This thesis presents a study of the design and optimisation of gallium-arsenide-based quantum cascade lasers (QCLs). Traditionally, the optical and electrical performance of these devices has been inferior in comparison to QCLs that are based on the InP material system, due mainly to the limitations imposed on performance by the intrinsic material properties of GaAs. In an attempt to improve the performance of GaAs QCLs, indium-gallium-phosphide and indium-aluminium-phosphide have been used as the waveguide cladding layers in several new QCL designs. These two materials combine low waveguide losses with a high confinement of the laser optical mode, and are easily integrated into typical GaAs QCL structures. Devices containing a double-phonon relaxation active region design have been combined with an InAlP waveguide, with the result being that the lowest threshold currents yet observed for a GaAs-based QCL have been observed - 2.1kA/cm2 and 4.0kA/cm2 at 240K and 300K respectively. Accompanying these low threshold currents however, were large operating voltages approaching 30V at room-temperature and 60V at 80K. These voltages were responsible for a high rate of device failure due to overheating. In an attempt to address this situation, two transitional layer (TL) designs were applied at the QCL GaAs/InAlP interfaces in order to aid electron flow at these points. The addition of the TLs resulted in a lowering of operating voltage by ~12V and 30V at 300K and 240K respectively, however threshold current density increased to 5.1kA/cm2 and 2.7kA/cm2 at the same temperatures. By utilising a high-reflectivity coating and epi-layer down bonding process, a QCL comprising an InGaP waveguide and double-phonon active region was observed to operate in continuous-wave mode up to a temperature of 80K, with an optical output power of 26mW.

Surface plasmon polaritons are highly confined electromagnetic waves which can be employed in developing miniaturised optical devices for bridging the size-mismatch between the nanoscale electronics and large diffraction-limited photonic devices. For this purpose, it is desired to develop silicon compatible plasmonic devices in order to achieve seamless integration with electronics on the silicon-on-insulator platform. Plasmonic devices such as modulators, detectors, couplers, (de)multiplexers, etc, would possess the advantages of having a small device footprint, low cost, low power consumption and faster response time. In this thesis, different silicon-based plasmonic devices were investigated using finite element simulations, including optical modulators, couplers and splitters. A metallised stub filled with SiGe/Ge multiple quantum wells or quantum dots in a silicon matrix, coupled to a dielectric waveguide was investigated. The modulation principles include ’spoiling’ of the Q factor and conversion of the electromagnetic mode parity, due to variation of the absorption coefficient of the stub filling. A CMOS compatible interference-based Mach-Zehnder modulator with each arm comprising a metal-insulator-semiconductor-insulator-metal structure, and a simpler single arm variant, were considered for electro-optic and electroabsorption modulation respectively. The electron density profiles in bias-induced accumulation layers were calculated with the inclusion of size-quantisation effects at the oxide-silicon interfaces. These were then used to find the complex refractive index profiles across the structure, in its biased and unbiased states, and eventually the modulator insertion loss and extinction ratio, and their dependence on various structural parameters. Finally, a silicon-based plasmonic nanofocusing coupler was investigated, which comprised symmetric rectangular grooves converging towards a central metal-silicon-metal nano-slit at the apex of the structure. The structure was optimised to achieve maximum coupling of light incident from a wide input opening, and coherent excitation and focusing of surface plasmons as they propagate towards the nano-slit waveguide. Application of the nanofocusing structure to achieve simultaneous coupling and splitting was also investigated, whereby incident light was focused into two nano-slits separated by a metal gap region at the apex. Such a plasmonic coupler or splitter can be used for coupling light directly from a wide fibre grating opening into nanoplasmonic waveguides in future on-chip plasmonic-electronic integrated circuits, or into the two arms of a plasmonic Mach-Zehnder modulator.

Terahertz (THz) quantum cascade lasers (QCL) have stimulated significant interest in THz laser imaging systems due to their compact size, broad spectral coverage (~1.2-5.2 THz) and high output power ( > 1 W). Due to their continuous wave (CW) narrowband emission and quantum noise limited linewidths, THz QCLs are particularly suited to coherent detection, but the majority of previously reported imaging systems have employed incoherent detection. Furthermore THz detectors typically fall into one of two categories (thermal or electrical), both of which have downsides (slow response rate or limited frequency range respectively). Self-mixing (SM) can be seen as a solution to these problems while also gaining the advantages of a reduced experimental set-up and cost, and increased sensitivity. SM occurs when radiation emitted from a laser is re-injected into the laser cavity by reflection from a remote target. The re-injected field interferes with the intracavity field, resulting in perturbations to both the measured output power and laser terminal voltage that depend on both the amplitude and phase of the reflected field. In this work, new SM imaging and modulation techniques were developed for both two- (2D) and three-dimensional (3D) imaging systems, including improvements leading to improved acquisition speed and depth resolution. Other techniques were developed to identify parameters of the QCL spectral emission and tunability, and SM was also exploited for extraction of optical parameters of explosive materials; a precursor to identification of such materials, something that is very important to national security and public safety. Further work was also developed in the areas of phase-nulling for the purpose of vibromacy measurements and extraction of laser parameters, and near-field (NF) spectroscopy, which has led to a massively improved lateral imaging resolution (~1 um).

The development of the optical frequency comb technique has transformed the field of spectroscopy, allowing the measurement of atomic transition frequencies to unprecedented levels of accuracy. In this thesis a frequency comb has been used to make absolute frequency measurements of optical transitions to highly excited Rydberg levels in Rb. The reliable measurement of these levels plays an important role in improving the accuracy of atomic models and the widely used Rb atom is an excellent candidate for such studies. A laser system has been constructed and optimised for resolving these highly excited states in an ordinary vapour cell, using a Doppler-free technique of purely optical detection. After several developments to the apparatus, the absolute energies of a collection of Rydberg levels have been measured to an accuracy of 3 parts in 1010, demonstrating the first sub-megahertz accuracy optical Rydberg spectroscopy. A vapour cell is a convenient and straightforward solution for finding Rydberg levels and these findings show that cell-based detection techniques could potentially permit rapid advances in the field. Along the way, the vapour cell sample has also highlighted many interesting areas of exploration: For example, it has allowed long term laser stabilisation to Rydberg levels for experiments such as the micromaser. Also, the Rydberg atoms in the cell have been manipulated by microwaves, allowing the study of high ` = 4 states, which has illuminated a whole range of new experiments. It has even been found that one of the limiting factors of these cell-based schemes may be the knowledge of the frequency of lower lying transitions, which has ultimately led this research into a secondary area, involving the measurement of the Rb D lines with a frequency comb. Together, these findings have exposed a large variety of atomic physics to be investigated in the future.

No description available.

Wave based imaging methods aim to build an accurate reconstruction of the physical properties of an object by recording the scattered field caused by illumination from multiple directions. Classically the minimum distance between the characteristics of the object that can be resolved by an imaging method is limited by the wavelength, λ, of the interrogating field. In order to improve the resolution shorter wavelengths can be propagated; however, due to material absorption, this limits the penetration depth of the wave which consequently reduces the potential imaging range. Any imaging technique which can overcome the resolution limit is of great practical and academic interest and represents the subject of this thesis. Subwavelength characterisation has become well established in the field of Near- field Scanning Optical Microscopy which requires part of the probing system to be within λ of the object being illuminated (near field), in order to detect the nonpropagating evanescent waves. The super-oscillatory properties of the evanescent waves are subsequently used to achieve subwavelength resolution. However, access to the near field of an object is not always feasible and since evanescent waves decay exponentially they cannot be directly detected in the far field (greater than λ from the object). The aim of this thesis is to define and investigate an imaging strategy that will allow super resolution to be achieved from the far field. Conventional imaging techniques, which are constrained by the resolution limit, neglect the distortion of the scattered field caused by the internal structure of the object. This thesis will show that a more accurate description of the interaction of the incident field with the object, which includes the multiple scattering of evanescent waves, can lead to subwavelength resolution from the far field.

This thesis describes the development of a Mach-Zehnder interferometric filter based planar Doppler velocimetry (MZI-PDV) flow measurement technique. The technique uses an entirely new optical system, an unbalanced MZI incorporating glass blocks for wavefront-matching, to replace the iodine cell currently used in conventional PDV. The free spectral range of the interferometric filter can be selected by adjusting the optical path difference of the MZI. This allows the velocity measurement range, sensitivity and resolution to be varied. This system offers no restricts to the choice of laser wavelength of operation which is not the case with most techniques. Two techniques to process the interference fringe images are presented. The first uses the shift of the fringe pattern to determine the Doppler frequency shift along profiles. The second provides a full-field measurement by normalising the received light intensity at each pixel in the image. With the single camera MZI-PDV scheme, exact alignment of the two output images on the active area of the camera is automatic. This eliminates the pixel-matching problem in conventional two camera PDV systems. The technique allows the measurement of up to three components of the flow velocity across a plane defined by a laser light sheet. The construction of a single velocity component MZI-PDV system that incorporates a phase-locking system designed to stabilise the filter is described. Measurements are made on the velocity field of a rotating disc with maximum velocities of ~±70ms-1 and an axis-symmetric air jet (with a nozzle diameter of 20mm) with an exit velocity of ~85ms-1. Standard deviations in the measured velocities were found to be about 2.9 and 2ms-1 for the two processing methods respectively. The system was then modified to make 3-component velocity measurements using imaging fibre bundles to port multiple views to a single detector head, and the standard deviation of the velocity error is around ±3ms-1 for a maximum velocity of ~±30ms-1 in the field of view. The factors that will affect the quality of the interference fringe image are investigated including polarisation sensitivity of the two beam splitters and flatness of the optical components. The inclination angle and the optical path deviation have little effect on the contrast of the interference fringes since collimated light beams, rather than divergent ones, are used in the interferometer.

The temporal characteristics and the spectral content of light can be manipulated and modified by harnessing linear and nonlinear interactions with a dielectric medium. Optical fibres provide an environment in which the tight confinement of light over long distances allows the efficient exploitation of weak nonlinear effects. This has facilitated the rapid development of high-power fibre laser sources across a broad spectrum of wavelengths, with a diverse range of temporal formats, that have established a position of dominance in the global laser market. However, demand for increasingly flexible light sources is driving research towards novel technologies and an improved understanding of the physical mechanisms and limitations of existing approaches. This thesis reports a series of experiments exploring two topical areas of ongoing research in the field of nonlinear fibre optics: mode-locked fibre lasers and fibre-based supercontinuum light sources. Firstly, integration of novel nano-materials with existing and emerging fibre-based gain media allows the demonstration of ultrafast mode-locked laser sources across the near-infrared in a conceptually simple, robust, and compact scheme. Extension to important regions of the visible is demonstrated using nonlinear conversion. Scaling of pulse energies in mode-locked lasers can be achieved by operating with purely positive dispersion for the generation of chirped pulses. It is shown unequivocally, through a direct measurement, that the pulses generated in ultra-longmode-locked lasers can exist as highly-chirped dissipative soliton solutions of the cubic (and cubic-quintic) Ginzburg Landau equation. The development of a numerical model provides a framework for the interpretation of experimental observations and exposes unique evolution dynamics in extreme parameter ranges. However, the practical limitations of the approach are revealed and alternative routes towards achieving higher-energy are proposed. Finally, an experimental and numerical study of the dependence of continuous-wave pumped supercontinua on the coherence properties of the pump source shows an optimum exists that can be expressed as a function of the modulation instability period. A new and simplified model representing the temporal fluctuations expressed by continuous wave lasers is proposed for use in simulations of supercontinua evolving from noise. The implications of the experiments described in this thesis are summarised within the broader context of a continued research effort.

The aim of plasmonics is to exploit the strong coupling between photons and collective electron oscillations in metals, so-called surface plasmon polaritons, which enable a strong confinement of the electromagnetic field to metal-dielectric interfaces. The interaction of confined optical states with electronic transitions within matter accelerates these otherwise slow light-matter interactions. This work's purpose is to investigate accelerated light-matter interactions within plasmonic lasers, which arise due to optical confinement, and how these influence laser dynamics. In particular, this work focuses on the fabrication, demonstration and characterisation of plasmonic lasers. The devices investigated in this work consist of semiconductor nanowires made from zinc oxide (ZnO) placed in the proximity of a silver substrate. In this geometry the metal allows for strong optical confinement, whereas the semiconductor delivers the necessary gain to achieve lasing. Operating at room temperature, the emission from ZnO lies near the surface plasmon frequency, where confinement and loss become maximal, leading to accelerated spontaneous recombination, gain switching and gain recovery compared with conventional - photonic - ZnO nanowire lasers. To assess the lasing dynamics, in this work a novel double-pump spectroscopy technique is used, which exploits the non-linearity of the laser process to allow the investigation of accelerated light-matter interactions. This novel technique is necessary, as the speed of plasmonic devices is too fast for electrical detection, and the emission of single devices is too weak for non-linear spectroscopic techniques. Comparing photonic and plasmonic devices reveals contrasting dynamics between both, highlighting the benefits of plasmonic confinement, but also exposing an important limitation. Plasmonic devices could potentially be faster, but are ultimately limited by internal relaxation processes of the chosen gain medium. The findings of this work will improve the understanding of plasmonic lasers and their limitations, but also lead to improved knowledge of internal semiconductor processes.

In this thesis the extension of the depth of field of optical systems is investigated. The problem of achieving extended depth of field (EDF) while preserving the transverse resolution is also addressed. A new expression for the transport of intensity equation in the prolate spheroidal coordinates system is derived, with the aim of investigating the phase retrieval problem with applications to EDF. A framework for the optimisation of optical systems with EDF is also introduced, where the main motivation is to find an appropriate scenario that will allow a convex optimisation solution leading to global optima. The relevance in such approach is that it does not depend on the optimisation algorithms since each local optimum is a global one. The multi-objective optimisation framework for optical systems is also discussed, where the main focus is the optimisation of pupil plane masks. The solution for the multi-objective optimisation problem is presented not as a single mask but as a set of masks. Convex frameworks for this problem are further investigated and it is shown that the convex optimisation of pupil plane masks is possible, providing global optima to the optimisation problems for optical systems. Seven masks are provided as examples of the convex optimisation solutions for optical systems, in particular 5 pupil plane masks that achieve EDF by factors of 2, 2.8, 2.9, 4 and 4.3, including two pupil masks that besides of extending the depth of field, are super-resolving in the transverse planes. These are shown as examples of solutions to particular optimisation problems in optical systems, where convexity properties have been given to the original problems to allow a convex optimisation, leading to optimised masks with a global nature in the optimisation scenario.