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A detailed study has been conducted into the effect of laser process parameters on the production of laser induced periodic surface structures in 316L stainless steel using a pulsed picosecond fibre laser. Periodic structures were produced via scanning of the incident beam at varying speeds in order to control the pulse overlap. Feature depth and uniformity were found to rely heavily upon both the scan speed and pulse energy. Inhomogeneous feature production across the track cross-section was observed and attributed to the Gaussian nature of the incident beam. Features were observed oriented perpendicularly to the direction of polarization for linearly polarized light with a period approximately equal to the incident light wavelength. This period was observed to alter with incident angle and a dependency on laser scan direction was also found. A threshold intensity value for the production of laser induced surface ripples was calculated from a Gaussian analysis of measured track width. The threshold value when machining at 200 kHz, 750 mm/s, and 2 µJ was calculated to be 2.35 x 1014 W/m². [ABSTRACT FROM AUTHOR]

Two-dimensional (2-D) laser forming (LF) can currently control bend angle, with reasonably accurate results, in various materials including aerospace alloys. However, this is a different situation for three-dimensional (3-D) LF. To advance this process further for realistic forming applications and for straightening and aligning operations in a manufacturing industry it is necessary to consider larger scale controlled 3-D LF. The work presented in this paper uses a predictive and adaptive approach to control the LF of mild steel and aluminium sheet into a desired surface. Key to the control of the process was the development of a predictive model to give scan strategies based on a required geometry and the surface error. The forming rate and distribution of the magnitude of forming across the surface were controlled in the closed loop by the process speed. 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 and distortion control and removal is possible. [ABSTRACT FROM AUTHOR]

Laser forming offers the industrial promise of controlled shaping of metallic and non-metallic components for prototyping, correction of design shape or distortion and precision adjustment applications. In order to fulfil this promise in a manufacturing environment the process must have a high degree of controllability, which can be achieved through a better understanding of its underlying mechanisms. One area of limited understanding is that of the variation in bend angle per pass observed during multi-pass laser forming along a single irradiation track, notably the decrease in bend angle per pass after many irradiations. Finite element (FE) modelling can be used to ascertain which of the various process parameters (such as graphite burn-off, geometrical effects, variation in absorption, etc.) contribute towards this phenomenon and subsequently the magnitude of their contribution. [ABSTRACT FROM AUTHOR]

Spatial light modulator addressed with computer generated holograms is used to diffract femtosecond laser pulses into > 15 parallel beams and focused simultaneously inside poly(methyl methacrylate) (PMMA) for high throughput 3D refractive index modification. Uniform modification throughout the structure is demonstrated and used to create a 19 μm pitch, 5 × 5 × 3.5 mm³ thick volume grating in < 50 minutes with first order Bragg diffraction efficiency > 65%. Thick volume phase gratings are created by carefully stitching filamentary modifications. PMMA shows a remarkable time dependent Δn, related to the photochemistry during and after exposure. [ABSTRACT FROM AUTHOR]

Particulate silver inks are finding increasing use in the direct manufacture of electronic circuitry, for example in Direct Write (DW). Laser curing of these inks as opposed to traditional oven-curing methods is advantageous for reasons of speed and the protection of nearby heat-sensitive components, and particularly for the construction of circuitry on conformal surfaces and 'on the fly' modification or production. To fully appreciate the benefits provided by using a laser-based process, it is important that the final-cured component has electrical and physical properties that are comparable to, if not better than, those that are produced using the conventional technique. Currently, in some single pass curing techniques, cavities are formed in the track due to rapid boiling off of solvents that leads to a higher resistivity and surface roughness than that of oven-cured tracks. While mass loss is similar, overall density is decreased through porosity of the tracks. By going from a single pass curing process to a multiple pass process and controlling the power regime, it is possible to improve the track quality by allowing the gradual release of solvent vapour from the ink and achieve lower resistivities than that of oven-cured samples and a surface roughness that is comparable. This incremental process improves settling of the silver particulates leading to a resistance of the cured inks that is lower than that achievable through oven curing. This creates the possibility of fabricating a surface-mounted or -embedded damage sensor without thermally affecting the substrate. The effect of substrate type thermal conductivity on the laser curing speed of silver inks is investigated. It was found that a noticeable factor in determining the resulting resistance of the conductive track was the substrate thermal conductivity. Both CO2 and Er:YAG laser radiation was investigated. Although CO2 lasers are in widespread use, the increasing demand for portability and ease of integration into existing DW systems favours fibre delivered laser radiation such as that from an Er:YAG fibre laser. This source was also investigated here to determine if fibre delivered processing could be achieved. [ABSTRACT FROM AUTHOR]

Utilizing wireless data buses in aircraft has the potential to increase reliability, reduce weight, and improve flexibility and adaptability. In this article, the implementation of wirelessly connected interconnects in aircraft is introduced from a broad perspective, considering the issues of radio systems, propagation, antennas, and applications. Existing avionics systems are discussed briefly in order to understand the requirements placed on a wireless system and the specification it must meet. A review of existing wireless protocols and commercial off the shelf systems is provided, explaining where systems including ZigBee and IEEE 802.11 are suitable and unsuitable for a wireless aircraft application, and it is found that no existing system can be used exclusively. It is shown quantitatively that the propagation environment is expected to have high delay spreads and the negative impact of this on system reliability is shown through simulation and measurements. The application and benefits of a direct write, in-situ, metallic deposition procedure are discussed for the placement of antennas, sensors, and their associated electronics. By writing electronics onto a surface, fitting and retrofitting are made much easier without the need to affect the aircraft structure, and this enables a more flexible wireless system. The opportunities, advantages and challenges for such a technology within the wireless aircraft are discussed. [ABSTRACT FROM AUTHOR]

The use of laser energy to ignite gas and liquid based fuel-air mixtures has been the subject of a number of studies and laboratory experiments at a fundamental level over the past 30 years. Yet, the practical implementation of this laser application has still to be fully realised in a commercial automotive application. Laser Ignition (LI), as a replacement for Spark Ignition (SI) in the internal combustion (IC) engines of automotive vehicles, offers several potential advantages including extending lean burn capability, reducing the cyclic variants between combustion cycles and reducing the overall ignition package costs, weight and energy requirements. The continued development of increasingly compact and efficient laser sources and new associated laser beam delivery techniques have provided the basis for significant steps forward in research towards practical proof-of-concept demonstration of LI in engines for automotive vehicles. This paper reports on some results of research recently undertaken in the Department of Engineering, University of Liverpool, in which a Q-switched Nd:YAG laser operating at 1064 nm wavelength has been used to successfully ignite and run (for extended periods) one cylinder of a 4-cylinder internal combustion (IC) test engine. The variation of several laser parameters and their effect on the engine performance are reported; namely, pulse energies of 5–20 m J, pulse lengths of 6–15 ns and focused beam waist diameters at the combustion point of 40–100/μm. The engine performance was measured in terms of changes in Coefficient of Variant (COV) in both Indicated Mean Effective Pressure (IMEP) and the Peak Cylinder Pressure Position (PPP). Further experiments on the focal position of laser ignition were undertaken. [ABSTRACT FROM AUTHOR]

High speed patterning of a 30 nm thick Aluminium thin film on a flexible Polyethylene Terephthalate substrate was demonstrated with the aid of Computer Generated Holograms (CGH׳s) applied to a phase only Spatial Light Modulator. Low fluence picosecond laser pulses minimise thermal damage to the sensitive substrate and thus clean, single and multi-beam, front side thin film removal is achieved with good edge quality. Interestingly, rear side ablation shows significant Al film delamination. Measured front and rear side ablation thresholds were F th =0.20±0.01 J cm −2 and F th =0.15±0.01 J cm −2 respectively. With laser repetition rate of 200 kHz and 8 diffractive spots, a film removal rate of R >0.5 cm 2 s −1 was demonstrated during patterning with a fixed CGH and 5 W average laser power. The effective laser repetition rate was f eff ~1.3 MHz. The application of 30 stored CGH׳s switching up to 10 Hz was also synchronised with motion control, allowing dynamic large area multi-beam patterning which however, slows micro-fabrication. [ABSTRACT FROM AUTHOR]

Abstract: The Laser Assisted Direct Write (LADW) method can be used to generate electrical circuitry on a substrate by depositing metallic ink and curing the ink thermally with a laser. Compared to conventional oven curing, laser curing can be used in-situ, over complicated 3D contours, and selectively cure over heat sensitive substrates. The intensity distribution of the laser beam used has significant influence on many surface heating processes. A conventional Gaussian beam would lead to uneven heating across the beam path. To overcome this issue, Wellburn has designed a novel beam shaping device, which allows one to convert a Gaussian beam profile into an annular ring with a variable level of central plateau fill [1]. Shang et al. have developed and verified a predictive Finite Element Method (FEM) model to simulate the laser curing process for a generic combination of processing parameters. Laser modes can be flexibly varied within this model. Hence it can be used to predict the performance of curing with differently intensity distributions. This study investigates the curing process under different intensity profiles obtained with the beam shaping device. Electrical resistivity and scratch resistance of cured samples are tested as measures of the curing quality achieved. Results obtained are then compared with the FEM model predictions, and indicate the best intensity distribution for this particular application. [Copyright &y& Elsevier]

Parallel beam frequency doubling of 170 fs, NIR pulses is demonstrated by placing a thin beta barium borate (BBO) nonlinear crystal after a spatial light modulator. Computer-generated holograms applied to the spatial light modulator create 18 parallel diffracted beams at the fundamental wavelength λ=775 nm, then frequency doubled to λ=387 nm and focussed inside the poly(methyl methacrylate) (PMMA) substrate for refractive index structuring. This procedure, demonstrated for the first time in PMMA, requires careful attention to phase matching of multiple beams and opens up dynamic parallel processing at UV wavelengths where nematic liquid crystal devices are more sensitive to optical damage. By overlapping filamentary modifications, an efficient, stable volume phase grating with dimensions 5×5×2.0 mm and pitch Λ=15 μm was fabricated in 18 minutes and reached a first-order diffraction efficiency of 70 % at the Bragg angle. [ABSTRACT FROM AUTHOR]

The use of a fast-response, transmissive, ferroelectric liquid-crystal device for real-time control of the polarisation direction of a femtosecond laser beam, and the benefits for various aspects of ultra-short pulse micro-machining, are discussed. Several configurations have been used to drive the polarisation in real-time. Our microscopic investigations of the resulting features revealed significant improvements in process efficiency and quality, compared to static linear and circular polarisations. Following our successful micro-machining tests, real-time polarisation control could emerge as a powerful tool in laser engineering. [ABSTRACT FROM AUTHOR]

Laser ablation of a solid target material in a liquid environment provides with an easy, straightforward and environmentally friendly method for nanoparticles synthesis as well as with the unique possibility of directly controlling the type of the nanoparticles surface ligands through the liquid choice. In this paper, laser ablation (10.4 ps, 1064 nm and 50 kHz) of a bulk silver target in deionized water, was carried out for nanoparticles synthesis. The synthesised nanoparticles are either pure Ag or AO or a mixture of the two materials. Their size distribution follows log-normal function with a statistical median diameter of ≈5 nm. The nanoparticles colloidal solutions were directly mixed after synthesis, with the polymer solution poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) for the formation of polymer-nanoparticles nanocomposite. The nanoparticles readily form bonds with the sulphur atom of PEDOT which results in their uniform distribution within the polymer matrix as well as in a replacement by the nanoparticles of the PSS as the counteranions to the PEDOT. These effects result in the reduction of the effective insulation of the polymer blend particles by the insulating PSS and furthermore in the electrical conductivity of the nanocomposite becoming higher (by ∼1.5 times) as compared with that of the pure polymer. [ABSTRACT FROM AUTHOR]

Abstract: Picosecond pulsed laser (10.4ps, 1064nm, 5 and 50kHz) patterning studies were performed, of PEDOT:PSS thin films of varying thickness deposited by spin coating on glass substrates, by ablating the films or by changing locally by laser irradiation the optical and electrical properties of the polymer. From a detailed observation of the morphology of single pulse ablated holes on the surfaces of the films, in combination with simple calculations, it is concluded that photomechanical ablation is the likely ablation mechanism of the films. The single pulse ablation thresholds were measured equal to 0.13–0.18J/cm2 for films with thicknesses in the region of ∼100–600nm. The implications on ablation line patterning of the films using different fluences, scanning speeds and pulse repetition rates, were investigated systematically. Laser irradiation of the films before ablation induces a metal–insulator transition of the polymer because of the formation of charge localization due to a possible creation of molecular disorder in the polymer and shortening of its conjugation length. [Copyright &y& Elsevier]

Parallel femtosecond refractive index laser inscription of clinical grade poly(methyl methacrylate) (PMMA) at 775 nm, 170 fs pulselength is demonstrated with multiple low fluence beams generated with the aid of a spatial light modulator. Using optimised computer-generated holograms (CGHs), 16 diffracted near identical beams were focused simultaneously within bulk PMMA to create a series of 19 μm pitch, 5 mm×5 mm×1-4 mm thick volume phase gratings at high speed. First order diffraction efficiency rises with grating thickness in accord with diffraction theory, reaching 75% at the first Bragg angle (4 mm thick) with fabrication time around 1 hour. By carefully stitching filamentary modifications while eliminating effects such as pulse front tilt during inscription, gratings exhibit high uniformity, which has not been achieved previously using femtosecond inscription. Highly uniform modification is exhibited throughout the material consistent with the observed excellent angular selectivity and low background scatter and quantitative comparison with first order diffraction theory is satisfactory. The diffraction efficiency and hence refractive index profile shows a temporal behaviour related to the material response after exposure. Simultaneous 3D modification at different depths is also demonstrated, highlighting the potential of creating complex 3D integrated optical circuits at high speed through the application of CGHs. [ABSTRACT FROM AUTHOR]

Abstract: Laser ablation of a solid target material in a liquid offers a direct, quick and easy method for nanoparticles synthesis. Titanium monoxide (and especially in the disordered cubic phase) is a material difficult to synthesize because it requires long annealing times or quenching from high temperatures and it is usually formed in the form of irregular shape chunks. In this Letter we report the synthesis of titanium monoxide nanoparticles (disordered phase) by laser ablation of a bulk titanium metal target in deionized water (picosecond laser source: 10.4ps, 1064nm, 50kHz). Nanoparticles with sizes from ∼2 up to ∼200nm were observed to be formed in the colloidal solutions. Their size distribution follows log-normal function with statistical median diameter of ≈10nm. The synthesized nanoparticles exhibit an optical absorption edge which extends in the visible range of the spectrum (at ∼600nm, energies ∼2eV). [ABSTRACT FROM AUTHOR]

Silicon nanoparticles were generated by femtosecond laser [387 nm, 180 fs, 1 kHz, pulse energy = 3.5 μJ (fluence = 0.8 J/cm2)] ablation of silicon in deionized water. Nanoparticles with diameters from ~5 up to ~200 nm were observed to be formed in the colloidal solution. Their size distribution follows log-normal function with statistical median diameter of ≈20 nm. Longer ablation time leads to a narrowing of the nanoparticle size distribution due to the interaction of the ablating laser beam with the produced nanoparticles. Raman spectroscopy measurements confirm that the nanoparticles exhibit phonon quantum confinement effects and indicate that under the present conditions of ablation they are partially amorphous. [ABSTRACT FROM AUTHOR]

Picosecond laser (10.4 ps, 1064 nm) ablation of the nickel-based superalloy C263 is investigated at different pulse repetition rates (5, 10, 20, and 50 kHz). The two ablation regimes corresponding to ablation dominated by the optical penetration depth at low fluences and of the electron thermal diffusion length at high fluences are clearly identified from the change of the surface morphology of single pulse ablated craters (dimples) with fluence. The two corresponding thresholds were measured as F=0.68±0.02 J/cm2 and F=2.64±0.27 J/cm2 from data of the crater diameters D1,2 versus peak fluence. The surface morphology of macroscopic areas processed with a scanning laser beam at different fluences is characterised by ripples at low fluences. As the fluence increases, randomly distributed areas among the ripples are formed which appear featureless due to melting and joining of the ripples while at high fluences the whole irradiated surface becomes grainy due to melting, splashing of the melt and subsequent resolidification. The throughput of ablation becomes maximal when machining at high pulse repetition rates and with a relatively low fluence, while at the same time the surface roughness is kept low. [ABSTRACT FROM AUTHOR]

Abstract: Laser ablation of a solid target in a liquid environment offers an easy, fast, and environmentally friendly method for the generation of nanoparticles with desired properties. In this Letter we report modification of the electrical properties of PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)) by the incorporation into it of ZnO nanoparticles which were synthesized by laser ablation. By forming the nanocomposite, change of the chemical structure of the polymer from a mixture of benzoid and quinoid to a mostly quinoid and a conformational change of the polymer chains from coil to expanded-coil or linear was observed. Furthermore, these changes result in an increase by almost twice of the electrical conductivity of the polymer. [Copyright &y& Elsevier]

Abstract: 180 femtoseconds (1kHz) and 10 picoseconds (1–50kHz) ultrafast laser micro-structuring of the metals Ti alloy, Al and Cu have been studied for the purpose of industrial application. The effects of some key laser operating parameters were investigated. The evolution of surface morphology revealed that laser pulses overlap in a range around the spatial FWHM can help to achieve optimal residual surface roughness. While observed ablation rate (unit: μm3 per pulse) changed dramatically with repetition rate due to the combined effects of plasma absorption, residual thermal energy and phase transition, higher throughput can be achieved with higher repetition rate. This study also indicated that residual surface roughness is almost independent of repetition rate at 10ps temporal pulse length. The ablation depth is approximately proportional to the number of overscan; however, machining accuracy deteriorates, especially for femtosecond laser processing and metals with low thermal conductivity and short electron–phonon coupling time. [Copyright &y& Elsevier]