David C. Emmony

Laser mirror damage in germanium at 10.6 μm

Damage in the germanium output mirror of a TEA CO2 laser shows a periodicity of 10.6 μm, the laser wavelength. A mechanism is proposed in which scattered and cavity radiation interfere. An interference mechanism is confirmed by experiments outside of the laser cavity. Periodic damage is obtained in thin films irradiated at angles other than the normal where the fringe spacing is modified.

The role of 'splashing' in the collapse of a laser-generated cavity near a rigid boundary

Vapour cavities in liquid flows have long been associated with cavitation damage to nearby solid surfaces and it is thought that the final stage of collapse, when a high- speed liquid jet threads the cavity, plays a vital role in this process. The present study investigates this aspect of the motion of laser-generated cavities in a quiescent liquid when the distance (or stand-off) of the point of inception from a rigid boundary is between 0.8 and 1.2 times the maximum radius of the cavity. Numerical simulations using a boundary integral method with an incompressible liquid impact model provide a framework for the interpretation of the experimental results. It is observed that, within the given interval of the stand-off parameter, the peak pressures measured on the boundary at the first collapse of a cavity attain a local minimum, while at the same time there is an increase in the duration of the pressure pulse. This contrasts with a monotonic increase in the peak pressures as the stand-off is reduced, when the cavity inception point is outside the stated interval. This phenomenon is shown to be due to a splash effect which follows the impact of the liquid jet. Three cases are chosen to typify the splash interaction with the free surface of the collapsing cavity: (i) surface reconnection around the liquid jet; (ii) splash impact at the base of the liquid jet; (iii) thin film splash. Hydrodynamic pressures generated following splash impact are found to be much greater than those produced by the jet impact. The combination of splash impact and the emission of shock waves, together with the subsequent re-expansion, drives the flow around the toroidal cavity producing a distinctive double pressure peak.

Characterization of laser cleaning of limestone

Abstract The precise and controlled cleaning of polluted limestone sculpture by a Q-switched Nd:YAG laser is described. Acoustic monitoring of the cleaning process is used to demonstrate the selective removal of black crusts from stone surfaces. Results are also presented which show that by applying a thin layer of water to the surface of the crust prior to irradiation, cleaning can be achieved at a lower energy than with normal laser cleaning. The probability of damage to the underlying stone is, therefore, reduced.

Analysis of the laser-induced reduction mechanisms of medieval pigments

This study is concerned with the effect of infrared Q-switched laser radiation on four of the most commonly used medieval pigments. Vermilion, red lead (minium), white lead and yellow lead (massicot) have been irradiated with Nd:YAG laser radiation at 1064 nm. Visual observation, reflectance spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy analyses have been employed to study the colour alteration, identity the chemical composition and observe the surface of the irradiated pigments. It was shown that, contrary to what was generally accepted, the mechanism responsible for the discoloration is reduction. It was suggested that under the intense laser ablation conditions and the plasma formation, oxidation is not favourable and instead the pigments decompose to their constituent elements. This scenario can also explain the temporary blackening on red and white lead as it is a well established tendency for metallic lead to become oxidised as soon as favourable conditions are obtained. Recovery mechanisms are currently being studied to prove the model.

Direct observation of the pressure developed in a liquid during cavitation‐bubble collapse

A Mach–Zehnder interferometer with a pulsed dye‐laser light source is used to observe the collapse of laser‐produced cavitation bubbles in water near a rigid transparent polymeric boundary. Vapor bubbles with a maximum radius of about a millimeter are formed using a Q‐switched Nd:YAG laser operating at 1.06 μm with a pulse energy of 4.6 mJ and a pulse duration of 8 ns. The pressure in the liquid around the bubbles is determined directly from the resulting interferograms using a fringe tracing method. Prior to bubble rebound, a region of compressed water develops adjacent to the cavity and a weak acoustic wave is directed towards the rigid boundary.

Experimental observations of the interaction of a laser generated cavitation bubble with a flexible membrane

In this paper we present new observations of a laser generated cavitation bubble interacting with an inertial boundary. Employing schlieren photography and Mach–Zehnder interferometry techniques, we present photographic sequences of a cavity interacting with a flexible membrane. During expansion the membrane is deformed away from the bubble centroid but during the collapse phase, instead of moving with the expected fluid motion caused by the contracting bubble, the flexible membrane introduces a certain asymmetry into the problem which in turn results in novel fluid motions around the bubble.

THE INTERACTION OF A SINGLE LASER-GENERATED CAVITY IN WATER WITH A SOLID SURFACE

This paper reexamines the collapse of a cavity in water and its role in possible damage processes. Laser‐generated cavities were formed near to a water–solid boundary and simultaneous observations made of the collapse processes and stresses on the solid surface. Using schlieren photography and a thin film transducer on the solid, the importance of fluid flow to induced material stress was determined. The stresses in the solid were also observed using dynamic photoelasticity. The cavity deformation in the collapse phase was observed with high spatial and temporal resolution. Two distinct surface stresses were observed associated with bubble collapse near the boundary. The results indicate that one was due to a shock wave. It is postulated that the second stress was associated with fluid flow through the bubble, set up by the so‐called ‘‘liquid jet’’ as opposed to jet impact on its own.

Laser-induced shock waves in liquids

High‐speed schlieren photography of the interaction of a TEA CO2 laser pulse with the surface of absorbing liquids shows the formation of a complex shock structure in the liquid. The shock structure in liquids with different absorption coefficients at 10.6 μm is compared. In carbon tetrachloride a cylindrical wave is generated in the bulk of the liquid in addition to the hemispherical wave which is formed at the liquid surface. The cylindrical wave leads the hemispherical one, and this separation is interpreted as differential attenuation in the initial phase. This implies a shock velocity above the sound speed within the first 0.5 μs.

The effect of Nd:YAG laser radiation on medieval pigments

Very often traces of paint and pigment have been found on medieval sculpture. Presently, little is known about the effects of laser radiation on polychromy. It is important to protect and preserve these traces during any cleaning or restoration process. The absorption properties of polychrome materials are likely to be different from those of the underlying stone or substrate and as a result special care must be taken to avoid any damaging or discolouring phenomena associated with laser cleaning. Spectroscopic methods (XPS, AES, reflectance spectroscopy) together with optical mi- croscopy, digital photography and X-ray diffraction analysis have been used in a series of experiments on simulated samples of common medieval pigments (vermilion, red and white lead and ochres in linseed oil) on limestone plates, in order to study the reaction of these materials to infrared laser radiation.

The topography of laser-induced ripple structures

The authors present results which show the topography of the laser-induced periodic surface structures formed upon samples of germanium, which had been irradiated at a wavelength of 248 nm, and fused quartz irradiated at 10.6 mu m. Surface profiles are obtained from phaseshifting interferometry measurements. A slight modification to a theory which models the development of the surface contour during resolidification of a molten surface can be successfully employed to explain their observations upon germanium. To explain the profiles obtained on fused quartz they suggest that plasma oscillations at the surface may be involved.