TEA-CO2 laser generation of ultrasound in non-metals

TEA-CO2 laser generation of ultrasound in non-metals G.S. Taylor*, D.A. Hutchins% C. Edwards* and S.B.Palmer* * Department of Physics and $ Department of Engineering, University of Warwick, Coventry CV4 7AL, UK

Received 29 September 1989; revised 22 January 1990 An industrial TEA-CO2 laser, operating at a wavelength of 10.6 #m, has been used to produce broadband ultrasonic pulses in polymers. The generation mechanism falls into three categories. At low power densities ~<107 W c m - 2 a thermoelastic regime predominates. As the power density is increased in the range (1-5) x 107 W cm 2 ablation of the material surface plays an increasingly important role in the acoustic generation. Thirdly, at greater power densities, plasma breakdown just above the material surface serves as the means of generation. This paper describes the acoustic sources for these types of generation mechanism and presents theoretically calculated acoustic waveforms to match those recorded experimentally.

Keywords: TEA-CO2 lasers; polymers; non-metals

Introduction

Experimental

The generation of ultrasound by pulsed lasers is a subject which has received much attention over the past decade 1,2. The bulk of previous work has involved lasers at wavelengths ranging from the ultraviolet and visible, typically nitrogen at 0.337 ~tm 3 and ruby at 0.694 #m 4, through to the near infrared using Nd : YAG a 1.06/~m 5. At these wavelengths a reasonable proportion of the laser energy is absorbed by most metals. For Nd : YAG a figure of between 9 % and 20 % is absorbed at an aluminium target dependent on surface quality. The absorption increases with decreasing wavelength, leading to enhanced ultrasonic generation although the material ablation arising from the extra absorption will result in increased material damage. In a previous publication we have demonstrated that TEA CO 2 laser pulses can be used to generate ultrasonic transients in metals, using an air breakdown method 6. This mechanism arises because of the high reflectivity of infra-red radiation at the relevant wavelength (10.6 ~m). In other media, however, this wavelength may be strongly absorbed, as has been shown by studies in various liquids 1. Work in this area has included the visualization of ultrasonic wavefronts generated in water using Schlieren optics 7, and the analysis of wavefronts that are generated following vaporization of the liquid surface 8. In the present work, the interaction of CO2 laser radiation with polymer materials, where the optical penetration depth is significant, is reported. It will be shown that this factor has an effect on the resultant acoustic waveforms. Theories are discussed as well as experiments to indicate the importance of this phenomenon.

The laser used in this investigation was an industrial T E A - C O 2 laser (Lumonics Lasermark), operating at a wavelength of 10.6 #m. The laser was pulsed in a single shot mode; however such lasers can be pulsed at repetition rates of several hundred hertz, enabling signal averaging or rapid scanning to be carried out. The laser used a modified gas mixture which had a reduced nitrogen content. This removes the long tail of the pulse which normally lasts between 1-2 #s. The modified laser pulse is shown in Figure 1, which has an energy of about 1 J and a rise time of 50 ns. The laser beam profile is, to a first approximation, a top-hat function of diameter 25 mm. The radiation is focused via a 10 cm focal length anti-

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Laser generation of ultrasound. G.S. Taylor et al. reflection coated (ARC) germanium lens and the degree of focus depends on the desired type of acoustic source. In the case of thermoelastic generation, a high degree of beam attenuation is required to prevent ablation of the sample under inspection. Filters, made from varying thicknesses of Mylar film, attenuated the beam by at least 80% to ensure a purely thermoelastic interaction. The sample materials used were 12 mm thick Perspex and glass. A modified wideband Michelson interferometer 9 and an out-of-plane electromagnetic acoustic transducer (EMAT) 1° were used to detect the ultrasonic arrivals. Figure 2a shows the experimental arrangement of laser generation and interferometer detection. Figure 2b shows a schematic diagram of the EMAT used in some of these experiments. The interferometer uses a stabilized 5 mW polarized HeNe laser, operating at 638.2 nm, as its light source. One beam of the interferometer is reflected the sample surface, the other from a reference mirror. The typical amplitude of acoustic transients arising from laser generation is of the order of a few hundred picometres, which represents a fraction of a fringe shift in the interferometer. In order to negate the effect of large amplitude, low frequency background vibrations on the interferometer-sample path length which would swamp the desired acoustic signal, a feedback loop from the detection system to the reference mirror constantly adjusts the reference path length. This minimizes environmental effects. The bandwidth of the interferometer, which is limited by the response of the detection photodiodes, extends from 1 kHz to approximately 25 MHz. The interferometer output is proportional to sin(2uk) where u is the surface displacement and k the wavenumber of Circular aperture F[ terSLens S~mpe Metallic coating

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Theory Thermoelastic generation In the case of laser generation of ultrasound on metals, it is usual to assume that to a first approximation, the source is at the material surface. Rose's paper xl describes the ultrasonic source as a surface centre of expansion,

the result of which produces an on-epicentral waveform typical of those observed experimentally :'z'5. The theoretical model differs from those obtained experimentally by the absence of a small initial outward displacing arrival travelling at the longitudinal velocity. Doyle 12 explained this anomaly by considering the effect of thermal diffusion of heat from the surface. In this model, the diffusion of heat produces a source which develops in time, and hence has a buried component which produces the outwardly displacing motion. In the case of CO 2 pulses incident on materials such as Perspex, the radiation penetrates to a depth determined by the optical absorption coefficient, which is the reciprocal of the optical penetration depth. For a non-scattering material, the radiation intensity follows a Beers law exponential decay into the material. Thus the temperature distribution, assuming the material to have a low thermal conductivity, can be written as

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HeNe radiation at 638.2 nm. For small displacements the output is can be approximated to 2uk, while for large amplitude displacements the output is increasingly nonlinear. The second transducer used in these experiments, the EMAT, operates on non-magnetic conducting solids via a Lorentz force interaction on the conduction electrons moved by the acoustic waves in a static magnetic field. The eddy currents generated by this interaction are detected by a suitably orientated coil. To detect out-ofplane motion, the static magnetic field which in our case was generated with a N d - F e - B permanent magnet, must be parallel to the material surface and orthogonal to the coil. Due to the nature of this interaction the EMAT is a velocity sensor 1°. To enhance the inteferometer's sensitivy and to allow the EMATs to operate on non-metals, the samples were coated with a thin aluminium layer on the detection face. Ultrasonic waveforms were captured and stored using a Lecroy 9400 digital oscilloscope.

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where A is the source strength q(t) the normalized laser pulse energy and 9 n the Greens function which describes the displacement field due to a centre of expansion at a depth d below the surface. On application of appropriate boundary conditions this yields the epicentral displacement waveform, shown in Figure 3, if it is assumed that q(t) = &(t), a Dirac delta function. The form of this waveform may be explained as follows. Consider a point source, at a distance d below the surface. This produces

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effectively two longitudinal waves, the first travelling into the bulk, while the second propagates towards the surface (Note that Rose used his solution to calculate the resultant surface displacement by having an acoustically matched overlay to simulate a buried source.) The direct wave consists only of a longitudinal component having an initial delta pulse followed by a low amplitude positive displacement. There is no shear wave, as isotropic expansion in all directions is assumed as the source. The second longitudinal wave undergoes a phase change of ~ on reflection at the top surface and also a degree of mode conversion to produce a shear component. The resultant waveform, shown in Figure 3, thus contains two longitudinal transients, with the shear step being associated with the second of these. It is interesting at this stage to compare this result to that obtained from a point source at the solid surface, which is shown in Figure 4. The major difference is that the longitudinal signal is now a step, and does not have the b-function component in both directions that was observed in the buried source. This is because there is no vertical dipole component to the surface thermoelastic source because this is forbidden by boundary conditions. Thus, instead of an omnidirectional centre of expansion, the stresses now exist only in directions parallel to the surface. The resultant acoustic displacement from a laser source decaying with depth, as described by equation (1) is a summation of equation (2) over depth as given in equation (3) below: d"

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adopted by Chang and Sachse 13 to analyse distributed acoustic sources in solids; however the authors used Rose's surface solution as their initial Greens function and not that of a buried point source. Reflections at the top surface of the solid were thus not considered. Because the laser pulse is not a b-function in time, as is assumed in the usual development, the resultant waveform can be convolved with q(t) to account for the real laser pulse shape, which in our case is shown in Figure 1. Figure 5a shows the result of the spatial summation given by equation (3), with q(t) being assumed to be a delta function, and Figure 5b gives the result of convolution with the laser profile. It is evident that the extended nature of the CO 2 laser pulse has a marked effect on the longitudinal arrival. In both cases, however, the longitudinal arrival contains a prominent pulse-like precursor. The spatial extent of the laser beam, varying from 2-12 mm in diameter, will also contribute to the overall ultrasonic waveform. The effect of this may be illustrated using a surface source 14. For a completely uniform, 'top hat' spatial source distribution, Figure 6a, the resultant displacements originate purely from the edge of the source, hence producing effectively delayed wave arrivals when compared to the point source of Figure 4. For a non-uniform source, Figure 6b, c each point within its limits contributes to the overall wave field, thus reducing the rise times of the longitudinal and shear arrivals. It should be noted that the effect of finite width is to delay the arrival of a particular mode; it does not contribute to any pulse-like precursor on the longitudinal signal.

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As the laser power density is increased, for example by focusing, ablation of the material surface will occur beyond a certain critical power density. The onset of ablation in polymer materials is marked by a fine cloud of ablated matter being ejected from the surface. This produces a normal stress component by momentum

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Figure 6 (a) Displacement waveform for a uniform source distribution at a solid surface 13, showing the transit times to be measured from the edge of the laser source. ( b ) , (c) Solutions for non-uniform intensity distributions for source diameters of 5 and 11 mm, respectively transfer. Increasing the power density enhances the reactive force produced by the ejection of material. In addition, at TEA CO 2 laser power densities of the order 5 x 10 7 W cm -2 air breakdown occurs. At a wavelength of 10.6 #m, the photon energy is normally insufficient to produce multiphoton ionization of the air. Breakdown may occur at high energy densities, of the order 10 9 W c m - 2 , via ionization of impurities in the air, such as hydrocarbons 15. In our case, the occurence of air breakdown at such comparatively low power densities is due to the presence of the material surface a6 where the process is seeded by initial ablation of polymer material. This produces a sufficiently high electron concentration to allow breakdown to occur. The onset of air breakdown prevents any further damage as the radiation is absorbed by the plasma. Two modes of breakdown exist depending upon the incident power density/7. Firstly, at intensities just above breakdown threshold, a three-dimensional plasma expansion occurs and this produces a Heaviside step type time dependence for the normal force at the surface. Secondly, for higher still intensities, a laser detonated wave is generated. This is a one dimensional expansion which travels along the path of the laser beam towards the laser. This produces a delta type normal force time dependence at the surface. The waveforms generated by these types of source may be predicted using the theory of Knopoff TM, who derived the epicentral displacement obtained when a Heaviside force acts normally on a surface. Figure 7a shows

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Knopoff's solution convolved with the rise time of the laser pulse, and would be expected from a three dimensional plasma expansion. In addition Figure 7b and c gives the first and second derivatives of Figure 7a, with respect to time. Here Figure 7b would be expected to be the displacement produced by a 6-function normal force, i.e. that produced by a one dimensional plasma expansion, whereas Figure 7c would be that detected by an EMAT, which is a velocity sensor.

Results and discussion Thermoelastic results As discussed earlier, two overall parameters effect the source produced by a laser. These are the spatial extent of the beam in terms of depth and width, at the sample surface. In an attempt to deconvolve these two source properties, glass was chosen as a reference material because it has a low optical penetration depth. Glass has an optical penetration depth at a wavelength of 10.6 #m of ~< 50 ~m, which is roughly an order of magnitude greater than the thermal skin depth of aluminium but much less than that of polymers such as Perspex. Figure 8a and b shows thermoelastic waveforms generated in glass. Both were recorded at a constant power density but with different source diameters of 4 and 11 mm. N o significant change in longitudinal (denoted as P) and shear (denoted as S) transit times can be detected. This

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indicates that the source is not totally uniform across its area, as a comparison with the waveforms of Figures 6b and c will indicate. However, the shear wave arrival is marked as a distinctive positive surface displacement for smaller source sizes, rather than a simple change in gradient, which is the case for large source size. The initial positive displacement contribution to the longitudinal signal is of low amplitude, as expected from a material with such a high absorption coefficient (The source would be almost at the surface, where theory predicts the absence of a positive displacing precursor.) In contrast are the waveforms on Perspex, which has an optical penetration depth of ~ 200 #m. Figure 9a shows the comparable waveform on Perspex to that in glass shown earlier in Figure 8b. Two distinct differences are observed. The amplitude of the initial longitudinal precursor is increased and the shear arrival is less pronounced. Both effects were predicted by theory for an increased penetration depth, as shown in Figure 5b. This serves to confirm that the theoretical development is a valid approach for this situation. For a thermo-elastic source at a solid surface, the shape of the displacement waveform is expected to be independent of the incident energy, although the overall amplitude will change. It has been observed that for incidence of T E A - C O 2 laser radiation on Perspex, the nature of the waveform and its amplitude both change. Increased optical power density elevates the amplitude of the source strength, A. The result is to produce significant source contributions from deeper within the material. Thus the rise time of the initial positive precursor and the overall acoustic amplitude are increased, as shown by the waveform

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Ablative and b r e a k d o w n results As optical power densities are increased, the critical value for material ablation is reached. For optical power densities in the range ( 1 - 5 ) x 10 7 W cm -2 significant ablation of the target material surface occurs. Because it is dependent on the degree of ablation, the acoustic source now becomes a superposition of thermoelastic and ablative type interactions. Figure lOa is an epicentral displacement waveform obtained experimentally within this power

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density range, using the Michelson interfermeter as detector. It has a prominent longitudinal arrival, as expected from a source containing normal forces. However, it also contains a significant thermoelastic contribution. This may be predicted theoretically, by the superposition of an ablative and thermoelastic contribution in the relevant proportions 19. The result of such an operation is shown in Figure lOb, which is a 1 : 1 addition ratio of both source types. It shows good agreement with that obtained experimental in Figure lOa. Above these power densities, optical breakdown occurs just above the sample surface. For power densities just above this second threshold a 'laser plasmotron '17 is generated, creating a Heaviside force time dependence. Figure lla shows an epicentral waveform on Perspex recorded under the action of such a source using the E M A T shown in Figure 2b. Due to the large amplitude of such signals the interferometer was unable to detect such displacements without passing through a fringe shift. For a HeNe laser, passing through such a fringe corresponds to a displacement of at least 2/4 ~ 160 nm. The ultrasonic arrivals were thus detected with the out-of-plane sensitive EMAT, which detected eddy currents generated in the metallic coating applied to the sample surface. These waveforms show a monopolar longitudinal pulse followed by a much lower amplitude shear arrival. Comparison with Figure 7b indicates that this is the expected result, following detection with a velocity sensor. Increasing the power density still further causes the laser plasmotron to convert into a laser detonated wave. As mentioned earlier, this produces a delta type force at the material surface, which, when viewed using an EMAT, gives a signal characteristic of the second differential of Knopoff's solution (see Figure 7c). Figure lib shows an experimental waveform detected on Perspex. G o o d agreement is again observed, demonstrating that the proposed models have sound physical basis.

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F i g u r e 11 Experimental, on epicenter waveforms, recorded using an EMAT for (a) three dimensional plasma breakdown and ( b ) a o n e dimensional laser detonated wave

Conclusions It has been shown that pulsed TEA CO2 lasers can be used to generate ultrasonic transients in polymer materials. At low power densities thermoelastic mechanisms dominate and cause no damage to the material. The presence of an initial positive displacing precursor, a feature of the extended nature of the thermoelastic source within the material, has benefits when precise timing measurements are required. Ablation of material can also be used, if desired, to increase acoustic longitudinal generation efficiency. Both sources have been modelled theoretically. In the thermoelastic case, the source was shown to decay exponentially into the material. This technique should find application to the testing of a range of polymer materials.

Acknowledgements The authors would like to thank the Atomic Energy Authority Laboratories, Risley for financial help and Lumonics Ltd for their technical assistance in the installation of the laser. G.S.T. acknowledges the provision of an SERC studentship.

References 1 2

Hutehins, D.A. Ultrasonic generation by pulsed lasers in Physical Acoustics (Ed. Mason, W.P. and Thurston, R.N.) Academic Press New York (1988) Ch 18, 21-123 Scruby, C.B., Dewhurst, R.J., Hutchins, D.A. and Palmer, S.B. in Research Techniques in Non-Destructive Testing (Ed. Sharpe, R.S.) Academic Press, London (1982) vol. 5, 281-327

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5 6 7 8

Tam, A.C. and Leung, W.P. Measurement of small elastic anisotropy in solids using laser-induced ultrasonic pulses Appl Phys Lett (1984) 45 1040-1042 Hutchins, D.A., Wilkins, D.E. and Lake, G. Electromagnetic acoustic transducers as wideband velocity sensors Appl Phys Lett (1985) 46 634-636

Aindow,A.M., Dewhurst, R.J. Hutchins, D.A. and Palmer, S.B. Laser-generated ultrasonic pulses at free metal surfaces J Acoust Soc Am (1981) 69 449-453 Edwards, C., Taylor, G.S. and Palmer, S.B. Ultrasonic generation with a pulsed TEA~:~O 2 laser Appl Phys Lett in press Emmony, D.C. Interaction of IR laser radiation with liquids Infrared Phys (1985) 25 133-139 Sigrist, M.W. and Kneubuhl, F.K. Laser generated stress waves in liquids J Acoust Soc Am (1978) 64 1652-1663

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Dewhurst,R.J., Edwards, C., McKie, A.D.W. and Palmer, S.B.

l0

Comparative study of wide-band ultrasonic transducers Ultrasonics (1987) 25 315-321 Kawashima, K. Quantitative calculations and measurement of longitudinal and transverse ultrasonic pulses in solids IEEE Trans Son Ultrason (1984)SU-31 83-94

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19

Rose, L.R.F. Point source representation for laser-generated ultrasound J Acoust Soc Am (1984) 75 723-732 Doyle, P.A. On epieentral waveforms for laser-generated ultrasound J Phys D (1986) 19 1613-1624 Chang, C. and Saehse, W. Analysis of elastic wave signals from an extended source in a plate J Acoust Soc Am (1985) 77 1335-1341 Bresse, L F . and liutehim, D.A. Transient generation by a wide thermoelastic source at a solid surface J Appl Phys (1989) 65 1441-1447 Grey Morgan, C. Laser induced breakdown phenomena Sci Prog: 0xf(1978) 65 31-50 Weyl, G., Pirri, A. and Root, R. Laser ignition of plasma off aluminium surfaces AIAA J (1981) 19 460-469

Barehukov, A.I., Bunkin, F.V., Koaov, V.I. and Lyabin, A.A. Investigation of low-threshold gas breakdown near solid targets by CO2 laser radiation Soy Phys J E P T (1974) 39 469-477 Kno0off, L Surface motions of a thick plate J Appl Phys (1958) 29 661-670

Dewharst,R.J., Hatehias, D.A., Palmer, S.B. and Scruby, C.B. Quantitative measurements of laser-generated acoustic waveforms J Appl Phys (1982) 53 4064-4071

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