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Multispectral and hyperspectral imaging are efficient methods of measuring spectral reflectance at high spatial resolution. This non-invasive technique has been applied to the imaging of paintings over the last 20 years. PRISMS (Portable Remote Imaging System for Multispectral Scanning) was designed specifically for imaging wall paintings. Optical Coherence Tomography (OCT) is a low coherence interferometric technique capable of fast non-invasive imaging of subsurface microstructure. This paper shows the first application of in situ OCT imaging of a wall painting. The combination of PRISMS and OCT gives information on the varnish and paint layer structure, pigment identification, the state of degradation of the paint and varnish layers and informing curators on the painting schemes and techniques.

The analysis of entrapped debris provides a useful complementary method of investigating the laser ablation mechanism in laser processing of polycrystalline metal samples using a femtosecond laser (Clark MXR, CPA2001). Morphological investigations of the laser- processed areas, for a range of laser fluences and pulse number, were recorded using optical and scanning electron microscopies (SEM) and white light interferometry. Data obtained on ablation rates, ejected particle sizes, and crater morphologies prove that ablation changes from a smooth to an explosive process at high fluences, as identified with changes in the material removal mechanisms. The build-up of laser-induced mechanical stresses, due to the heating and cooling of the samples between successive laser shots, plays an important role in the material modification process, leading to the observed dependence of ablation threshold on shot number. The strength of the dependence is governed by the incubation coefficient, S, which has been measured for all materials studied. In this paper, additional insight is derived from the analysis of the debris generated for metal samples, which can be attributed to laser ablation mechanisms based on vaporization, spallation, phase explosion, and fragmentation.

The aim of the current work is two-fold: First, the aim is to investigate the transition, for a number of metals, from a smooth ablation process to an explosive one. Secondly we aim to study the dependence of the ablation threshold in metals on the applied laser shot number. Ablation of polycrystalline metal samples was performed with multiple pulses from a femtosecond laser (Clark MXR, CPA2001). Mor phological investigations of the laser processed areas were recorded using optical and scanning electron microscopies (SEM) and white light interferometry. The investigations have been carried out on sample matrices which were processed for a range of laser fluences and applied laser shots for four metals. Data obtained on ablation rates, ejected particle sizes and crater morphologies prove that ablation changes from a smooth to an explosive process at high fluences, as identified with changes in the material removal mechanisms. Threshold fluences were measured for both the smooth and explosive ablation processes. The ablation threshold fluence depends on the number of pulses applied to the same spot. It was found that the build up of laser induced mechanical stresses, due to the heating and cooling cycles of the samples between consecutive laser shots, plays an important role in the material modification process. It leads to the observed dependence of ablation threshold on shot number, which is described by a power law based on a mechanical fatigue model. The strength of the dependence is governed by the incubation coefficient, S, which has been measured for all materials studied. It is expected that the build up of laser energy or incubation leads to the accumulation of material defects and residual stresses which has the effect of lowering the energy required to cause ablation using a large number of incident laser shots. Keywords: femtosecond pulse laser, ablation threshold, material removal mechanisms, laser materials processing, metals. 1. INTRODUCTION Ultrashort pulsed laser ablation has become a very active area of research in recent years. It is definitely one of the most industrially driven technologies to have arisen from femtosecond laser applications because, through careful tailoring of the appropriate parameters, it allows practically any material to be processed with extremely high precision and with minimal collateral damage. By depositing the laser energy into the electrons of the material on a time-scale which is short compared with the transfer time of this energy to the bulk of the material, the ablation efficiency is improved and the ablation threshold is reduced and more well defined. The fundamental physical mechanisms of material removal during laser ablation are not clearly understood as yet

We present theoretical calculations and experimental measurements of silicon micromachining rates, efficiency of laser pulse utilization, and morphology changes under UV nanosecond pulses with intensities ranging from 0.5 GW/cm2 to 150 GW/cm2. Three distinct irradiance regimes are identified based on laser intensity. At low intensity, proper gas dynamics and ablation vapor plume kinetics are taken into account in our theoretical modeling. At medium high intensity, we incorporate the proper plasma dynamics, and predict the effects of the laser generated vapor plasma and the electron hole plasma on the laser-matter interaction. At even higher intensity, we attribute the observed increased ablation rate to energy re-radiation from the laser heated hot plasma, the strong shock wave, and the accompanied strong shock wave heating effects. Experimentally measured data in these regimes agree well with our calculations, without changing parameters in the calculations used for the three regimes. Our results can be applied toward quantitatively characterize the behavior of ablation results under different laser parameters to achieve optimal results for micromachining of slots and vias on silicon wafers.

The generation of debris is critical in the future application of laser technology in IC, MEMS, MOEMS manufacture. Re-deposition of debris is also critical in optimising throughput of multi-pass laser ablative processes. In this study, the debris formed in laser micromachining of wafer grade silicon is investigated. Details of the laser workstation, based on a UV DPSS laser, will be presented and the development of real time diagnostic capabilities and off-line techniques will then be described. A real time imaging capability has been used to monitor plasma and shock front propagation with nanosecond resolution. The detection system is also used to monitor spectral emission of debris and micron-sized particulate ejected from the silicon surface. Emission spectroscopy of the laser ablated silicon in the plasma show spectral features that are characteristic of atomic and molecular species on timescales of nanoseconds and microseconds, respectively, after the laser pulse. Off-line characterisation techniques have focused on investigating the distribution and chemical composition of entrapped particulate. A number of novel experimental configurations for particulate entrapment, both adjacent to and remote from the laser-ablated surface, will be described. EDX results indicate that debris generated in air is composed principally of oxygen and silicon. Additional SEM results indicate that the particulate size grows through aggregation and depends on the environment in which they are generated.

The focussed spot size of industrial laser beams is a critical processing parameter in most laser machining applications as it determines the machined feature size and the irradiance produced by the laser at the material interface. There are a number of standard methods available for accurately measuring and analysing the focussed spot. These methods often require expensive equipment that can be time consuming and difficult to set up in a production environment. This paper presents an investigation into a cost effective and straightforward method for the measurement of focussed laser spot sizes based on drilling of holes in mylar film. It can be shown that the slope of a plot of the square of the hole diameter versus the natural log of the laser pulse energy is equal to twice the square of the spot radius. A measure of the laser spot size can be calculated by generating laser-drilled holes at number of laser pulse energies. The practicality and accuracy of this method is investigated in this paper for a number of laser types including a diode pumped solid state laser (UV DPSS) operating at the third harmonic (355nm), a femtosecond laser and a flash lamp pumped Nd:YAG laser. A comparison between the measured results and the results generated with other available techniques is also presented.