Physics - Doctoral Theses

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https://hdl.handle.net/10468/430

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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.
Nanostructured magnetic materials for integrated magnonic devices
(University College Cork, 2024) Samanta, Arindam; Roy, Saibal
This 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.
Surface dynamics in III-V epitaxy and its device implications
(University College Cork, 2024) Ozcan Atar, Ayse; Pelucchi, Emanuele; Juska, Gediminas; Science Foundation Ireland
Metalorganic vapour phase epitaxy (MOVPE) is a well-established, industry-compatible, compound semiconductor crystal growth technique, allowing for efficient and controllable material deposition. A wide range of semiconductor devices, both from III-V and nitrides families, are commonly fabricated for a broad range of applications in photonics, electronics and related fields, due to the reproducibility, scalability and overall excellent control over the growth process. Nevertheless, despite the technique’s popularity, there persists a large number of unresolved issues (mostly related to growth process/dynamic understanding) effectively hindering some of potential developments of III-V devices. A major unresolved technological issue, is the reported long range “leakage” of the dopant Zn into intrinsic layers during (and post) epitaxy, including the device processing steps. Zn is used as a typical p-type dopant for III-V materials and devices, but it is reportedly highly diffusive and historically very problematic. To bypass the Zn related problems, the large majority of InP based photonic devices, such as lasers and modulators, are fabricated with an n-i-p design, using p-type dopant at the top of the device. This approach essentially limits the design degree of freedom and stands in the way of the novel advanced stacked device architectures. This work reevaluated the Zn doping issues with unprecedented and surprising findings on Zn dopant behavior. The secondary ion mass spectrometry (SIMS) experiments show that Zn (or its precursors) can behave as a surfactant; accumulating on the sample surface during the growth of intentional doping layer and gradually incorporating into the nominally undoped layers even after the Zn source is shut off. Experimental findings are modelled by combining the surfactant and diffusion behavior with good qualitative agreement. Also, we demonstrated that this phenomenology can be suppressed and controlled either by introducing growth interruption steps or, even more effectively, introducing a competing surfactant species (Sb or its precursors). Our results highlight the relevance of (often overlooked) multi-faceted surface processes during MOVPE epitaxy, and help the implementation of robust solutions for novel device designs, crucially enabling next-generation integrated III-V applications.
Towards a GaN-on-sapphire photonic integrated circuit via micro-transfer printing
(University College Cork, 2024) O'Brien, Megan; Corbett, Brian; Peters, Frank H.; Science Foundation Ireland
Photonic integrated circuits (PICs) can alleviate the pressure on existing infrastructure for increasing bandwidth requirements by potentially offering faster data transmission with low energy consumption all on the same chip. The ability to integrate active lasing sources to waveguide platforms while also maintaining low-loss coupling is key for efficient PICs.This research focuses on the development of an AlGaN/GaN-on-sapphire platform and the integration of 1.3 µm edge-emitting InP-based multi-quantum well (MQW) and GaAs-based quantum dot (QD) lasing devices to the platform via micro transfer printing (µTP). Gallium nitride (GaN) can be used alternatively to traditional Si-based photonics due to its broad spectral transmission, birefringence, refractive index suitable for fiber coupling, and the possession of both the linear electro-optic effect and non-linear properties. First, we design an AlGaN/GaN structure that creates a single-mode, broad passive spot size for relaxed alignment tolerance of the transfer-printable laser-to-waveguide. Coupling losses as low as 0.6 dB were calculated for the optimal positioning of the laser. We also outline a double-tapered mode adapter that converts the dimensions to more suitable structures for light-routing. The integration of LiNbO3 coupon to the AlGaN/GaN platform is explored through simulations in order to enhance the electro-optic property, and a Mach-Zehnder modulator (MZM) design is proposed. Additionally, the simulations of (i) directional couplers, (ii) polarization converters and (iii) grating couplers on an AlGaN/GaN platform are presented in this work. Next, the fabrication of AlGaN/GaN waveguides devices through BCl3-based etching recipes, along with the challenges of achieving smooth and vertical waveguide sidewalls, are discussed. A BCl3 dry etch for forming a trench into sapphire is also developed for µTP alignment purposes. The AlGaN/GaN waveguides were then characterized, and we distinguish the differences in propagation losses between the TE and TM polarizations.The average propagation losses ranged between 2.4-14.7 dB/cm for both polarizations. Following on from this, both the InP-based MQW and the GaAs-based QD laser coupons were characterized and transfer-printed and coupled to the fabricated GaN waveguides. A coupling output of 2.3 mW at 80 mA was achieved for the QD laser, giving a coupling efficiency of 10.3 %. The results show promise for GaN to be a suitable platform for integrated photonics.
Micro-transfer print integration of high-speed photodetectors to SOI platform
(University College Cork, 2024) Muthuganesan, Hemalatha; Corbett, Brian; Peters, Frank H.; Science Foundation Ireland
Photonic integrated circuits (PICs) offer a pivotal solution to meet the escalating demands of data bandwidth and used in data centres, LIDAR and optical sensing. Among various approaches, micro-transfer printing emerges as a rapid and universally compatible integration technology to generate PICs. To implement this technology, it is necessary to separate the devices from their original substrate using a release layer and transfer print them to designated location on target wafer. This enables the heterogeneous integration of numerous active devices from multiple wafers onto a single wafer, resulting in a compact and highly functional PIC. In addition to being a room temperature process, it is also cost-effective allowing reuse of expensive III-V substrates. This thesis demonstrates the application of micro-transfer printing technology in integrating ultra-thin, high-speed InGaAs photodetectors with silicon waveguides through various coupling mechanisms. The thesis begins with optimizing the optical power coupled between the waveguide and the photodetector (PD), as even a small gap between them leads to optical mode loss at the interface. A co-planar in-fill idea is experimented by transfer printing the PD’s absorber region at the same height as silicon waveguide. To fill the gap, evaporated amorphous silicon (a-Si) with a refractive index of 3.1 in the telecom wavelength range is utilized. This strategy aims to achieve maximum coupling. In transfer printing process, it is crucial to efficiently release the PD coupons along with smooth interface, for successful printing on target wafer. This work introduces for the first time, a combination of InGaAs and AlInAs as release layers for InP based devices yielding double the etch rate and almost isotropic etch compared to individual release layers (InGaAs or AlInAs). This facilitates direct bonding of the PD coupons to the target wafer since the interface roughness is extremely low as 0.2 nm over an area of 10 µm x 10 µm, contributing to high-speed of the PD. An excellent selectivity of 5410 is obtained with InP, which is 3.4 times higher than AlInAs and 7 times higher than InGaAs as release layers. These outcomes are accomplished at room temperature thus saving time, cost, and energy for the entire process. The above optimized processes are used in integration of high-speed InGaAs photodetector to SOI platform, by direct bonding, with 100 % yield. An ultra-thin 675 nm epitaxial stack is grown with dual release layer on InP substrate, where a PD of size 21 µm x 57 µm is fabricated. Such small dimension has the potential to fit 1 million devices on a 75 mm InP wafer. The same photodetectors are coupled to silicon waveguides via evanescent, grating and butt coupling mechanisms on the same target wafer. These PDs exhibit a maximum responsivity of 0.6 A/W, 47 nA dark current and with a data communication rate of 50 Gbps with on-off keying. In future, this unique integration approach can be universally applied to any III-V active device like lasers and modulators to realise compact and high performance photonic integrated circuits.

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