Brillouin Imaging Demonstrated
Daniel S. Burgess
Scientists at Arizona State University in Tempe have developed a confocal Brillouin imaging microscope that enables the rapid collection of information about the elastic properties of condensed matter systems. The system has potential applications in the testing of safety glass and in the measurement of blood flow velocity in vessels close to the surface of the skin.
Jeff Yarger, director of the Magnetic Resonance Research Center and a professor of chemistry and biochemistry at Arizona State, explained that the apparatus measures the speed of acoustic waves traveling through a sample based on the frequency shift the waves induce in scattered laser light, a Doppler effect known as Brillouin scattering. He developed the system with Kristie J. Koski, a former student of his who is now a graduate student at the University of California, Berkeley.
Unlike the tandem scanning Fabry-Perot interferometer system designed by John R. Sandercock, the new single-pass, angle-dispersive setup can collect thousands of spectra per second, Yarger said. Although it lacks the finesse of the Sandercock spectrometer, its acquisition speed enables the user to perform time-resolved experiments or to assemble spectra from many points across a sample to form images. And unlike ultrasonic techniques, it does not introduce acoustic waves into the sample, so it is less invasive.
In the Brillouin imaging setup, which is essentially a Fabry-Perot interferometer, a confocal arrangement with an adjustable slit enables the spatial filtering of the backscattered signal from the sample and the rejection of unwanted components such as Raman scattered light and specular reflection, which are further controlled using an interference filter. A Coherent Inc. CW Nd:YAG laser, operating at 532 nm, serves as the illumination source, and a cooled CCD detector from Andor Technology collects the spectra. To enable 1- and 2-D scanning, the sample is mounted on an X-Y-Z stage driven by linear actuators made by Zaber Technologies Inc.
To test the apparatus, the researchers produced a 5-mm-long scan across a mixture of oil and water and a 0.4 × 0.5-mm image of a piece of polystyrene in methanol. The acquired longitudinal acoustic velocities of the materials were in good agreement with those recorded previously using ultrasound or a Sandercock spectrometer. The spatial resolution was approximately 2 µm, limited by the focused spot size of the laser, and the spectral resolution of the Brillouin frequency shifts was approximately 0.05 GHz, corresponding to an acoustic velocity of approximately 10 m/s.
Yarger envisions the application of Brillouin imaging in the experimental study of materials failure. Safety glass for automobiles, for example, could be monitored under applied stress to verify that strain profiles develop as desired so that the glass will break as it should. Similarly, time-resolved measurements of the preferential buildup of strain in homogeneous structures such as aircraft wings would offer data to predict failure.
Before the technique is ready for such work, however, a means must be found to reject Rayleigh scattered light, which can swamp the Brillouin-shifted signal. The problem is particularly acute with highly reflective materials such as metals, Yarger said. Proper filter design may address this challenge.
Alternatively, he suggested, the illumination source could be replaced with one emitting at a wavelength that is not reflected well by the materials to be studied. Brillouin imaging instruments for biomedical applications, for example, might feature an infrared laser.
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