A batch fabrication process for nano-electro-mechanical systems (NEMS) based on sidewall transfer lithography (STL) is developed and demonstrated. The STL is used to form nanoscale flexible silicon suspensions entirely by conventional lithography. A two-step process is designed for single-layer STL to fabricate simple electrothermal actuators, while a three-step process is designed to allow nanoscale features intersecting with each other for more complicated device lay-outs. Fabricated nanoscale features has a minimum in-plane width of approx. 100nm and a high aspect ratio of 50 : 1. Combined structures with microscale and nanoscale parts are transferred together into silicon by deep reactive etching (DRIE). Suspensions are achieved either by plasma undercut or HF vapour etch based on BSOI. The STL processes are used to form nanoscale suspensions while conventional lithography is used to form localised microscale features such as anchors. A wide variety of demonstrator devices have been fabricated with high feature quality. Analytic models have been developed to compare with experimental characterization and finite element analysis (FEA) predictions. Lattice structures fabricated by multi-layer STL have also be investigated as a novel type of mechanical metamaterial. Thus, the process could allow low-cost and mass parallel fabrication of future NEMS with a wider range of potential applications.

Due to the increasing demand for new wireless services and applications, the high level of integration and the coexistence of multi-standard (MS) or multi-band operations into a single device are becoming defining trends in designing microwave filters. This has driven considerable technological advances in reconfigurable/tunable and miniaturized filters. More specifically, reconfigurable/tunable filters that tune to different frequency bands instead of classical filter banks have great potential to significantly reduce the system size and complexity; while reducing the filter size becomes essential to achieve the highest degree of integration density in compact and portable wireless devices. In the light of this scenario, the objective of this dissertation is to develop the new design technologies, concepts and filtering configurations for tunable microstrip filters and compact passive microwave filters. To this aim, this dissertation is divided into two main parts. The first part (Part I) focuses on the designs of novel varactor-tuned microstrip filters with advanced performances. In this aspect, new topologies for realizing tunable lowpass and highpass filters are firstly developed. State-of-the-art performances, including wide tuning range, high selectivity with multiple transmission zeros, low insertion loss and compact size for all the tuning states are obtained in both of these filters. Secondly, two novel classes of tunable bandpass filters are presented. One of them is designed based on varactor-loaded parallel-coupled microstrip lines (PCML) and short-circuited stubs, which allows the lower passband edge together with two transmission zeros located around the lower passband skirt to be reconfigured separately. While the other tunable bandpass filter is constructed by the combination of tunable bandpass and lowpass filters, featuring both centre frequency and bandwidth tunabilities, as well as high selectivity with abundant transmission zeros. Furthermore, a new concept of tunable lossy filter is demonstrated, which attempts to achieve an equivalent high-Q tunable performance by using low-Q resonators. This concept makes the presented tunable combline filter interesting for some frequency-agile applications in which the low in-band loss variation and high selectivity are much desired while the absolute insertion loss can be a tradeoff. The second part (Part II) is devoted to the design of miniaturized passive microwave filters with improved characteristics. For this, the concept of artificial right-handed and left-handed transmission lines are applied to the signal interference filtering topology, which results in a compact circuit size and good out-of-band performance. In particular, for a further size reduction, such filter is implemented in the forms of multilayered structure by using liquid crystal polymer (LCP) technology. Additionally, another two types of miniaturized bandpass filters using stepped impedance resonators are demonstrated, which are implemented based on different fabrication processes (i.e. LCP bonded multilayer PCB technology and a standard planar PCB technology). Among their main features, the compact size, wide passband, broad stopband with multiple transmission zeros and circuit simplicity are highlighted. For all the proposed design techniques and filtering structures, exhaustive theoretical analyses are done, and design equations and guide rules are provided. Furthermore, all the proposed schemes and/or ideas have been experimentally validated through the design, implementation and measurement of different filters. The fabrication processes of multilayer technology utilized: liquid crystal polymer (LCP) technology and liquid crystal polymer (LCP) bonded multilayer printed circuit board (PCB) technology, are also demonstrated for reference. All of the results achieved in this dissertation make the proposed filters very attractive for their use in modern wireless communication systems.

The research presented in this thesis addresses the development of directly diode pumped Yb3+:KY(WO4)2 and Yb:fibre lasers, operating at several-hundred-MHz repetition frequencies and investigates their suitability as the basis of high-efficiency, convenient, low-cost and moderate precision optical frequency combs. The design, construction, and characterisation of a 1042-nm 1-GHz Yb3+:KY(WO4)2 femtosecond laser is presented, achieving pulses with an average power output of 770 mW, bandwidths of 3.8 nm, and durations of 278 fs. The laser achieved an opticalto- optical conversion efficiency and slope efficiency of 61% and 69%, respectively, and the relative intensity noise was <0.1%. Spectral broadening of the output pulses in a photonic crystal fibre with a negative dispersion wavelength of 975 nm and a core diameter of 3.7 m resulted in a supercontinuum with a bandwidth of 400 nm, which was insufficient to enable f-2f referencing. A re-designed Yb:KYW laser was demonstrated, operating at a pulse repetition frequency of 666 MHz and producing pulses with reduced durations of 220 fs and increased bandwidths of 5 nm, while maintaining an average output power of >700 mW. Repetition-frequency locking was implemented on this laser and had the effect of reducing its relative intensity noise from 1.1% to 0.5%, with limitations on the locking stability being traced to cantilever-like vibrational modes of the mirrormount assemblies. A fully stabilised 1030-nm Yb:fibre frequency comb operating at a pulse repetition frequency of 375 MHz was developed. The comb spacing was referenced to a Rb-stabilised microwave synthesiser and the comb offset was stabilised by generating a supercontinuum containing a coherent component at 780.2 nm, which was heterodyned with a 87Rb-stabilised external cavity diode laser to produce a radiofrequency beat used to actuate the carrier-envelope offset frequency of the Yb:fibre laser. The two-sample frequency deviation of the locked comb was 235 kHz for an averaging time of 50 seconds, and the comb remained locked for over 60 minutes with a root mean squared deviation of 236 kHz.

Optoelectronic integration on silicon is an area of increasing interest for both physicists and the microelectronics industry. Due to the limitations of silicon as an optical gain medium, the integration of III-Vs with silicon microelectronics has become a prominent area of research. However, the fundamental physical differences between these materials has caused such lasers to be strongly limited by non-radiative recombination. Studies of these mechanisms are therefore essential for solutions to be developed that will allow commercially viable III-V/Si lasers to be fabricated. This thesis presents such studies for three of the four leading approaches to producing III-V/Si lasers (quantum dots on silicon are not studied), with conclusions on the relative performance of each presented in the final chapter. AlGaInAs/InP laser active regions wafer bonded onto pre-processed silicon-on-insulator waveguides have exhibited strong performance, with electrical injection lasing demonstrated at room temperature. However, large and temperature sensitive threshold current densities of ~ 3.4-6.16 kAcm⁻² indicate that the devices are not yet optimised. Defect current fractions of 22-39% suggest that significant densities of threading dislocations propagate to the active region during bonding. In addition low T0 and T1 values, a lack of carrier density pinning and Z parameter values in excess of three suggest the presence of both carrier leakage and inter valence band absorption. Development of the GaNAsP active material, has allowed lattice matched, direct epitaxial growth on silicon. The polar/non-polar III-V/Si interface and thermal expansion coefficient mismatch however, cause large densities of defects to form. As such, up to 68% of carriers are found to recombine non-radiatively via defect states. Improvements to performance are achieved by the use of MQW structures and by optimising the silicon surface orientation to minimise the formation of anti-phase domains, leading to an 18% increase in radiative current fraction. However, defect densities remain large, with additional thermally activated defects potentially caused by the diffusion of nitrogen from the QW, forming defect states at the QW/barrier interface. These states may also form the carrier leakage path, responsible for up to 27% of recombination. Optimisation of the GaNAsP lasers by optical simulation predicts a potential increase in optical confinement factor from 0.35% to 0.6% and 1.33% to 1.73%, corresponding to reductions in threshold current of 41% and 27% for single and multiple quantum well structures, respectively. Poor electrical performance was investigated by SEM of the contacts. This also identified limitations to the lithography, etching and metalisation, which caused among other effects, the burning of contacts under electrical injection. A processing optimisation study eliminated the contact burning, improved the IV characteristics and increased the facet output power by almost an order of magnitude. The addition of a post metalisation annealing step was also found to reduce the p and n-type contact resistances by 64% and 20% respectively. A final method studied in this thesis is the use of GaInSb composite quantum wells grown on GaSb. III-Sbs have been demonstrated to accommodate significant strain as well as tending to prevent dislocations from propagating in the growth direction. Lasing at room temperature with a threshold current density of 426 Acm⁻² is observed, with a radiative current fraction of 41%. Carrier leakage is found to be the dominant source of loss, accounting for 39% of recombination. However, based on pressure dependence studies an increase in carrier confinement of only 20 meV may be enough to halve this. The remaining 20% of carriers are thought to recombine via Auger processes, particularly CHSH due to the small difference between the bandgap and the spin-orbit splitting.

Most ultra wideband (UWB) target detections are mainly carried out in the radiated near field or far field. However, the success of the detection mainly relies on the distance between the sensor and the target which may require a large measurement space. This research investigates the ability to sense targets in the reactive near field in case that the measurement space is constrained. Food detection in a smart fridge is chosen as the main application and test platform. At present, food can be detected by leveraging radio frequency identification (RFID) technology for intelligent fridge. Despite promising results have been shown, it may cause potential health risk and be costly due to tags being affixed to the food to obtain detailed information. Besides, RFID technology lacks of the ability to know the exact food amount such as the level of drink. There is therefore a need for developing new approaches being self content-aware in a low cost and reliable manner. Due to the nature of low cost, relatively high accuracy and immunity to noise, UWB technology provides the potential to detect food as an alternative to RFID. Egg quantity determination, which is an initial and accessible platform of intelligent fridge will be investigated in this thesis. Egg quantity can be well determined in terms of polarisation information in the far field region. However, the challenges arise by taking practical fridge size into account in which the information of eggs will be known in the reactive near field. New approaches are proposed based on investigation of reflection and coupling coefficient correlation of in fridge sensors. Both simulations and measurements are conducted to study the feasibility of sensing the number of egg in the free space environment. Further to this, the effect of other food placed around and above the egg box is investigated in order to verify the robustness of the proposed approaches. Finally, the study is extended to examine the capability of determination of liquid volume. In which, S-parameters are measured related to a variety type of drink in their unique topology and liquid level. The correlation coefficients are evaluated and analysed in both magnitude and phase domain exploiting the amount of liquid information that will be of great significant in the development of future smart fridge.

The purpose of this thesis is to determine whether hydrogen can be implanted into elec- tronic components for the goal of investigating low ionising dose rate sensitivity, and using this to suggest whether hydrogen implantation can be used as an accelerated method to detect ELDRS (Enhanced Low Dose Rate Sensitivity) susceptability. Current ground testing methods for total ionising dose irradiate using cobalt-60 at dose rates greater than 10mGy(Si)/s up to 200Gy. It has been found that bipolar devices show an increased susceptibility to radiation induced damage at dose rates below 10mGy(Si)/s known as ELDRS. Current research has linked ELDRS susceptibility with hydrogen content within the integrated circuit and experiments based upon hydrogen soaking de-lidded bipolar devices demonstrate this relationship, however this has not led to an accepted method for testing ELDRS susceptibility in previously un-tested devices. In this thesis, a novel proposal is put forward whereby bipolar devices are directly implanted with hydrogen using a targeted ion beam in order to accelerate the testing process. Hydrogen implantation via a 600keV ion beam has been achieved to a level of 10^17 H/cm^2 in Analog Device’s AD590KF temperature transducer, and 10^14-15 H/cm^2in National Semiconductor’s LM124 quad operational amplifiers. Devices were decapped, optically analysed, and targeted with a focussed proton beam. These devices were then irradiated at 15mGy/s, 5mGy/s and 15mGy/s. Increased degradation was seen at lower dose rates which was matched by high dose rate irradiation of the implanted devices followed by a room temperature anneal. The use of ion implantation for the development of an accelerated ELDRS test method is proposed. This thesis demonstrated that hydrogen can be succesfully implanted into devices, established an upper bound for the LM124 for implantation and a lower bound for hydrogen remaining in the target area and the effect of hydrogen implantation on the AD590 temperature transducer is discussed. This thesis concludes by suggesting hydrogen implantation as a method for use by manufacturers during the design and investigation of intrinsically ELDRS-free technologies.

Although difficult to notice initially, examples of bioinspired technology have now become commonplace in society today. Construction materials, aerodynamic transport design, photography equipment and robot technology are among many research fields which have benefitted from studying evolution-driven solutions to common engineering problems. One field of engineering research which has recently begun to take inspiration from the natural world is that of acoustical systems such as microphones and loudspeakers. Specifically, to solve the problems involved in the miniaturisation of these systems, the auditory organs of insects are inspiring new design strategies. In this thesis, one such insect auditory system, that of the desert locust Schistocerca gregaria, was extensively studied beginning with a comprehensive review of the historical observations of the system. Micro-scanning laser Doppler vibrometry was then used to characterise the response of the locust ear, providing an explanation for the method behind frequency discrimination in the ear. Afterwards, finite element models, simulating the ear's features, were constructed with a view to furthering the understanding of each component of the hearing system. Directionality of the locust hearing system was also briefly investigated through computational modelling. All of these studies were performed with the overall aim of feeding into the future design of bioinspired acoustic sensors. Devices constructed using micro-electro-mechanical systems fabrication techniques, with similar dimensions to the ear of the parasitoid fly, Ormia ochracea, were then experimentally tested using laser vibrometry and simulated using finite element analysis. Although not originally designed to operate as such, one MEMS structure exhibited some element of mechanical directionality in its response, found to be both predictable and repeatable. The objective of this section of the PhD research was to test the hypothesis that any system with sufficient degrees of freedom is capable of displaying an element of directionality in its vibrational response.

No description available.

A novel electronic nanodevice, the self-switching diode (SSD), is explored in this work. This includes exploration of its ability to operate as an ultra-high-speed detector at room temperature, its low-frequency noise properties, and its application to terahertz (THz) imaging. The SSDs have been realised using two novel nanolithography techniques, known as atomic-force microscope lithography and electron-beam lithography.The SSD is a unipolar two-terminal device. It has a nonlinear current-voltage characteristic which resembles the behaviour of a conventional diode. The planar structure of the SSD provides intrinsically low parasitic capacitance that enables signal rectification at higher speed than a standard vertical diode. It also allows the fabrication of a large number of SSDs in a single lithography step without the need for interconnection layers, which may introduce parasitic elements. Indeed, this is the key feature of the SSD that makes the whole fabrication process simpler, faster and lower cost, when compared with other conventional electronic nanodevices. By using large arrays of SSDs connected in parallel, the overall resistance of the devices can be reduced.A large SSD array, fabricated onto a two-dimensional electron gas (2DEG) in an InGaAs/InAlAs heterostructure material, has been defined within the fingers of an interdigital structure, located in the gaps of a coplanar waveguide. Despite of the large impedance mismatch between the SSD array and the measurement systems, the device successfully converted radio-frequency (RF) signals with frequencies up to 3 GHz (i.e., the highest frequency of the instruments used in the RF experiments) into usable DC power which may be employed in many RF applications. The obtained room-temperature results are matched very well with the theory.The development of the SSD-based THz detectors is a key objective of this work. The SSDs, coupled with either spiral or bow-tie antennas, have been fabricated onto a 2DEG in an AlGaAs/GaAs heterostructure material. Room-temperature detection of free-space radiation up to 1.5 THz using a free-electron laser has been achieved by the SSDs-based detector at unbiased condition. To the best knowledge of the author, this is the highest speed reported in room-temperature nanorectifiers to date.The first experimental study on low-frequency noise properties of the SSDs was also performed. The measurements were carried out at room and elevated temperatures using a two-channel cross-correlation technique. The noise performances of the SSDs, which are important in any detector, are discussed in terms of noise-equivalent power and corner frequency. Both parameters are comparable to those reported for state-of-the-art Schottky diodes. The observed noise in the SSDs is described using Hooge’s mobility fluctuation theory. Other properties extracted from the results obtained at elevated temperatures, such as activation energy, are also presented. Based on the excellent noise properties measured, an active THz imaging experiment using an SSD-based detector was carried out successfully. A low-cost blackbody radiation source (rather than a laser) was used as a continuous-wave THz generator. Several THz images of hidden objects (e.g., a USB connector underneath its plastic cover) have been obtained by means of raster scanning.

The field of nonlinear optics is now more than fifty years old, since the beginnings with the observation of second harmonic generation by Franken and co-workers in 1961. Since then a lot of progress has been made in this field which has many applications in frequency conversion devices and optical networks. My original contribution to knowledge in this field is the proposition of a natural quasi phase matching (NQPM) scheme, for efficient optical frequency conversion using second order nonlinearity in thin crystalline film waveguides and the development of the theory behind this scheme as an extension of the well-known quasi phase matching theory. The scheme is based on the periodic polarization inversion of the light field as it circulates in a whispering gallery (WG) micro-disc resonator. By placing the optical axis of the crystalline film material in the disc plane, this arrangement naturally produces the necessary 1t phase shift between the nonlinear polarization and the second order wave to compensate for the phase shift between the pump and harmonic waves, which is realised in conventional quasi phase matching schemes by periodically inverting the crystal domains in ferroelectric crystals such as Lithium Niobate (LN). I derive the coupled wave equations for second harmonic generation (SHG) in circular resonator structures and use these to describe the evolution of the second harmonic (SH) wave in the resonator and the efficiency of the frequency conversion. Lithium Niobate is used as an example material, for which a coefficient for the second harmonic generation in an x-cut resonator is calculated for the in-plane (TE) polarized guided mode. Optimization parameters are presented for enhancing the efficiency of the frequency conversion. A general model is derived with the use of temporal coupled mode theory which describes the second harmonic generation in any doubly resonant (at the fundamental and SH frequencies) resonator coupled with a pair of waveguides. Finally the procedure for designing such devices is presented where the conditions for NQPM are satisfied by choosing specific whispering gallery modes (WGM) for the fields and subsequently applying temperature tuning to ensure double resonance at the fundamental and SH frequencies is achieved. LN micro-disc resonators in which NQPM is feasible are designed.