Laser Cleaning II

The following sections are included:

• LIST OF CONTRIBUTORS

• PREFACE

• TRIBUTE TO PROFESSOR BORIS LUK'YANCHUK: TO MARK HIS CONTRIBUTIONS TO PHYSICS ON THE OCCASION OF HIS 60TH BIRTHDAY

• ACKNOWLEDGEMENTS

• CONTENTS

This article gives an overview of the fundamentals of laser-matter interactions with regard to the cleaning of solid surfaces from either tiny particulates or extended contamination layers. Various different applications and limitations of the technique are discussed together with surface modifications that open up completely new possibilities in nano- and biotechnology.

An overview of the literature on laser cleaning experiments is provided in a tabulated form. The details of a large cross-section of experimental studies on the removal of particles and contaminants from various material surfaces by pulsed laser cleaning is provided and discussed. The tables aim to provide a useful, clear and quick comparison of the numerous different sets of experimental parameters used in laser cleaning studies, and to enable a better understanding of the removal of particulates from surfaces.

A laser irradiated particle on a surface produces a complicated intensity distribution on the substrate. In the case of a transparent particle, and normal incidence of radiation, the intensity has near-field focusing near the center and additional ring(s) around the basic maximum. This ring of intensity produces a cylindrical convergent surface acoustic wave, which can enhance the particle removal. Another effect is discussed for metallic particles. The intensity on the surface under a metallic particle is typically diminished due to the screening effect, in contrast to transparent particles, which act as a near-field lens. Nevertheless, if one uses radiation frequencies near the surface plasmon resonance, the conditions for efficient coupling of radiation with a metallic surface can be provided. This plasmonic effect can permit laser cleaning of metallic nanoparticles from a silicon surface.

The field enhancement by spherical particles plays a fundamental role in nanopatterning and in dry laser cleaning where such particles are used as model contaminants. We describe the general vectorial axially symmetric and strongly aberrated focusing by matching the solution of geometrical optics with a wave field built by the integral canonical for this topology, i.e., the Bessoid integral. For the focusing by transparent spheres with a few wavelengths in diameter, the results are much more compact and intuitive than the Mie theory. The results of the latter are reproduced with good agreement down to Mie parameters of about 30. Analytical expressions for the intensity on the axis and the position of the diffraction focus are derived.

The laser-shock cleaning (LSC) method based on the photomechanical effect of a high-power laser pulse has been shown to be effective for removal of nanoscale contaminants from solid surfaces. In the dry LSC technique, laser-induced breakdown of the ambient gas creates a strong shock wave, and impingement of this shock wave onto a solid surface removes submicron-sized particles from the surface. However, removal of nanoscale contaminants is still a challenging task and it is important to develop cleaning methods effective for particulates less than 100 nm in size. In this work, a novel method entitled "liquid-assisted laser shock cleaning (LLSC)" is proposed for removal of nanoscale particles that cannot be efficiently removed by conventional laser cleaning techniques. The LLSC process utilizes the combined effect of the laser-induced shock impingement and explosive liquid-film evaporation. The key to the technique is synchronizing the onset of bubble nucleation and the timing of shock-wave collision with the solid surface. Both the pressure force created in the explosive vaporization of the liquid film and the synchronized impact by the shock wave in the ambient air increase the cleaning force to detach the bound particles which could not be removed otherwise. It is shown that the novel process leads to substantial enhancement of particle-removal efficiency, compared with the conventional laser cleaning methods.

The 'clean' surface of silica glass is usually covered with a quasi-layer of hydroxyl groups. These groups are significant as their concentration on a surface affects surface adhesion and chemical reactivity. Removal of hydroxyl groups from the surface by a UV pulsed laser treatment has been demonstrated to be an alternative technique to the dehydroxylation of glass by the traditional oven heat treatment. Silica so treated has improved resistance to particulate adhesion. Dehydroxylation using this UV laser treatment has key advantages of being: a much faster process; largely limited to heating the surface not the bulk of the silica; and which allows selective spatial patterning of the dehydroxylation of the silica surface. This work outlines a technique developed to allow systematic, quantitative measurements of the dehydroxylation of UV fused silica. The removal of hydroxyl groups using laser irradiation is shown to be a thermal process.

Laser cleaning of 5.0 μn diameter silica spheres from silica surfaces was investigated. It was found that for three different slide preparation methods that the particles did not adhere strongly to the silica substrate. Experimental laser cleaning particle removal thresholds were 4 orders of magnitude less than that predicted by theoretical modeling based on thermal expansion and Van der Waals adhesion. There are two reasons put forward to explain this discrepancy. The first is that there are other forces acting on the particle besides the Van der Waals force. This is supported both by the present study which found that silica spheres did not adhere strongly to silica slides for three different preparation methods and by previous atomic force microscope (AFM) and surface force apparatus (SFA) studies which showed the existence of a short range repulsive force between silica spheres and silica substrates. The second explanation is that the spheres have a greater effective absorption coefficient than that of bulk silica. This was supported by the fact that at high fluences small hillocks of silica remained after irradiation indicating that temperatures in excess of 1900 K had been achieved. Both factors taken together can account for the discrepancy between theoretical predictions and experimental results in the present study.

Prior research in the field of laser cleaning has suggested that shorter pulses are preferable to achieve low laser cleaning threshold fluences. These predictions were based mainly on exponential pulse shapes. In the present work the three-dimensional model of laser cleaning developed by Boris Luk'yanchuk (B. S. Luk'yanchuk et al., Appl Phys A, 77, 2, 209) which accounts for near-field focussing, has been used to calculate the laser cleaning threshold hence for three different pulse shapes. These were rectangular, sinusoidal and exponential. For each pulse shape, the threshold fluence was determined as a function ofpulse width (1- 200 ns) and height (1-15 GW/cm²).

It was found that the threshold fluence is strongly dependent on the laser pulse shape, particularly for pulses greater than 100 ns in width. The threshold fluence of the rectangular pulse oscillated with a period equal to that of the period of oscillation of the particle on the substrate. In contrast, for both the exponential and sinusoidal pulses, the threshold fluence increases monotonically with pulse length.

Sigismund Chapel of the Krakow Castle was built in 1525 by Bartholo Berecci. The latest total renovation was executed using laser ablation in order to reconstruct the original appearance of the chapel. Sandstone and gypsum samples from Sigismund Chapel were cleaned using the 'ReNOVALaser 5' system. Air contaminants which emerge during this laser ablation often cause health risks if released at the workplace and a decrease of laser cleaning quality if redeposited at the material surface. At the same time, nanoparticles are generated if short pulses are applied. Consequently, a description of the nanoparticle aerosol generation is given in the presented investigation. Though the emission rates for nanosecond laser ablation are remarkably lower than for conventional macro laser technologies such as cutting or welding, the high respirability of particles can pose health risks. A clear shift of the mean aerodynamic diameter of the aerosols to smaller diameters compared to conventional lasers is observed, so that suitable capture systems near to the processing zone or personal protective equipment such as respiratory masks are required to avoid possible health risks.

Corrosion products and other contaminations of metallic artworks can often not be removed sufficiently by conventional conservation techniques. A laser cleaning method for antique metallic artworks using a femtosecond laser technology is presented. Due to the small thermal impact of this technology, compared to nanosecond laser ablation, damage or discolouring of the original surface is avoided. The selective removal of corrosion products from original objects made of copper, bronze alloy and silver, using a Titan-Sapphire femtosecond laser, is shown. Moreover, the influence of the laser fluence and of the number of cleaning cycles on the specific removal efficiency is presented leading to specific fluence thresholds. The maximum surface-specific cleaning rates are evaluated from the viewpoint of restoration in practice, so that an economic balance sheet including operating and labour costs is given for this technology.

A new method of laser ablation, having high pulse-rate of the order of MHz, with short ps and sub-ps pulses, has been recently developed at the ANU. As a preliminary test for the short-pulse ablation applied to art conservation for selective removal of unwanted surface layers, we used the ps laser to clean brass samples coated with contaminant layers such as rust, paint and wax. We demonstrated the laser cleaning system with 2^nd (λ ₂=532nm) and 4^th (λ₄=266nm) harmonics of a Nd:YVO₄ ultra-fast laser, combined with a constant-velocity scanning system. Great flexibility in cleaning the surfaces selectively was demonstrated by removal of the wax coating from the black-painted surface without altering the black paint surface sub-structure. Major conservation and restoration challenges were considered where ultrafast laser cleaning might play a vital role.

A new mode of ultra-fast pulsed laser deposition has been recently proposed as a solution for deposition of highest quality optical films, such as those required for fabrication of optical waveguides. This mode of laser ablation employs short ps and sub-ps pulses with rather low energy, of the order of μJ, delivered at MHz repetition rates, as opposed to conventional low-repetition-rate ns pulse deposition. The low energy pulses have to be tightly focused to a focal spot of the order of tens of microns to achieve the ablation conditions on the target surface. This tight focusing together with short time between the pulses, results in much broader expansion of the laser plume when compared to a narrow-shaped plume in conventional ns-pulse laser deposition. An advantage of broad plume expansion is a more homogeneous film coating, however this comes at the cost of subsequent deposition of the ablated material on the laser beam entry window. To overcome this problem, a laser cleaning technique was developed for in-situ deposition cleaning, and applied to deposition of chalcogenide glasses comprised of Ge-As-Se and As-S used for creation of nonlinear optical films for photonics applications.

Short pulsed, high peak power VUV output has been obtained from pulsed voltage excitation of high pressure dielectric barrier discharges (DBD). Output with these characteristics has demonstrated advantage over longer pulse DBD VUV sources for several materials processing and surface modification applications. Results using a Xe₂^* DBD source at 172 nm for removing optical mountants from optical surfaces, for removing hydrocarbon contamination from optical and polymer surfaces, and dehydroxylation of fused silica are presented. Guidelines.

The interaction of short laser pulses with matter is an interesting study in its own right. Such investigations are preferably performed under well-defined, reproducible conditions. It is by no means straightforward to transfer the knowledge from basic investigations to the "real world" of laser machining. In this paper, a brief overview of the challenges that meet real-world applications will be given, and different solutions to improving important parameters like reproducibility and throughput will be presented.

Optical surface profilometry is a technique that has advantages over other profilometry techniques (stylus profilometry, AFM) of being non-contact and being able to profile comparatively large areas in a single "z-scan". Thus, it is employed in monitoring surface quality and measuring surface form in high technology manufacturing processes and quality assurance, as well as being applied as a diagnostic in research and development contexts. Its application to optical materials has been limited due to issues relating to the low reflectance of the surfaces. A feasibility study for profiling laser induced optical damage and "loose" microscopic sized pieces of optical material (particles introduced by design) on optical substrates is reported. Progress on profiling these difficult samples has been achieved.