INTERFEROMETRY

INTERFEROMETRY
Watching Ripples on Crystals
With a Sagnac Interferometer
Yoshihiro Sugawara, Oliver B.Wrightand Osamu Matsuda

aser acoustic techniques, because of
their non-contact nature, are attractive vehicles for probing the mechanicaland elastic properties of solids. Particularsensitivity to near-surface
properties or to thin films
can be obtained by optically generating and detecting surface acousticwaves (SAW). Althoughprogress in this field has
been promising,1 the signal-to-noise ratio has notbeen sufficient to allow
real-time imaging of SAW
focusing patterns in elastically anisotropic materials. With the increasinguse of SAW on crystals inapplications such as theSAW filters used in communication systems andcellular phones, this is aproblem of significant
practical importance.
We describe a new
method for imaging SAW
in real time using an ultrafast optical techniqueinvolving a Sagnac interferometer.2 We image
SAW at frequencies of up
to 1 GHz generated at apoint source in isotropic
and anisotropic solids
and make animations of
their propagation.
The samples used
are metallic thin films
on transparent substrates[see Fig. 1(a)]. We excite the SAW with atrain of blue optical pulses, each of a duration of about one picosecond, arriving at
the surface periodically at intervals ofabout ten nanoseconds. This light is focused through the substrate onto a micrometer-sized spot on the sample, thus
stimulating SAW through thermal expansion. After each blue light pulse, we fire
two infrared probe pulses onto the front
surface of the film. The highly stable
Sagnac interferometer 3 is designed toLFigure 1. (a) Sagnac interferometer used for surface acoustic wave imaging in real time. PBS, NPBSand /4 mean polarizing beam splitter, non-polarizing beam splitter and quarter-wave plate. Theprobe light enters the interferometer with its
plane of polarization at 45° to the plane of the diagram. (b) SAW image for the (001) surface ofTeO2 coated with a 40-nm Au film. (c) 3D rendering of the central portion of (b). (d) and (e): SAW
and optical reflectivity images for a crown glass
substrate coated with a 140-nm Cr film and Aupyramids.The excitation optical pulses are incident
at the top right.

20 Optics & Photonics News ■ December 2002

monitor the optical phase difference between these probe pulses and to follow thetiny surface displacement induced by theSAW, typically only 0.2 atomic diameters.
By scanning the lateral probe position andprobe delay time, SAW wave fronts can beimaged with micrometer spatial and picosecond temporal resolutions.
Figure 1(b) corresponds to a SAW image of a 150 m  150 m region of tellurium oxide, a tetragonal crystal coatedwith a 40-nm polycrystalline gold film.
The pattern is exquisitely intricate owing
to the strong elastic anisotropy and showsthe expected four-fold symmetry andcaustic behavior for the (001) crystal cut
used.2 Figure 1(c) shows a 3D rendering of
the central portion of Fig. 1(b).
Figures 1(d) and (e) show, respectively,
SAW and optical reflectivity images for a90 m  90 m region of a microstructure consisting of an array of flat-topped
gold pyramids (1-m-high polycrystalline) deposited on a 140-nm polycrystalline chromium film on a glass substrate.
The acoustic refraction clearly visible atthe pyramid boundaries, which produces
an inversion of the SAW wave-front curvature inside the pyramid, is mainly
caused by the difference in SAW propagation velocities in gold (~1 km s-1) and inglass (~3 km s-1).
Direct dynamic optical visualization ofSAW wave fronts provides new insights
into the elaborate underlying mechanisms
for SAW focusing, dispersion, refraction
and scattering. There are opportunities fora wide variety of studies on subjects suchas the animation of phonon ripples on
samples of surface phononic crystals (theacoustic analogue of photonic crystals),phonon waveguides and phonon resonators. Practical applications of this ultrafast optical imaging technique in thenondestructive testing of SAW devices orin thin film thickness monitoring are also
expected.

References
1.
2.

3.

M. Clark, S. Sharples and M. Somekh, Meas. Sci.Technol. 11, 1792 (2000).
Y. Sugawara, O. B.Wright, O. Matsuda, M.Takigahira,Y.Tanaka, S.Tamura and V. E. Gusev, Phys. Rev. Lett.
88, 185504 (2002).
D. H. Hurley and O. B.Wright, Opt. Lett. 24, 1305
(1999).

Yoshihiro Sugawara, Oliver B. Wright (assp@eng.
hokudai.ac.jp) and Osamu Matsuda are with the Department of Applied Physics, Faculty of Engineering,
Hokkaido University, Sapporo, Japan. For more information, SAW animations can be viewed at kinoap.eng.hokudai.ac.jp/ripples.html.