David Whitehead

The influences of particle number on hot spots in strongly coupled metal nanoparticles chain.

In understanding of the hot spot phenomenon in single-molecule surface enhanced Raman scattering (SM-SERS), the electromagnetic field within the gaps of dimers (i.e., two particle systems) has attracted much interest as it provides significant field amplification over single isolated nanoparticles. In addition to the existing understanding of the dimer systems, we show in this paper that field enhancement within the gaps of a particle chain could maximize at a particle number N>2, due to the near-field coupled plasmon resonance of the chain. This particle number effect was theoretically observed for the gold (Au) nanoparticles chain but not for the silver (Ag) chain. We attribute the reason to the different behaviors of the dissipative damping of gold and silver in the visible wavelength range. The reported effect can be utilized to design effective gold substrate for SM-SERS applications.

Laser micro/nano fabrication in glass with tunable-focus particle lens array

Based on medium-tuned optical field enhancement effect around a self-assembled particle-lens array (PLA) irradiated with a femtosecond (fs) laser source, we demonstrated that high-precision periodical array of micro/nano-structures can be readily fabricated on glass surface or inside glass in large areas in parallel without any cracks or debris. The technique has potential for rapid fabrication of three-dimensional structures in multiple layers inside glass.

Large-area laser nano-texturing with user-defined patterns

Writing nano-sized features less than the diffraction limit of the lasers efficiently over a large area requires special technology development. This paper reports the use of a self-assembled particle lens array with near-field enhancement effect to write millions of nano-sized user-defined features, e.g. English letters, lines, curves, simultaneously by angular beam scanning. About a 5 mm × 5 mm area can be written with a single shot of a laser beam or few scans for up to 100 million identical features of nano or sub-micro scales. With the help of certain environmental conditions, such as the use of a suitable chemical solution in conjunction with the particle lens array, the characteristic of the features produced can be further controlled, including the generation of reversed (e.g. pits become hills and grooves become walls) features of laser-written patterns. The technical challenges, experimental findings and theoretical analysis/simulation are presented.

Generation of metal-oxide nanoparticles using continuous-wave fibre laser ablation in liquid

In recent years, laser ablation in liquid has become an increasingly important technique for the fabrication of nanoparticles (NPs). To date, only pulsed lasers have been used. This paper reports our recent studies on the generation of Ti-oxide and Ni-oxide NPs by the ablation of metal targets in aqueous environments using a high-power, high-brightness continuous-wave (cw) fibre laser at a wavelength of 1070 nm. Owing to the high and uniform irradiation, the fibre laser provides an alternative approach for NP generation with well-controlled phase, size and size distribution, along with high production rate. Characterization of the NPs, in terms of morphology, size and size distribution, chemical composition and phase structure, by means of high-resolution transmission electron microscopy (HRTEM), high-angle annular dark field (HAADF) in scanning-transmission (STEM) mode, energy-dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD), has been presented. In addition, limitations of the cw fibre laser process have been discussed in comparison with pulsed laser process.

Laser stripping of TiAlN coating to facilitate reuse of cutting tools

The potential of using the laser ablation process to perform controlled stripping of titanium aluminium nitrite (TiAlN) coating from tungsten carbide (WC) substrate is explored in this paper. TiAlN coatings are extensively used in cutting tools to improve machining capability, and to extend the life of the tools. However, if any error is detected on the coated tools, or when the tooling needs to be reused, it is mandatory to remove the existing coating to facilitate reshaping/recoating. The existing coating removal process uses chemical stripping methods, which are not environmentally friendly and not suitable for selective coating removal. In the present work, excimer laser removal of TiAlN from coated WC flat plates has been studied and demonstrated as a viable alternative to existing chemical stripping methods. The ablation thresholds of the TiAlN coating and WC substrate were identified as 1.85u2009J/cm2 and 2.3u2009J/cm2 respectively. The paper also presents experimental and theoretical evidence of the process mechanism responsible for laser stripping of TiAlN coatings

Effect of surface morphology changes of Ti-6Al-4V alloy modified by laser treatment on GDOES elemental depth profiles

An aerospace grade Ti-6Al-4V alloy, treated by an excimer laser, has been analysed by elemental depth profiling using glow discharge optical emission spectrometry (GDOES). Laser exposure may modify the surface morphology of the alloy due to melting and generation of microcracks. Melting reduces the roughness over an area of 1 × 1 μm, whereas the roughness over an area of 100 × 100 μm increases. The influence of such modification of the surface morphology on changes in the shapes of the GDOES elemental depth profiles is discussed.