, as the electrons are quickly accelerated to higher energy, where they can scatter from the $\Gamma$ valley with low density of states (DOS) to the L valleys, with their much higher DOS. We calculate that the contribution of alloy scattering causes the mobility to drop below that of Ge for all values of $x$ considered once the field exceeds 50 V cm$^{-1}$.Item Novel Vertical-External-Cavity Surface-Emitting Lasers (VECSEL) systems and applications(University College Cork, 2024) Bondaz, Thibault Alain Georges; McInerney, John G.; Laurain, Alexandre; Jones, R. Jason; Moloney, Jerome V.The development of laser technologies represents a pivotal milestone in human technological history, with semiconductor lasers playing a particularly transformative role due to their versatility, efficiency, and tunability. This thesis focuses on Vertical-External-Cavity Surface-Emitting Lasers (VECSELs), a subset of semiconductor lasers that offer unique advantages for high-power, single-mode operation and the integration of intracavity optical elements. These properties enable their application across a broad spectrum of fields, including terahertz (THz) generation and nonlinear imaging. This work presents two novel VECSEL-based systems. The first is a bicolor VECSEL designed for room-temperature THz emission via difference frequency generation (DFG). This system achieves milliwatt-level THz power and employs a two-stage stabilization method to enhance operational stability. The second system is a 1.5 GHz broadband pulsed laser developed for multiphoton microscopy. This high-repetition-rate laser minimizes photodamage and photobleaching. By using photonic crystal fibers and a Multiphoton Intrapulse Interference Phase Scan (MIIPS) scheme, the system generates a wide supercontinuum spectrum, enabling advanced pulse shaping for nonlinear imaging.Item Nanostructured magnetic materials for integrated magnonic devices(University College Cork, 2024) Samanta, Arindam; Roy, SaibalThis PhD thesis explores the fabrication, characterization, and application of advanced exchange spring (ES)/exchange coupled (EC) nanoheterostructures and their magnetic properties. Current challenges in the field of magnetics include the development of materials that demonstrate tunable magnetic properties, particularly in terms of controlling anisotropy and spin dynamics at the nanoscale. This thesis addresses these challenges by utilizing an electrodeposition technique, we have been able to develop for the first time in situ amorphous/nanocrystalline cobalt-phosphorus (CoP) thin films at room temperature. These films exhibit a unique transverse exchange spring structure due to the interplay between in-plane (IP) and out-of-plane (OOP) anisotropies. The inherent IP anisotropy of the amorphous phase competes with the OOP anisotropy of the nanocrystalline structure, producing characteristic stripe domain structures that evolve into novel corrugated stripe domain shapes. Systematic investigations reveal the evolution of hysteresis loops in these thin films, showing a transition from low coercivity non-ES loops to staircase-ES loops with multiple coercivities in thicker films. The First Order Reversal Curve (FORC) distributions demonstrate various reversal mechanisms within the samples, confirming the transition between non-ES and ES states based on the prevalent interfacial exchange coupling. Additionally, the field-dependent Brillouin Light Scattering (BLS) spectra unveil distinct spin wave modes, with ES films showing well-resolved bulk and Damon-Eshbach surface spin wave modes, while non-ES films exhibit mode doublets below a certain applied field threshold. These findings indicate a linear dependence of mode frequencies on magnetic field intensity and enhanced exchange coupling in thicker ES films. On the other hand, ultrafast magnetization dynamics studies highlight the in-plane magnetic orientation (φ) dependent ultrafast demagnetization and precessional dynamics of electrodeposited non-exchange spring nanostructured CoP alloys. The precession frequency shows dominant two-fold anisotropy superposed with moderate four-fold anisotropy, while the Gilbert damping coefficient exhibits four-fold anisotropy. The ultrafast demagnetization remains nearly isotropic with φ, suggesting a significant role of spin-orbit coupling (SOC) in anisotropic precessional dynamics and isotropic spin-fiip scattering processes. These detailed studies of ultrafast spin dynamics reveal crucial dynamical properties for potential applications in high-frequency integrated magnetic passives for future monolithic on-chip power supplies. Furthermore, this thesis introduces novel "Magnon Microwave Antennas" (MMAs) for generating tunable microwave frequencies without external bias magnetic fields. The MMAs, comprising patterned arrays of magnetostrictive nanomagnets embedded in piezoelectric heterostructures, generate multimode microwave frequencies through the phonon-magnon coupling. Static magnetic studies elucidate various magnetization reversal processes within the nanowire and nanodot arrays, unveiling the critical role of demagnetization energy distribution in tuning the domain configuration and power-phase distributions of these MMAs. Functional tuneability has been proposed to be achieved through amplitude-dependent training using different combinations of nanowire and nanodot dimensions, topologies, material properties, and array configurations. The non-volatile nature of the spin textures generated in MMAs under bias-free conditions holds promise for energy-efficient logic and low-power computing applications. Thus, the comprehensive research presented in this thesis paves the way for the development and exploitation of next-generation nano-heterostructures for various cutting-edge magnetic vis-à-vis magnonic applications, including on-chip reservoir computing, leveraging the unique magnetic vis-à-vis magnonic properties and their tuneability in these advanced materials/devices.