Microactuators are active elements for microelectromechanical systems (MEMS), and are used to provide force to move the MEMS device in the way required. Therefore, their force characteristics, displacement capabilities and velocity ranges are of importance, and require to be investigated. Scratch drive actuators, thermal actuators, and comb drive actuators have separately been characterized, improved, and modified in this thesis. Asymmetric thermal actuators are current driven actuators that can generate relatively high force and can be operated bi-directionally. We have designed a novel structure that has a higher displacement than traditional asymmetric actuators. The increment of maximum displacement is about 20 % above the displacement of the ordinary asymmetric thermal actuators. Detailed electro - thermal heat dissipation and expansion - deflection mechanical analyses has been performed to advance the idea. Device prototypes have been designed and the fabricated devices were tested; the experimental results show a good match to the theoretical analysis. Scratch drive actuators (SDAs) have been modelled and characterized both on travel and force performances. Long linear travel SDAs have been designed and the fabricated devices characterized by using a high-speed camera. Detailed motion has been recorded. A theory for flexing of SDA plates has been developed. Voltage - step size relation has been obtained by theoretical analysis. Theoretical and experimental results have then been compared. Force characteristics of single and multi-plate SDAs driven by different voltages have been measured by means of micro box springs. The SDA typical step size has been measured to be 7 nm at 60 V and 100 HZ driving condition. Typically, a 4 stage SDA driven with 200 Volts produces a force of 850 µN. Comb drive actuators are commonly used in resonators, which need high Q factors. However in some applications, such as optical choppers, they require low Q factors so that they can be operated over a large frequency range. We have designed a comb drive actuator with spring that can be operated from a few Hz up to 5 kHz. The static and dynamic testing and theoretical analysis have been undertaken in this thesis. A variable optical attenuator (VOA) has been designed, and fabricated by surface micromachining using PolyMUMPs (Polysilicon Multi-User MEMS Processes) foundry process. The principle is simply interrupting the light beam by a vertical microshutter. An array of SDAs have been used to drive the microshutters. Microhinges are employed to build the vertical microshutter. Stress-induced beams are used to self-assemble the microshutter. Optical simulation of the VOA has been performed using Rayleigh-Sommerfield diffraction theory. Devices have also been tested with single mode optical fibres. Testing results show a dynamic range of 34.2 dB. The VOA structure was driven at a speed of 1.6 µm/s at 150 volts and 100 Hz driving condition. An optical chopper has been realized by a pair of comb drive actuators driving two shutters. Two shutters are employed to double the response time. The device has been designed and fabricated using SOIMUMPs (Silicon On Insulator Multi-User MEMS Processes) foundry process. Completely design, simulation and testing of the chopper has been undertaken. The attenuation range of chopper has been measured to be from - 1.4 dB to - 29 dB. The response time of the chopper has been measured to be 90 µs.

The GaInAsSbP pentanary system has been utilised to grow epilayers on InAs substrates using Liquid Phase Epitaxy, and used to form the basis of. optoelectronic devices in the technologically important Mm spectral range (3-5 μm). The photoluminescence spectra of a single epilayer confirmed that the dominant radiative recombination mechanism was band-to-band in the pentanary layer. XRD analysis indicated the epilayers did not suffer from spinodal decomposition, and SEM and SIMS confirmed the layers were flat and abrupt. Raman spectroscopy was carried out over a wide range of lattice-matched InAsSbP compositions for the first time, before a further study on GaInAsSbP. Binary-like optical phonon signals were identified, and their position was found to directly relate to the composition of the alloy. Phonon signals resulting from alloy disorder were identified in the Raman spectra, which provides a valuable tool for future work on determining crystal quality. Prototype mesa diode devices were fabricated using wet etching with the addition of an InAsSbP window layer. Uncooled photodetectors were found to operate at room temperature, limited by diffusion current. Thermophotovoltaic cells using the same structure, designed for use with comparatively low temperature heat sources, were found to have a 33% fill factor. This is the first report of a pentanary alloy used for such an application. The corresponding photoresponse spectra exhibited two peaks, attributed to recombination in both the window layer and active region. Room temperature LEDs were demonstrated, operating with a 50% duty cycle, with their emission peaking at ~3.75 μm. The analysis of the excitation dependent electroluminescence allowed the electron effective mass of 0.018 mo to be calculated for the GaInAsSbP alloy. The devices were found to be limited by CHCC Auger recombination, even though the CHSH mechanism was suppressed by increasing the spin-orbit split-off band, as confirmed by high pressure measurements. The bandgap dependence of GalnAsSbP on pressure was found to be 10.7 meV/khar, which is believed to be the first such investigation for a III-v pentanary alloy. Multi-ring structures v/ere fabricated and current crowding effects were investigated. It was found that by employing multiple rings, rather than spot contacts, there was an improvement in the current spreading. and hence the output of the device. When only the outer-most contact was energised the current crowding under the contact was sufficient to facilitate whispering gallery modes. Lasing was achieved at 4K with drive currents of >300 mA, peaking at 3.3 μm.

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With many interesting electronic and optical properties, conjugated molecules have long been a subject for both theoretical and experimental studies, especially in recent applications in optoelectronics. Most of the properties are controlled by their electronic structures, with the low-lying states predominantly characterised by the 7!'-electrons. We have thus employed the semi-empirical Pariser-Parr-Pople model, whose Hamiltonian parameters are optimised against the excited states, in this study. Despite only considering the π-delocalised system, the model has been widely and successfully applied to study such molecules, giving fairly accurate results. The calculation of these strongly correlated systems requires an accurate computational method, which is usually limited to small molecules due to an enormous amount of resources demanded, as in ab initio calculations. Density Matrix Renormalisation Group (DMRG) provides a systematic way to truncate the size of the Hilbert space while being able to reasonably capture the correlation effect. We have compared three different approaches using both localised and delocalised basis states in the DMRG calculations. First, in the real-space method, atomic sites are aligned on the one-dimensional DMRG lattice as originally developed to study the fermionic systems in condensed matter physics. Alternatively, a collection of atoms, e.g. a monomer unit in conjugated polymers, can be used where an in situ optimisation technique is employed to pre-select the important states. Last, the correlation effect between the Hartree-Fock (HF) molecular orbitals on the lattice sites is calculated in the HF -DMRG scheme. In this post-HF method, the energy converges more slowly than the first but the implementation of t he codes is generic to any geometric structures. In addition to the spin-flip and particle-hole symmetries, which are available in the real-space method, the spatial symmetries can also be applied to target specific states when working in HF space. Calculations have been made on conjugated polymers, i.e. polyacetylene and poly(parophenylene), and aromatic compounds, i.e. pentacene and porphin. The results are compared to other methods, e.g. exact diagonalisation and Configurational Interaction Singles, as well as other theoretical and experimental results in the literature

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