- Email: [email protected]
Excitation of high-frequency surface acoustic wave pulses with tapered fibers
Surface Science 474 (2001) L191±L196
www.elsevier.nl/locate/susc
Surface Science Letters
Excitation of high-frequency surface acoustic wave pulses with tapered ®bers A. Frass, P. Hess * Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Received 16 September 2000; accepted for publication 27 November 2000
Abstract Tapered ®ber tips with 1±2 lm aperture were used to excite broadband surface acoustic wave (SAW) pulses with frequency spectra up to 800 MHz, this limit being set by interferometric detection. The spatial con®nement of the laser pulses by the ®ber source in the near ®eld is demonstrated by a shortening of the elastic pulse pro®le and extension of its frequency spectrum with decreasing tip±surface distance. The method provides a new imaging technique for detecting the con®nement of pulsed radiation and extends the SAW technique. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser methods; Photoacoustic spectroscopy; Acoustic waves
In contrast to narrowband laser-induced transient gratings (LITGs), where frequencies of several gigahertz have been achieved [1], pulsed laser excitation of broadband surface acoustic wave (SAW) pulses is currently limited to several hundred megahertz [2]. Short elastic surface pulses possess characteristic features that make them a unique tool in surface and materials science. For example: (1) the broad frequency range of 1±2 decades, which allows single pulse dispersion measurements in thin-®lm systems [3]. (2) A time resolution in the nanosecond range, which allows the cuspidal structure of phonon focusing in anisotropic crystals to be resolved [4]. (3) The realization of strongly nonlinear SAW pulses with frequency-up and frequency-down conversion,
* Corresponding author. Tel.: +49-6221-545205; fax: +496221-544255. E-mail address: [email protected] (P. Hess).
leading to the formation of shock fronts and fracture of crystals [5,6]. A further extension of the frequency spectrum of elastic surface pulses into the gigahertz region would considerably extend the capabilities of the SAW pulse technique. Short SAW pulses are usually excited by focusing laser pulses to the smallest possible point or narrowest line at the substrate surface. According to the theory of thermoelastic (ablative) excitation, the length of SAW pulses is essentially determined by the laser pulse duration when mR s a, where mR is the Rayleigh velocity, 2s the laser pulse duration, and a the half-width of the line focused on the surface. If, however, suciently short laser pulses, in the picosecond range, with mR s a are used the SAW pulse shape will be determined by the light intensity distribution over the spot cross section [7±9]. Light penetration into the solid does not in¯uence the surface displacement much in a strongly absorbing medium, i.e., as long as a 2p=a, where a is the optical absorption coecient.
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 1 0 5 2 - 9
CE RF A LE C E TT S E R C IE S N
SU
L192
A. Frass, P. Hess / Surface Science 474 (2001) L191±L196
This indicates that in focusing laser radiation with a spherical or cylindrical lens the potential of picosecond or even femtosecond laser pulses to launch ultrashort SAW pulses cannot be realized, since the practical width of the focused line, which lies in the micrometer range, limits the SAW bandwidth. With a special microscope objective a detection laser spot size of about 1 lm has recently been obtained [10]. An unprecedented con®nement of laser radiation at a surface is possible by near-®eld scanning optical microscopy (NSOM) [11,12]. With near®eld optical devices even the diraction limit of light can be surpassed and in principle pulsed thermoelastic or ablative sources below that limit may be realized in the future. However, before this goal can be reached the problems of destructionfree coupling of pulsed laser radiation into optical ®bers and transmission of sucient pulse energy through tapered and coated ®ber tips must be solved. Here the spatial narrowing of pulsed laser radiation by a tapered ®ber at a distance comparable to the aperture size is demonstrated by SAW experiments. In the present point-source point-probe measurements tapered ®ber tips with 1±2 lm aperture were applied to excite broadband SAW pulses and a stabilized Michelson interferometer was used to monitor the resulting transient surface displacement of the propagating pulse in inverse geometry from the back side of the coated substrate. This con®guration allowed measurements at propagation distances of several hundred micrometers. As the ®ber tip was moved toward the surface, the spectrum of the SAW pulses was observed to extend gradually to higher frequencies, due to the decreasing size of the thermoelastic (ablative) source, ®nally reaching the bandwidth of the detection device employed. Fig. 1 shows schematically the experimental setup employed. For SAW excitation the radiation of a passively mode-locked Nd:YAG laser (180 ps pulse duration FWHM) operating at the second harmonic frequency (532 nm) was coupled into optical ®bers using a mechanical three-axis tiptilt ®ber coupler. The ®ber tip was moved toward the strongly absorbing surface by means of a calibrated piezoelectric drive. The approach was
Fig. 1. Scheme of the experimental setup with pulsed excitation laser, ®ber optics, and distance control devices as well as a c.w. detection laser and a Michelson interferometer for probing from the back side.
monitored by an optical setup consisting of a CCD camera and a macroobjective, which allowed the ®ber tip to be imaged with a magni®cation factor of eight, providing a spatial resolution of 2 lm. This was sucient to control the position of the ®ber tip with respect to the surface in order to prevent damage of the tip resulting from mechanical contact. The SAW pulses launched by the laser pulses were detected from the back side of the transparent substrate with sub-angstrom sensitivity at distances of 200±500 lm from the source by an actively stabilized Michelson interferometer. The detection laser was a Coherent, model Compass 315 M, frequency-doubled diode-pumped Nd: YAG laser with 100 mW power. The focal width of the probe laser spot was about 2 lm, limiting the frequency range that could be detected to below 800 MHz. Note that this width was similar to that of the tip aperture of 2.1 lm. A smaller tip aperture did not further increase the frequency spectrum, as veri®ed experimentally. The photodiode and the ampli®er employed in the detection setup had bandwidths of 1.5 and 1.8 GHz,
SU N IE SC RS CE E A TT R F LE CE
A. Frass, P. Hess / Surface Science 474 (2001) L191±L196
respectively, while the Tektronix digital oscilloscope, model 680 B, had an analog bandwidth of 1 GHz, with a sampling rate of 5 GS/s. Thus, it was possible, in principle, to measure oscillation frequencies of up to 1 GHz in real time, using these high-speed electronic instruments. The sample investigated was a double-layer system with a SiCN ®lm (700 nm thick) of high damage threshold on top of an aluminum ®lm (700 nm thick) deposited onto a substrate of fused silica (Herasil). Aluminum was chosen as the ®lm to be evaporated onto the substrate because of its high re¯ectivity at 532 nm, which allowed sensitive interferometric detection through the substrate. In this work, tapered ®ber tips, produced by the focused ion beam (FIB) method, were used for thermoelastic SAW excitation. These ®ber tips were manufactured by completely coating the tapered ®ber end with aluminum and then milling away a well-de®ned part of the ®ber tip using a beam of strongly focused Ga ions [13]. The result of this process is a ¯at open end with a well-de®ned aperture, the size of which can be controlled with a precision of a few tens of nanometers. Great care had to be taken while coupling the pulsed laser radiation into the coated ®bers because the high power densities could easily destroy the aluminum coating with its low ablation threshold in the tip region or even the tip itself and hence increase the eective aperture size. The measurements were carried out using the visible wavelength of the picosecond Nd:YAG laser at 532 nm because at this wavelength the pulseto-pulse stability was better than in the UV at 355 nm. This reduced the risk of ¯uctuations in the laser pulse energy inadvertently damaging the metal coating. Fig. 2 shows measurements at two dierent tip± surface distances using the 2.1 lm FIB tip. The output energy measured at the tip aperture was about 150 nJ (transmission of several percent). The ®rst SAW pulse shown in Fig. 2(a) was launched with a tip±sample distance of about 7 lm and the second pro®le with a distance of 2 lm. This latter distance coincided with the spatial resolution of the CCD camera. Thus, the 2.1 lm aperture could be moved reproducibly to a distance from the sample comparable with the aperture size, where
L193
Fig. 2. (a) SAW pulse shapes obtained with the 2.1 lm FIB tip at distances of 7 and 2 lm from the sample surface. (b) Corresponding frequency spectra of the two pulse pro®les shown in Fig. 2(a).
the con®nement of the light beam started to have a measurable eect on the size of the source, and thus on the shape and frequency spectrum of the SAW pulses. Consequently, SAW pulses provide a new means to image directly the con®nement of light above and below the diraction limit. With decreasing tip±sample distance the bipolar SAW pro®les became shorter and their amplitude larger, as can be seen in Fig. 2(a). This is clear proof of the decreasing size of the thermoelastic or
CE RF A LE C E TT S E R C IE S N
SU
L194
A. Frass, P. Hess / Surface Science 474 (2001) L191±L196
ablative source. In the corresponding spectra, shown in Fig. 2(b), the maximum frequency components with a reasonable amplitude are shifted from about 650 MHz at the largest tip±surface distance studied (7 lm) to about 800 MHz at 2 lm. Moreover, the contributions of the frequency components in the central part of the spectra around 500 MHz increased by more than a factor of two. The observed increase of the SAW amplitude as the tip approaches is in agreement with the theory of thermoelastic excitation, which gives an optimal focal width of a mR s 0:3 lm for maximum signal generation under the present conditions [9]. Therefore, the signal will increase further on reduction of the tip aperture to 0.3 lm, but the light intensity also increases and may cause destruction of the sample [3]. It should be pointed out that the results of Fig. 2 were obtained by averaging 128 laser pulses. This relatively large number of pulses was possible only due to the high ablation threshold of the SiCN ®lms (>30 GW/cm2 ) compared to that of aluminum (100 MW/cm2 ) [3]. Thus, the commonly used aluminum ®ber coating is not suitable for pulsed operation. In order to measure the dependence of the SAW pulse amplitude on propagation, a series of measurements was performed on the SiCN/Al/Herasil sample using the smallest FIB ®ber tip examined in the present experiments, which had an aperture of 0.95 lm. As mentioned above, this reduction of the aperture did not further extend the frequency spectrum. The output pulse energy was only 120 nJ and the results were again averaged over 128 pulses to obtain a reasonable signal-to-noise ratio. In Fig. 3 the peak-to-peak amplitudes of the SAW pulses are plotted versus the radial distance between excitation and detection points. A theoretical ®t is shown in the same graph for comparison. The function describing the relationship found experimentally for p SAW attenuation can be written as f r / 1= r, which is the behavior expected for the two-dimensional cylindrical geometry of a point source at the surface in the far®eld [14]. This observation is in agreement with a detailed study of the extension of the near®eld and the far®eld behavior of broadband SAW pulses excited with a Gaussian laser-beam pro®le with halfwidth a [15].
Fig. 3. Comparison of the peak-to-peak amplitudes of SAW pulses launched by the FIB tip with 0.95 lm aperture versus radial distance between source and probe. The line shows the theoretical behavior for the two-dimensional cylindrical geometry.
In this paper it was shown that the inverse square root dependence expected for the far®eld breaks down only for r=a < 8. In the present experiments the r=a values were substantially larger. Another point of interest is the dependence of the spectrum of the SAW pulse on the propagation distance in the layered system investigated. For this purpose, some measurements with the 0.95 lm tip are presented in Fig. 4, which shows three SAW pulses recorded at 181, 293, and 436 lm from the source. The decay of the SAW pulse amplitude roughly obeys the inverse square root law mentioned above. The geometrical attenuation of the pulse amplitude is re¯ected in the reduction of the overall spectral amplitudes with propagation distance but no substantial decrease of the high frequency components was found if the radial dependence given above is taken into account (see Fig. 5). From this fact one may conclude that, for the propagation distances investigated here, no substantial attenuation of these high-frequency components due to additional loss mechanisms took place. It is important to note that the pulsed near-®eld technique developed here for elastic surface pulse generation holds great potential for future developments in laser ultrasonics if a tapered ®ber tip is used not only for SAW excitation, as considered
SU N IE SC RS CE E A TT R F LE CE
A. Frass, P. Hess / Surface Science 474 (2001) L191±L196
L195
Fig. 5. Comparison of the frequency spectra of the three pulses shown in Fig. 4.
pulses with a better time resolution, an extension to higher frequencies, stronger nonlinear eects, and the potential to determine the elastic constants of sub-micrometer ®lms by single-pulse dispersion experiments. It will also make possible the detection of SAWs at distances of centimeters from the source.
Acknowledgements
Fig. 4. SAW pulse shapes obtained with the 0.95 lm FIB tip at distances of 181, 293, and 436 lm from the source, indicating that about 100 nJ is sucient to excite measurable SAW pulses.
here, but also to probe SAW propagation. Such a development may open the door to picosecond hyperacoustics [16]. Fiber materials and metal coatings with higher destruction thresholds will allow the application of higher laser pulse energies and the investigation of smaller apertures below the diraction limit. This yields shorter SAW
We thank Dr. I. Sokolov, Ioe Institute, St. Petersburg, for his contributions to the problem of coupling pulsed laser radiation into optical ®bers. Supply of the tapered ®bers by the group of Prof. van Hulst, MESA Research Institute, Twente, is gratefully acknowledged. We thank the group of Dr. K.-H. Chen, IAMS, Taipei, for supplying the SiCN ®lms and A. Lomonosov for critically reading the manuscript. Financial support of this work by the German±Israeli Foundation (G.I.F.) for Scienti®c Research and Development is gratefully acknowledged.
References [1] A. Harata, H. Nishimura, T. Sawada, Appl. Phys. Lett. 57 (1990) 132. [2] M. Szabadi, P. Hess, A.J. Kellock, H. Coufal, J.E.E. Baglin, Phys. Rev. B 58 (1998) 8941.
CE RF A LE C E TT S E R C IE S N
SU
L196
A. Frass, P. Hess / Surface Science 474 (2001) L191±L196
[3] A. Lomonosov, A.P. Mayer, P. Hess, in: W. Sachse, A. Every (Eds.), Handbook of Elastic Properties of Solids, vol. II, Academic Press, New York, 2001. [4] A.A. Maznev, Al.A. Kolomenskii, P. Hess, Phys. Rev. Lett. 75 (1995) 3332. [5] Al.A. Kolomenskii, A.M. Lomonosov, R. Kuschnereit, P. Hess, V.E. Gusev, Phys. Rev. Lett. 79 (1997) 1325. [6] A. Lomonosov, P. Hess, Phys. Rev. Lett. 83 (1999) 3876. [7] A.A. Karabutov, Sov. Phys.-Usp. 28 (1985) 1042. [8] V.E. Gusev, A.A. Karabutov, Laser Optoacoustics, AIP Press, New York, 1993. [9] W. Arnold, B. Betz, B. Homann, Appl. Phys. Lett. 47 (1985) 672.
[10] J.V. Knuuttila, P.T. Tikka, M.M. Salomaa, Opt. Lett. 25 (2000) 613. [11] D. Courjon, C. Bainier, Rep. Prog. Phys. 57 (1994) 989. [12] B. Hecht, B. Sick, U.P. Wild, V. Deckert, R. Zenobi, O.J.F. Martin, D.W. Pohl, J. Chem. Phys. 112 (2000) 7761. [13] J.A. Veerman, A.M. Otter, L. Kuipers, N.F. van Hulst, Appl. Phys. Lett. 72 (1998) 3115. [14] C.B. Scruby, L.E. Drain, Laser Ultrasonics, Adam Hilger, Bristol, 1990. [15] C.M. Scala, P.A. Doyle, J. Acoust. Soc. Am. 85 (1989) 1569. [16] S.A. Akhmanov, V.E. Gusev, Sov. Phys.-Usp. 35 (1992) 153.
www.elsevier.nl/locate/susc
Surface Science Letters
Excitation of high-frequency surface acoustic wave pulses with tapered ®bers A. Frass, P. Hess * Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Received 16 September 2000; accepted for publication 27 November 2000
Abstract Tapered ®ber tips with 1±2 lm aperture were used to excite broadband surface acoustic wave (SAW) pulses with frequency spectra up to 800 MHz, this limit being set by interferometric detection. The spatial con®nement of the laser pulses by the ®ber source in the near ®eld is demonstrated by a shortening of the elastic pulse pro®le and extension of its frequency spectrum with decreasing tip±surface distance. The method provides a new imaging technique for detecting the con®nement of pulsed radiation and extends the SAW technique. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser methods; Photoacoustic spectroscopy; Acoustic waves
In contrast to narrowband laser-induced transient gratings (LITGs), where frequencies of several gigahertz have been achieved [1], pulsed laser excitation of broadband surface acoustic wave (SAW) pulses is currently limited to several hundred megahertz [2]. Short elastic surface pulses possess characteristic features that make them a unique tool in surface and materials science. For example: (1) the broad frequency range of 1±2 decades, which allows single pulse dispersion measurements in thin-®lm systems [3]. (2) A time resolution in the nanosecond range, which allows the cuspidal structure of phonon focusing in anisotropic crystals to be resolved [4]. (3) The realization of strongly nonlinear SAW pulses with frequency-up and frequency-down conversion,
* Corresponding author. Tel.: +49-6221-545205; fax: +496221-544255. E-mail address: [email protected] (P. Hess).
leading to the formation of shock fronts and fracture of crystals [5,6]. A further extension of the frequency spectrum of elastic surface pulses into the gigahertz region would considerably extend the capabilities of the SAW pulse technique. Short SAW pulses are usually excited by focusing laser pulses to the smallest possible point or narrowest line at the substrate surface. According to the theory of thermoelastic (ablative) excitation, the length of SAW pulses is essentially determined by the laser pulse duration when mR s a, where mR is the Rayleigh velocity, 2s the laser pulse duration, and a the half-width of the line focused on the surface. If, however, suciently short laser pulses, in the picosecond range, with mR s a are used the SAW pulse shape will be determined by the light intensity distribution over the spot cross section [7±9]. Light penetration into the solid does not in¯uence the surface displacement much in a strongly absorbing medium, i.e., as long as a 2p=a, where a is the optical absorption coecient.
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 1 0 5 2 - 9
CE RF A LE C E TT S E R C IE S N
SU
L192
A. Frass, P. Hess / Surface Science 474 (2001) L191±L196
This indicates that in focusing laser radiation with a spherical or cylindrical lens the potential of picosecond or even femtosecond laser pulses to launch ultrashort SAW pulses cannot be realized, since the practical width of the focused line, which lies in the micrometer range, limits the SAW bandwidth. With a special microscope objective a detection laser spot size of about 1 lm has recently been obtained [10]. An unprecedented con®nement of laser radiation at a surface is possible by near-®eld scanning optical microscopy (NSOM) [11,12]. With near®eld optical devices even the diraction limit of light can be surpassed and in principle pulsed thermoelastic or ablative sources below that limit may be realized in the future. However, before this goal can be reached the problems of destructionfree coupling of pulsed laser radiation into optical ®bers and transmission of sucient pulse energy through tapered and coated ®ber tips must be solved. Here the spatial narrowing of pulsed laser radiation by a tapered ®ber at a distance comparable to the aperture size is demonstrated by SAW experiments. In the present point-source point-probe measurements tapered ®ber tips with 1±2 lm aperture were applied to excite broadband SAW pulses and a stabilized Michelson interferometer was used to monitor the resulting transient surface displacement of the propagating pulse in inverse geometry from the back side of the coated substrate. This con®guration allowed measurements at propagation distances of several hundred micrometers. As the ®ber tip was moved toward the surface, the spectrum of the SAW pulses was observed to extend gradually to higher frequencies, due to the decreasing size of the thermoelastic (ablative) source, ®nally reaching the bandwidth of the detection device employed. Fig. 1 shows schematically the experimental setup employed. For SAW excitation the radiation of a passively mode-locked Nd:YAG laser (180 ps pulse duration FWHM) operating at the second harmonic frequency (532 nm) was coupled into optical ®bers using a mechanical three-axis tiptilt ®ber coupler. The ®ber tip was moved toward the strongly absorbing surface by means of a calibrated piezoelectric drive. The approach was
Fig. 1. Scheme of the experimental setup with pulsed excitation laser, ®ber optics, and distance control devices as well as a c.w. detection laser and a Michelson interferometer for probing from the back side.
monitored by an optical setup consisting of a CCD camera and a macroobjective, which allowed the ®ber tip to be imaged with a magni®cation factor of eight, providing a spatial resolution of 2 lm. This was sucient to control the position of the ®ber tip with respect to the surface in order to prevent damage of the tip resulting from mechanical contact. The SAW pulses launched by the laser pulses were detected from the back side of the transparent substrate with sub-angstrom sensitivity at distances of 200±500 lm from the source by an actively stabilized Michelson interferometer. The detection laser was a Coherent, model Compass 315 M, frequency-doubled diode-pumped Nd: YAG laser with 100 mW power. The focal width of the probe laser spot was about 2 lm, limiting the frequency range that could be detected to below 800 MHz. Note that this width was similar to that of the tip aperture of 2.1 lm. A smaller tip aperture did not further increase the frequency spectrum, as veri®ed experimentally. The photodiode and the ampli®er employed in the detection setup had bandwidths of 1.5 and 1.8 GHz,
SU N IE SC RS CE E A TT R F LE CE
A. Frass, P. Hess / Surface Science 474 (2001) L191±L196
respectively, while the Tektronix digital oscilloscope, model 680 B, had an analog bandwidth of 1 GHz, with a sampling rate of 5 GS/s. Thus, it was possible, in principle, to measure oscillation frequencies of up to 1 GHz in real time, using these high-speed electronic instruments. The sample investigated was a double-layer system with a SiCN ®lm (700 nm thick) of high damage threshold on top of an aluminum ®lm (700 nm thick) deposited onto a substrate of fused silica (Herasil). Aluminum was chosen as the ®lm to be evaporated onto the substrate because of its high re¯ectivity at 532 nm, which allowed sensitive interferometric detection through the substrate. In this work, tapered ®ber tips, produced by the focused ion beam (FIB) method, were used for thermoelastic SAW excitation. These ®ber tips were manufactured by completely coating the tapered ®ber end with aluminum and then milling away a well-de®ned part of the ®ber tip using a beam of strongly focused Ga ions [13]. The result of this process is a ¯at open end with a well-de®ned aperture, the size of which can be controlled with a precision of a few tens of nanometers. Great care had to be taken while coupling the pulsed laser radiation into the coated ®bers because the high power densities could easily destroy the aluminum coating with its low ablation threshold in the tip region or even the tip itself and hence increase the eective aperture size. The measurements were carried out using the visible wavelength of the picosecond Nd:YAG laser at 532 nm because at this wavelength the pulseto-pulse stability was better than in the UV at 355 nm. This reduced the risk of ¯uctuations in the laser pulse energy inadvertently damaging the metal coating. Fig. 2 shows measurements at two dierent tip± surface distances using the 2.1 lm FIB tip. The output energy measured at the tip aperture was about 150 nJ (transmission of several percent). The ®rst SAW pulse shown in Fig. 2(a) was launched with a tip±sample distance of about 7 lm and the second pro®le with a distance of 2 lm. This latter distance coincided with the spatial resolution of the CCD camera. Thus, the 2.1 lm aperture could be moved reproducibly to a distance from the sample comparable with the aperture size, where
L193
Fig. 2. (a) SAW pulse shapes obtained with the 2.1 lm FIB tip at distances of 7 and 2 lm from the sample surface. (b) Corresponding frequency spectra of the two pulse pro®les shown in Fig. 2(a).
the con®nement of the light beam started to have a measurable eect on the size of the source, and thus on the shape and frequency spectrum of the SAW pulses. Consequently, SAW pulses provide a new means to image directly the con®nement of light above and below the diraction limit. With decreasing tip±sample distance the bipolar SAW pro®les became shorter and their amplitude larger, as can be seen in Fig. 2(a). This is clear proof of the decreasing size of the thermoelastic or
CE RF A LE C E TT S E R C IE S N
SU
L194
A. Frass, P. Hess / Surface Science 474 (2001) L191±L196
ablative source. In the corresponding spectra, shown in Fig. 2(b), the maximum frequency components with a reasonable amplitude are shifted from about 650 MHz at the largest tip±surface distance studied (7 lm) to about 800 MHz at 2 lm. Moreover, the contributions of the frequency components in the central part of the spectra around 500 MHz increased by more than a factor of two. The observed increase of the SAW amplitude as the tip approaches is in agreement with the theory of thermoelastic excitation, which gives an optimal focal width of a mR s 0:3 lm for maximum signal generation under the present conditions [9]. Therefore, the signal will increase further on reduction of the tip aperture to 0.3 lm, but the light intensity also increases and may cause destruction of the sample [3]. It should be pointed out that the results of Fig. 2 were obtained by averaging 128 laser pulses. This relatively large number of pulses was possible only due to the high ablation threshold of the SiCN ®lms (>30 GW/cm2 ) compared to that of aluminum (100 MW/cm2 ) [3]. Thus, the commonly used aluminum ®ber coating is not suitable for pulsed operation. In order to measure the dependence of the SAW pulse amplitude on propagation, a series of measurements was performed on the SiCN/Al/Herasil sample using the smallest FIB ®ber tip examined in the present experiments, which had an aperture of 0.95 lm. As mentioned above, this reduction of the aperture did not further extend the frequency spectrum. The output pulse energy was only 120 nJ and the results were again averaged over 128 pulses to obtain a reasonable signal-to-noise ratio. In Fig. 3 the peak-to-peak amplitudes of the SAW pulses are plotted versus the radial distance between excitation and detection points. A theoretical ®t is shown in the same graph for comparison. The function describing the relationship found experimentally for p SAW attenuation can be written as f r / 1= r, which is the behavior expected for the two-dimensional cylindrical geometry of a point source at the surface in the far®eld [14]. This observation is in agreement with a detailed study of the extension of the near®eld and the far®eld behavior of broadband SAW pulses excited with a Gaussian laser-beam pro®le with halfwidth a [15].
Fig. 3. Comparison of the peak-to-peak amplitudes of SAW pulses launched by the FIB tip with 0.95 lm aperture versus radial distance between source and probe. The line shows the theoretical behavior for the two-dimensional cylindrical geometry.
In this paper it was shown that the inverse square root dependence expected for the far®eld breaks down only for r=a < 8. In the present experiments the r=a values were substantially larger. Another point of interest is the dependence of the spectrum of the SAW pulse on the propagation distance in the layered system investigated. For this purpose, some measurements with the 0.95 lm tip are presented in Fig. 4, which shows three SAW pulses recorded at 181, 293, and 436 lm from the source. The decay of the SAW pulse amplitude roughly obeys the inverse square root law mentioned above. The geometrical attenuation of the pulse amplitude is re¯ected in the reduction of the overall spectral amplitudes with propagation distance but no substantial decrease of the high frequency components was found if the radial dependence given above is taken into account (see Fig. 5). From this fact one may conclude that, for the propagation distances investigated here, no substantial attenuation of these high-frequency components due to additional loss mechanisms took place. It is important to note that the pulsed near-®eld technique developed here for elastic surface pulse generation holds great potential for future developments in laser ultrasonics if a tapered ®ber tip is used not only for SAW excitation, as considered
SU N IE SC RS CE E A TT R F LE CE
A. Frass, P. Hess / Surface Science 474 (2001) L191±L196
L195
Fig. 5. Comparison of the frequency spectra of the three pulses shown in Fig. 4.
pulses with a better time resolution, an extension to higher frequencies, stronger nonlinear eects, and the potential to determine the elastic constants of sub-micrometer ®lms by single-pulse dispersion experiments. It will also make possible the detection of SAWs at distances of centimeters from the source.
Acknowledgements
Fig. 4. SAW pulse shapes obtained with the 0.95 lm FIB tip at distances of 181, 293, and 436 lm from the source, indicating that about 100 nJ is sucient to excite measurable SAW pulses.
here, but also to probe SAW propagation. Such a development may open the door to picosecond hyperacoustics [16]. Fiber materials and metal coatings with higher destruction thresholds will allow the application of higher laser pulse energies and the investigation of smaller apertures below the diraction limit. This yields shorter SAW
We thank Dr. I. Sokolov, Ioe Institute, St. Petersburg, for his contributions to the problem of coupling pulsed laser radiation into optical ®bers. Supply of the tapered ®bers by the group of Prof. van Hulst, MESA Research Institute, Twente, is gratefully acknowledged. We thank the group of Dr. K.-H. Chen, IAMS, Taipei, for supplying the SiCN ®lms and A. Lomonosov for critically reading the manuscript. Financial support of this work by the German±Israeli Foundation (G.I.F.) for Scienti®c Research and Development is gratefully acknowledged.
References [1] A. Harata, H. Nishimura, T. Sawada, Appl. Phys. Lett. 57 (1990) 132. [2] M. Szabadi, P. Hess, A.J. Kellock, H. Coufal, J.E.E. Baglin, Phys. Rev. B 58 (1998) 8941.
CE RF A LE C E TT S E R C IE S N
SU
L196
A. Frass, P. Hess / Surface Science 474 (2001) L191±L196
[3] A. Lomonosov, A.P. Mayer, P. Hess, in: W. Sachse, A. Every (Eds.), Handbook of Elastic Properties of Solids, vol. II, Academic Press, New York, 2001. [4] A.A. Maznev, Al.A. Kolomenskii, P. Hess, Phys. Rev. Lett. 75 (1995) 3332. [5] Al.A. Kolomenskii, A.M. Lomonosov, R. Kuschnereit, P. Hess, V.E. Gusev, Phys. Rev. Lett. 79 (1997) 1325. [6] A. Lomonosov, P. Hess, Phys. Rev. Lett. 83 (1999) 3876. [7] A.A. Karabutov, Sov. Phys.-Usp. 28 (1985) 1042. [8] V.E. Gusev, A.A. Karabutov, Laser Optoacoustics, AIP Press, New York, 1993. [9] W. Arnold, B. Betz, B. Homann, Appl. Phys. Lett. 47 (1985) 672.
[10] J.V. Knuuttila, P.T. Tikka, M.M. Salomaa, Opt. Lett. 25 (2000) 613. [11] D. Courjon, C. Bainier, Rep. Prog. Phys. 57 (1994) 989. [12] B. Hecht, B. Sick, U.P. Wild, V. Deckert, R. Zenobi, O.J.F. Martin, D.W. Pohl, J. Chem. Phys. 112 (2000) 7761. [13] J.A. Veerman, A.M. Otter, L. Kuipers, N.F. van Hulst, Appl. Phys. Lett. 72 (1998) 3115. [14] C.B. Scruby, L.E. Drain, Laser Ultrasonics, Adam Hilger, Bristol, 1990. [15] C.M. Scala, P.A. Doyle, J. Acoust. Soc. Am. 85 (1989) 1569. [16] S.A. Akhmanov, V.E. Gusev, Sov. Phys.-Usp. 35 (1992) 153.