Stuart Edwardson

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.

Some recent developments in two-and three-dimensional laser forming for 'macro' and 'micro' applications

Laser forming (LF) is an emerging laser process with potential for use in line bending or spatial forming of metal components and in the precise adjustment of novel miniature on-board actuators. The process is achieved by introducing thermal stresses into a work-piece by controlled irradiation with a focused laser beam. Unlike mechanical forming, there is no spring-back effect. A number of mechanisms have been identified to explain the thermo-mechanical behaviour in LF, accounting for various part geometries, laser process conditions and the complex interaction of many thermal and mechanical factors. Although these interactions are not yet fully understood, with increased knowledge of the process (in terms of theoretical models and control strategies) and the continued development of adaptive and/or predictive LF systems, the process offers significant potential value to industry. This review describes a number of recent developments and new techniques in two-and three-dimensional LF, for both macro-and micro-scale applications, and gives suggestions on the current industrial promise of the process. Macro-scale applications include rapid prototyping in industries that have relied on using expensive stamping dies and presses for design evaluations, while micro-scale applications already include the precision alignment of components during product assembly.

Dynamic modulation of spatially structured polarization fields for real-time control of ultrafast laser-material interactions

The polarization state of an ultrafast laser is dynamically controlled using two Spatial Light Modulators and additional waveplates. Consequently, four states of polarization, linear horizontal and vertical, radial and azimuthal, all with a ring intensity distribution, were dynamically switched at a frequency ν = 12.5 Hz while synchronized with a motion control system. This technique, demonstrated here for the first time, enables a remarkable level of real-time control of the properties of light waves and applied to real-time surface patterning, shows that highly controlled nanostructuring is possible. Laser ablation of Induced Periodic Surface Structures is used to directly verify the state of polarization at the focal plane.

Complete wavefront and polarization control for ultrashort-pulse laser microprocessing

We report on new developments in wavefront and polarization control for ultrashort-pulse laser microprocessing. We use two Spatial Light Modulators in combination to structure the optical fields of a picosecond-pulse laser beam, producing vortex wavefronts and radial or azimuthal polarization states. We also carry out the first demonstration of multiple first-order beams with vortex wavefronts and radial or azimuthal polarization states, produced using Computer Generated Holograms. The beams produced are used to nano-structure a highly polished metal surface. Laser Induced Periodic Surface Structures are observed and used to directly verify the state of polarization in the focal plane and help to characterize the optical properties of the setup.

Strain gauge analysis of laser forming

Laser forming has become a viable process for the shaping of metallic components, as a means of rapid prototyping and of adjusting and aligning. The laser forming process is of significant value to industries that previously relied on expensive stamping dies and presses for prototype evaluations. This investigation aims to complement the considerable amount of work already completed on two-dimensional laser forming, offering an insight into the mechanical behavior of a part during the process using a strain gauge analysis technique. The investigation was performed on mild steel CR4 sheet using a CO2 laser source. It includes empirical investigations to determine optimum processing parameters using the temperature gradient mechanism, thermocouple analysis to locate ideal strain gauge placement for temperature compensation, and strain gauge analysis of the transverse localized strains at a number of locations on the surface of the sheet during single and multipass laser forming. The results of the investiga...

Tailored optical vector fields for ultrashort-pulse laser induced complex surface plasmon structuring

Precise tailoring of optical vector beams is demonstrated, shaping their focal electric fields and used to create complex laser micro-patterning on a metal surface. A Spatial Light Modulator (SLM) and a micro-structured S-waveplate were integrated with a picosecond laser system and employed to structure the vector fields into radial and azimuthal polarizations with and without a vortex phase wavefront as well as superposition states. Imprinting Laser Induced Periodic Surface Structures (LIPSS) elucidates the detailed vector fields around the focal region. In addition to clear azimuthal and radial plasmon surface structures, unique, variable logarithmic spiral micro-structures with a pitch Λ ∼1μm, not observed previously, were imprinted on the surface, confirming unambiguously the complex 2D focal electric fields. We show clearly also how the Orbital Angular Momentum(OAM) associated with a helical wavefront induces rotation of vector fields along the optic axis of a focusing lens and confirmed by the observed surface micro-structures.

Dynamic distortion measurements during laser forming of Ti¿6Al¿4V and their comparison with a finite element model

Abstract Laser forming is, potentially, an attractive flexible manufacturing technique for the controlled forming of aerospace alloys. Laser forming experiments using a continuous-wave CO2 laser were performed on coupons of material 80 mm × 80mm in area and 2mm thick, with sequential passes of the laser beam, at a surface scanning rate of 20 mm/s with 90 s of convective cooling between passes. A novel surface profilometer that was specifically developed to operate under the conditions of high vibration and stray light typically found in laser machining applications recorded transient surface shape changes during individual laser passes at frame rates of 4 and 0.2 Hz. A finite element model was developed using ABAQUS for the laser forming of linear bends in free Ti-6A1-4V sheets, with sequentially coupled thermal and elastic-plastic analysis incorporating temperature-dependent material properties. Transient heat source scanning was implemented to simulate the experiment. Good agreement was found between the experimental three-dimensional shape data and those predicted by the transient model. In particular, the formation of an unwanted ‘camber’ distortion perpendicular to the desired main bend was correctly predicted; its magnitude and temporal evolution throughout the three laser passes, and during the periods of convective cooling, agreed well with the experimental data. The model and the shape measurement technique will enable the future predictive controlled laser forming of more complex three-dimensional shapes.

Ultrafast laser parallel microdrilling using multiple annular beams generated by a spatial light modulator

Ultrafast laser parallel microdrilling using diffractive multiple annular beam patterns is demonstrated in this paper. The annular beam was generated by diffractive axicon computer generated holograms (CGHs) using a spatial light modulator. The diameter of the annular beam can be easily adjusted by varying the radius of the smallest ring in the axicon. Multiple annular beams with arbitrary arrangement and multiple annular beam arrays were generated by superimposing an axicon CGH onto a grating and lenses algorithm calculated multi-beam CGH and a binary Dammann grating CGH, respectively. Microholes were drilled through a 0.03mm thick stainless steel foil using the multiple annular beams. By avoiding huge laser output attenuation and mechanical annular scanning, the processing is ∼200 times faster than the normal single beam processing.

Iterative 3D laser forming of continuous surfaces

There has been a considerable amount of work carried out on two-dimensional laser forming, using multi-pass straight line scan strategies to produce a reasonably controlled bend angle in a number of materials, including aerospace alloys. However in order to advance the process further for realistic forming applications and for straightening and aligning operations in a manufacturing industry it is necessary to consider larger scale controlled 3D laser forming. The work presented in this paper uses a predictive and adaptive approach to control the 3D laser forming of 1.5mm Mild Steel sheet into a desired continuous surface. The surface considered in the study was the pillow (or dome) shape. Key to the control of the process was the development of a predictive model to give scan strategies based on a required geometry. When the geometry is not formed within one pass, an incremental adaptive approach is used for subsequent passes, utilising the error between the current and desired geometry to give a new scan strategy, thus any unwanted distortion due to material variability can be accounted for. The forming rate and distribution of the magnitude of forming across the surface were controlled by the process speedThere has been a considerable amount of work carried out on two-dimensional laser forming, using multi-pass straight line scan strategies to produce a reasonably controlled bend angle in a number of materials, including aerospace alloys. However in order to advance the process further for realistic forming applications and for straightening and aligning operations in a manufacturing industry it is necessary to consider larger scale controlled 3D laser forming. The work presented in this paper uses a predictive and adaptive approach to control the 3D laser forming of 1.5mm Mild Steel sheet into a desired continuous surface. The surface considered in the study was the pillow (or dome) shape. Key to the control of the process was the development of a predictive model to give scan strategies based on a required geometry. When the geometry is not formed within one pass, an incremental adaptive approach is used for subsequent passes, utilising the error between the current and desired geometry to give a new sca...

Laser forming: overview of the controlling factors in the temperature gradient mechanism

Abstract Laser forming (LF) offers the industrial promise of controlled shaping of metallic and non-metallic components for prototyping, the correction of design shape or distortion, and precision adjustment applications. The potential process advantages include precise incremental adjustment, flexibility of application, and no mechanical ‘spring-back’ effect. To date, there has been a considerable amount of work carried out on two-dimensional LF, using multi-pass straight-line scan strategies to produce a reasonably controlled bend angle in a number of materials, including aerospace alloys. A key area, however, where there is a limited understanding, is the variation in the bend angle per pass during multi-pass temperature gradient mechanism-based LF along a single irradiation track, in particular, the decrease in the bend angle per pass after many irradiations for a given set of process parameters. Understanding this is essential if the process is to be fully controlled for a manufacturing environment. The research presented in this paper through empirical data and numerical simulation of the LF of sheet mild steel, Ti6Al4V and AA5251, by CO2 laser offers a novel coherent picture of the key influencing factors and at which point in the bend evolution each is dominant, which has not been presented before.