As the demand for micro-patterned parts getting bigger, the need for molds with micro/nano scaled patterns to duplicate
these parts effectively and economically is increasing ever so rapidly. Over the years, numerous attempts have been
made to fabricate these molds using various approaches such as lithography, FIB, laser ablation, and precision diamond
turning. Amongst these approaches, diamond turning is by far the most commonly used method to generate the micropatterned
rollers for roll-to-roll fabricating of precision optical parts such as BEF and 3D films. However, micro-burrs
are frequently produced during the micro-cutting process which not only makes the mold un-usable but also increases the
cost of machining. Efforts have been made to study the burr formation process during the micro-cutting by FEM
simulation, micro-scratching and diamond turning. Influences of the machining parameters such as rake angle, cutting
edge radius, included angle and cutting speed on the burr formation were systematically investigated. Array of micropyramids
with 90° apex angle, 40*40μm<sup>2</sup> basal area and minimised burr were successfully produced on a OFCu roller of
270mm in diameter. The results showed that (i) tool rake angle, included angle and cutting edge radius have profound
effect on burr formation and achievable surface finish, (ii) simulation can supply very useful information for setting the
machining parameters to suppress the burr formation during micro-cutting process.
As the demand for precision optical components with sub-millimeter feature size steadily increasing, numerous efforts have been made in developing new techniques and in improving the existing approaches to efficiently and economically produce those components. Glass molding process (GMP) is one of these methods to enable mass production of precision glass optical components in recent years. One of the key issues in GMP is precision mold insert fabrication. Since the mould are normally made of hard and brittle materials such as tungsten carbide (WC) and silicon carbide (SiC), precision diamond grinding is by far the principal choice used to machine the GMP mould. As the feature size of optical component gets smaller, the size of mould and grinding wheel used to fabricate the mould gets smaller too. This makes the grinding process a very time consuming and expensive task. This research aimed to improve the small mold fabrication processes by developing an effective way of producing small diamond wheels and in-process monitoring wheel profile. Diamond wheels of around 0.2mm to 0.5mm in diameter after truing and WC aspheric mold insert of form accuracy around 0.47μm were successfully produced in this research.
193nm-ArF excimer laser irradiation of amorphous silicon (a-Si) thin film has great potential on the production of poly-crystalline silicon (poly-Si) thin film transistor liquid crystal displays (TFT-LCDs). The main reason for applying poly-Si thin films instead of a-Si thin films on fabricating high performance electronics devices is due to its higher electron mobility, which is strongly influenced by the film's surface morphology, grain size, microstructure and defect density. The specimens used in this study have a 100 nm-thick a-Si thin films deposited on glass substrate by plasma enhanced chemical vapor deposition (PECVD) technique. The effects of annealing parameters, such as shot number, repetition rate and the fluence, on the surface morphology and recrystallization behaviour of a-Si thin films were investigated. The results showed that the threshold fluence of partial melting of a-Si thin films at 1Hz and single shot was around 150 mJ/cm<sup>2</sup>. Further increasing the fluence and shot numbers leads to the formation of microvoids trapped inside poly-Si film due to the evolution of hydrogen gas during the laser annealing process. The surface morphology formed during the recrystallization of amorphous Si thin films is found depending upon the laser fluence and shot numbers. The super-lateral grown (SLG) poly-Si grain can be obtained at the fluence around 190 mJ/cm<sup>2</sup>.
Diamond, having many advanced physical and mechanical properties, is one of the most important materials used in the mechanical, telecommunication and optoelectronic industry. However, high hardness value and extreme brittleness have made diamond extremely difficult to be machined by conventional mechanical grinding and polishing. In the present study, the microwave CVD method was employed to produce epitaxial diamond films on silicon single crystal. Laser ablation experiments were then conducted on the obtained diamond films. The underlying material removal mechanisms, microstructure of the machined surface and related machining conditions were also investigated. It was found that during the laser ablation, peaks of the diamond grains were removed mainly by the photo-thermal effects introduced by excimer laser. The diamond structures of the protruded diamond grains were transformed by the laser photonic energy into graphite, amorphous diamond and amorphous carbon which were removed by the subsequent laser shots. As the protruding peaks gradually removed from the surface the removal rate decreased. Surface roughness (Ra) was improved from above 1μm to around 0.1μm in few minutes time in this study. However, a scanning technique would be required if a large area was to be polished by laser and, as a consequence, it could be very time consuming.
Optical properties and material microstructures of three InGaN/GaN quantum well (QW) samples with various silicon-doping concentrations in barriers were measured. From the high-resolution transmission electron microscopy images, quantum dots (QDs) of a few nm in size were observed in silicon-doped samples. The regularities of QDs in size, shape and distribution increased with doping concentration up to 5 x 10<sup>18</sup> cm<sup>-3</sup>. Such observations implied that the reduction of quantum-confined Stark effect in such a sample was due to the relaxation of strain energy in QDs with silicon doping, besides the carrier screen effect. In other words, the microstructures were crucially changed with silicon doping in barriers. Also, the carrier localization effect was actually enhanced although potential fluctuation indeed became less randomly distributed. The calibrated radiative lifetimes in both silicon-doped samples showed the consistent trend of the formation of 0-D structure upon silicon doping.
We compared the results of optical characterization between five InGaN/GaN quantum well samples of different well widths. Temperature dependencies of photoluminescence (PL) spectral positions, integrated PL intensities, and PL intensity decay times at PL peaks of all the five samples showed three temperature ranges of different variation trends. The radiative efficiencies of the samples in the high temperature range had the same decay slope, which is supposed to be determined by the defect structures outside clusters. The radiative efficiencies in the medium temperature range varied among samples, indicating different defect structures in the regions between coupled clusters in different samples. Consistent results of temperature dependent variations between the integrated PL intensity and PL decay time among these samples provided clues for reasonable interpretations. Also, we showed the strong dependencies of thermal annealing effects on quantum well (QW) width in InGaN/GaN QW structures. Thermal annealing at 800 °C of a narrow QW width (2 nm) structure led to improved optical quality. However, thermal annealing at the same temperature of a sample of larger QW width (4 nm) resulted in degraded optical quality.
It is shown that post-growth thermal annealing of such a sample with temperature ranging from 800 to 900 °C led to a better confinement of indium rich clusters near InGaN quantum well layers. Transmission electron microcopy (TEM) and energy filter TEM results manifested that the sizes of indium-rich QDs were reduced with increasing annealing temperature. Also, the size homogeneity was improved. Quasi-regular arrays of indium-rich QDs embedded in InGaN quantum wells were observed in the sample of 900 °C annealing. X-ray diffraction also showed the enhancement of InN relative intensity. Photoluminescence measurements revealed blue shifts of photon emission spectral peak, indicating stronger quantum confinement after thermal annealing.
Periodical perturbations along an optical fiber can cause power coupling between the core mode and cladding modes for the applications of spectral filtering and derivative operations. Such perturbations can be generated through periodical loading on fiber. By applying loading onto fiber with two face-to-face, identical periodical groove structures, it was found that the long-period grating effects were dependent on the relative phase of the two periodical corrugations. Particularly, when the relative phase was zero (crest-to-crest), spectral filtering effects disappeared completely. The comparisons of such results between the cases of jacketed and un-jacketed fibers led to the conclusion that geometric deformation, instead of direct pressure-induced effect, was the dominating mechanism for generating spectral filtering functions in the double-sided loading configuration. The same conclusion can be applied to the single-sided loading device.