APPLIED PHYSICS LETTERS 89, 194103 共2006兲
Detection of acoustically induced electromagnetic radiation Kenji Ikushima,a兲 Shunsuke Watanuki, and Susumu Komiyama Department of Basic Science, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan
共Received 1 August 2006; accepted 18 September 2006; published online 7 November 2006兲 Electromagnetomechanical response of materials is detected through electromagnetic radiation induced by ultrasound waves. Target specimens are placed in the focusing zone of the ultrasound waves at a distance of ⬃60 mm from an acoustic generator. The radiation is picked up by a narrow-band loop antenna tuned to the ultrasound frequency. Due to the delay time caused by ultrasound wave propagation in the medium 共water兲, the pulsed electromagnetic radiation from a target is well separated 共temporally by ⬃40 s兲 from the apparent signal attributed to the generator, allowing unambiguous and sensitive detection of true signals. Upon stimulation with ultrasounds, emission of electromagnetic radiation is found in bones, woods, plastics, and ferrites as well as a standard piezoelectric material 共GaAs兲. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2374847兴 Ultrasound imaging technique is widely applied as noninvasive probe to human bodies and material structures. One important advantage of the technique is that elastic waves are capable of propagating through opaque substances such as human bodies, metals, and concrete blocks, in which light does not propagate. Owing to the remarkable difference between the sound and the light velocities, elastic waves are featured by remarkably short wavelengths, being by about five digits smaller than those of electromagnetic 共EM兲 waves. It follows that sharp focusing on a millimeter/ micrometer scale is achievable in the megahertz/gigahertz range where real-time wave form analysis is performed in commercially available equipments. Despite these advantages, the majority of existing applications are restricted to diagnosing elastic properties of the targets, viz., electromagnetomechanical properties are not probed. Electromechanical imaging techniques are highly developed in scanning probe microscope systems,1,2 but only surface analysis is targeted. Here we propose and demonstrate a distinguishing method of probing electromagnetomechanical properties of matters via acoustic-wave excitation. Electromagnetic and mechanical properties are closely coupled in many materials. A familiar example is the piezoelectricity, which ionic crystals exhibit due to the lack of inversion symmetry. It is known that a variety of crystallized biological molecules such as bones, tendons, muscles, etc., also shows piezoelectricity.3–6 A number of studies on medical applications using stress, electromagnetic fields, and ultrasound waves report biological effects of electromechanical response.6–8 Since the equation of acoustic waves in piezoelectric materials include electric-field terms, EM waves will be generated when a piezoelectric slab is in resonance with acoustic waves, as theoretically suggested in Ref. 9. Measurements of such EM radiation are performed in wireless operation of acoustic-wave devices,10,11 but detection of extremely weak EM radiation from materials to be studied has been untackled.12,13 Utilizing a pulsed acoustic technique, we show that EM waves are emitted from a variety of materials when they are acoustically stimulated and that the emission takes place even without the resonance condition. a兲
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Figures 1共a兲 and 1共b兲 depict the experimental setup. We prepared two schemes, a water-immersion method 关Fig. 1共a兲兴 and a nonimmersing probe method with an acoustic transmission tube 关Fig. 1共b兲兴. In both schemes, a target sample is placed in a focused zone at a distance 共50– 70 mm兲 from a medical-use polyvinylidene fluoride 共PVDF兲 transducer, in which rectangular 50 ns wide pulses are applied at a repetition rate of 100– 500 Hz by a pulser/receiver 共PanametricsNDT, 5077PR兲. Noting the ultrasound velocity in water 共1500 m / s兲, one expects that EM waves acoustically induced and emitted by the sample are temporally separated by 33– 47 s from the generator excitation pulses. EM waves are detected through a loop antenna tuned to the center frequency 共f 0兲 of ultrasound waves with a bandwidth of ⬃200 kHz. Using a broad-band hydrophone, we confirm that the ultrasound waves are concentrated on a focal spot of less than 2 mm diameter, with a spectrum around f 0 = 9.25 MHz 关Fig. 1共c兲兴. Signals picked up by the antenna are fed to a low-noise preamplifier and averaged over pulses by using a digital oscilloscope. Though not shown here, heterodyne detection of EM waves is also carried out by using a double balanced mixer. The first target sample is GaAs with piezoelectric comGaAs 兩 = 2.7 pC/ N.14 Longitudinal acoustic waves ponent of 兩d14 are expected to generate EM waves when their wave vector k is parallel to the piezoelectric axis of 具110典. A nondoped 0.35 mm thick GaAs crystal with the 关110兴 axis aligned to k of the incident acoustic waves is studied by the waterimmersion method 关Fig. 1共a兲兴. In addition to familiar acoustic echo signal 关Fig. 2共a兲兴, EM waves are also detected as shown in Fig. 2共b兲, where the EM signal is amplified by 82 dB and averaged over 200 pulses 共1 s兲. The prominent two EM signals coincident with the excitation 共t = 0 s兲 and the echo 共t = 88 s兲 signals of acoustic waves are attributed to EM waves generated by the PVDF transducer. We find, in addition, a weaker EM signal 共68 V兲 occurring at the middle point 共t = 44 s兲 between the excitation and the echo pulses. This is identified as the EM radiation emitted by the GaAs crystal 关Fig. 2共b兲兴. A secondary EM signal due to GaAs is also visible at t = 132 s. The envelope of the EM signal clarified by heterodyne detection is shown in Fig. 2共c兲. When the target is replaced with a 共nonpiezoelectric兲 pure
0003-6951/2006/89共19兲/194103/3/$23.00 89, 194103-1 © 2006 American Institute of Physics Downloaded 07 Nov 2006 to 133.11.199.17. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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Ikushima, Watanuki, and Komiyama
Appl. Phys. Lett. 89, 194103 共2006兲
FIG. 1. Schematic representation of measurements for acoustically induced EM radiation: 共a兲 Waterimmersion method and 共b兲 nonimmersing probe method with a plastic tube instead of water tank. 共c兲 Spectrum of ultrasound wave produced by a PVDF transducer.
silicon crystal of similar geometry, we find no discernible EM signal ascribable to the target, as displayed in Fig. 3共a兲. The study of a similar GaAs crystal with the 关100兴 axis aligned to k confirms that the piezoelectrically active propagation, k 储 关110兴, yields significantly larger EM signals than for k 储 关110兴 as demonstrated in Figs. 3共b兲 and 3共c兲. These findings definitely support the interpretation that the observed EM radiation is generated via the piezoelectricity of GaAs crystal. EM radiation spectrum studied by the heterodyne detection is shown in the inset of Fig. 3. The distinct peak of the spectrum at 7.60 MHz is suggested to be due to mechanical
resonance of the GaAs crystal, the thickness of which 共0.35 mm兲 is comparable to the half-wavelength 关the sound velocity of 4730 m / s in GaAs 共Ref. 15兲兴. The enhancement factor due to the resonance is ⬃10, but we note that EM waves are detectable also in the nonresonant condition. Owing to the ubiquity of electromechanical coupling in biological materials,3–6 this measurement scheme may find broad application in biological and clinical researches. We have carried out preliminary studies on bone. Principal ingredients of bone are hydroxyapatite crystal 共⬃70% 兲 and highly oriented collagen fibers 共⬃20% 兲, where the latter is known to contribute to piezoelectricity 共兩dbone兩 ⬇ 0.1pC/ N兲.3 We take costae of swine as the sample of bone and prepare ⬃2 mm thick square plates, respectively, out of the outer hard layer and the inner soft tissue, where the fiber axis is parallel to the plate surface. The samples are washed in eth-
FIG. 2. Typical time traces obtained for a GaAs 共110兲 plate: 共a兲 Acoustic FIG. 3. Time traces for 共a兲 Si 共100兲 plate, 共b兲 GaAs 共100兲 plate, and 共c兲 echo signals, 共b兲 EM signals, and 共c兲 an envelope of EM waves obtained via GaAs 共110兲 plate. Longitudinal ultrasound wave is normally incident on the heterodyne detection. A scale of the signal intensity for 共b兲 is denoted on the plate. The inset shows the spectrum of EM waves emitted from a GaAs left axis. 共110兲 plate. Downloaded 07 Nov 2006 to 133.11.199.17. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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Appl. Phys. Lett. 89, 194103 共2006兲
Ikushima, Watanuki, and Komiyama
FIG. 4. EM signals obtained for plates of 共a兲 bone 共outer hard layer兲, 共b兲 wood, 共c兲 plastic 共polypropylene兲, and 共d兲 ferrite 共SrO / Fe2O3兲.
anol solution for 1 h by ultrasonic cleaner. The acoustic wave vector is perpendicular to the fiber axis for all the samples. The signal is amplified with a gain of 97 dB and averaged for 10 min with 500 Hz repetition. We find acoustically induced EM signal from the outer layer of bone as shown in Fig. 4共a兲, where result obtained via the waterimmersed method is displayed. Similar result is obtained from the samples of inner tissues with no discernible difference in the signal intensity. It is reported that piezoelectricity of bone is considerably reduced by water due to ionic screening.16 In the present experiments, however, the screening effect might be negligible because the process of screening is usually slower than in the megahertz range.17 The measurements of immersed bones here make us expect possible application of the present method to noninvasive probing of living bones. Using the nonimmersing probe method 关Fig. 1共b兲兴, we have extended the studies to different materials such as wood and plastic.18 Acoustically induced EM radiation is found in wood as shown in Fig. 4共b兲. The mechanism is supposed to be due to the piezoelectricity of crystallized cellulose,5 which is the basic component of wood. Though extremely weak, EM signal is detected in an amorphous polymer compound of commodity plastic 共composed of a piezoelectric polypropylene兲 as shown in Fig. 4共c兲, suggesting the presence of crystallized grains. Additional experiments on thicker slabs of bone and wood reveal that the enhancement takes place primarily at the leading edge of each EM wave pulse. This suggests that the EM wave emission occurs predominantly in a region close to the front face of each specimen. This may be reasonable because if a regular sinusoidal distribution of alternating electric polarization is assumed over a region substantially larger than a half-wavelength of acoustic waves, the EM radiation will be canceled out at a distance well larger than the acoustic wavelength.19 In addition to piezoelectricity, magnetomechanical coupling is expected to generate acoustically induced EM radiation. Figure 4共d兲 represents the EM waves emitted from ferrite 共SrO / Fe2O3兲, where a cylindrical ferrite magnet
共20 mm diameter⫻ 15 mm height兲 is studied by the immersion method. The comblike structure in the EM wave form indicates a multiple reflection of acoustic waves at the front and back faces of the specimen.20 In summary, we have demonstrated a unique method of exciting a wide variety of materials with acoustic waves and probing induced EM waves. The materials studied include biological tissues, plastics, and ferrites as well as piezoelectric GaAs. It is a distinct advantage of the present method that pulsed EM signals are temporally well separated from the excitation and the echo signals/noises generated by the ultrasound transducer. When combined with ultrasound scanning technique, the method will make it possible to image electromagnetomechanical properties of a wide variety of matters. This work was supported by the Industrial Technology Research Grant Program in 2006 from New Energy and Industrial Technology Development Organization 共NEDO兲 of Japan and the Solution Oriented Research for Science and Technology 共SORST兲 from Japan Science and Technology Corporation 共JST兲. 1
A. Gruverman, O. Auciello, and H. Tokumoto, Annu. Rev. Mater. Sci. 28, 101 共1998兲. 2 S. V. Kalinin, E. Karapetian, and M. Kachanov, Phys. Rev. B 70, 184101 共2004兲; S. V. Kalinin, B. J. Rodriguez, S. Jesse, T. Thundat, and A. Gruverman, Appl. Phys. Lett. 87, 053901 共2005兲. 3 E. Fukada and I. Yasuda, J. Phys. Soc. Jpn. 12, 1158 共1957兲. 4 E. Fukada and I. Yasuda, Jpn. J. Appl. Phys. 3, 117 共1964兲. 5 E. Fukada, J. Phys. Soc. Jpn. 10, 149 共1955兲. 6 E. Fukada, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 47, 1277 共2000兲. 7 A. A. Marino and R. O. Becker, Nature 共London兲 228, 473 共1970兲. 8 Electromagnetics in Medicine and Biology, edited by C. T. Brighton and S. R. Pollack 共San Francisco Press, San Francisco, 1991兲. 9 J. J. Kyame, J. Acoust. Soc. Am. 21, 159 共1949兲. 10 H. Ogi, H. Hiho, and M. Hirao, Appl. Phys. Lett. 88, 141110 共2006兲. 11 M. Thompson, S. M. Ballantyne, L.-E. Cheran, A. C. Stevenson, and C. R. Lowe, Analyst 共Cambridge, U.K.兲 128, 1048 共2003兲. 12 Studies of EM radiation originated from piezoelectricity of rock are reported in geophysical researches, but the EM signals are produced by rock fracture or high-pressure experiments. For instance, see Ref. 17. 13 Although the inversed phenomenon, EM-wave-induced acoustic radiation, is applied to biomedical imaging, the quantity obtained is not piezoelectric response but thermal expansion response generated by absorption of EM waves. See M. Xu and L. V. Wang, Rev. Sci. Instrum. 77, 041101 共2006兲. 14 S. Adachi, J. Appl. Phys. 58, R1 共1985兲. 15 O. Madelung, Semiconductors Group IV Elements and III-V Compounds 共Springer, Berlin, 1991兲. 16 S. R. Pollack, E. Korostoff, W. Starkbaum, and W. Iannicone, Electrical Properties of Bone and Cartilage 共Grune & Stratton, New York, 1979兲, pp. 69–81; E. Fukuda, Ferroelectric Polymers 共Dekker, New York, 1995兲, pp. 434. 17 S. Yoshida and T. Ogawa, J. Geophys. Res. 109, B09204 共2004兲. 18 To avoid apparent EM signal from the plastic tube, the edge of the tube should not touch the specimen stimulated acoustically. 19 EM radiation may not be strictly canceled in the present measurements, where pulse waves are focused on the specimen. Hence EM waves emitted from the bulk region will be detectable if the sensitivity is improved. 20 Detection time of EM-signal peak in the comblike structure corresponds to a half of arrival time of echo signal reflected on the top or the bottom face of the ferrite. Owing to the large impedance mismatching with water, the long-time response is allowed by ultrasound waves confined in the ferrite.
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