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Brillouin Microfluidics

June 28, 2013

The French physicist, Léon Brillouin (1889-1969), who came of scientific age during a golden era of physics, contributed to many of its topic areas. He studied under Arnold Sommerfeld and Paul Langevin, and Marie Curie was part of the academic committee that certified him for his Docteur ès Science in 1920. Phonons, which are quantized acoustic vibrations in solids, were a part of Brillouin's doctoral dissertation.

Marcel and Leon Brillouin

Like father, like son.

Léon Brillouin's father, Marcel (left), was also a physicist.

(Source images, left and right, via Wikimedia Commons.)

Phonons will gently expand and compress regions of an optical crystal, and this will affect its refractive index. Such a change in the refractive index causes a small scattering of light, now known as Brillouin scattering. Since acoustic waves have a wavelength, they will produce a periodic variation in refractive index, effectively producing a shallow diffraction grating. Additionally, if the acoustic wave is not part of a resonator, it's not a standing wave, so the grating is moving and it will Doppler shift the light.

Brillouin scattering is quite weak, but the advent of lasers, modern light detectors and advanced signal processing make many Brillouin scattering experiments practical. I wrote about one of these in an article earlier this year (Spider Silk Mechanics, February 15, 2013). A research team from the Department of Chemistry and Biochemistry, Arizona State University (Tempe, Arizona) has been investigating strands of spider silk by Brillouin light scattering.[1-2]

A mechanical engineer from the University of Illinois at Urbana-Champaign has teamed with electrical engineers and biomedical engineers from the University of Michigan, Ann Arbor, to apply Brillouin scattering to the coupling of phonons and photons in liquids.[3-4] Such experiments have been hitherto challenging, since immersion of optomechanical components into a liquid dampens the acoustic vibrations. The acoustic energy leaks out to the liquid surrounding the device.

The Illinois-Michigan research team has gotten around this problem by putting the liquids into hollow capillaries. This allowed the optical excitation of mechanical whispering-gallery modes at frequencies from 2 MHz to 11 GHz.[3-4] The light is coupled to the liquid from the outer, dry side of the capillary, and the liquids are brought into the capillary by a standard microfluidic inlet.[3] This experiment builds on their experience in a previous study in which they excited whispering-gallery type mechanical resonances in 100 micrometer diameter silica microspheres in a frequency range of 49 MHz to 1.4 GHz using forward Brillouin scattering .[5]

A microfluidic optomechanical resonator

Scanning electron micrograph of a microfluidic optomechanical capillary resonator with false color highlighting the capillary resonator.

(University of Illinois image.)[4)]

Mounted microfluidic optomechanical capillary resonator

Microfluidic optomechanical capillary resonator in a mounting with its fluid control tubing.

(University of Illinois image.)[4)]

The capillaries care made from fused silica glass, and the optical whispering gallery modes enhance the intensity of the force resulting from the optical excitation by many orders-of-magnitude. This allows a greater interaction between photons and phonons. Strong phonon-photon coupling is interesting, since it enables experiments in quantum information storage, optomechanical cooling, and ultra-sensitive force measurements.[4]

Says Gaurav Bahl, an assistant professor of mechanical science and engineering at the University of Illinois and lead author of the paper describing this work,
"Optomechanics is an area of research in which extremely minute forces exerted by light (for example: radiation pressure, gradient force, electrostriction) are used to generate and control high-frequency mechanical vibrations of microscale and nanoscale devices... In particular, the high frequency, high quality-factor mechanical vibrations demonstrated in this work may enable strongly localized, high-sensitivity, optomechanical interaction with chemical and biological samples."[4]
This technology has some potential applications, such as optomechanical biosensors for measuring the optical and mechanical properties of single cells, rapid analysis of fluids, and optical control of fluid flow.[4] The Illinois research team is investigating this technology for building biosensors.[4] Brillouin scattering has been shown also to attenuate the Brownian motion of microscopic acoustic resonators.[6-7]


  1. Kristie J. Koski, Paul Akhenblit, Keri McKiernan and Jeffery L. Yarger, "Non-invasive determination of the complete elastic moduli of spider silks," Nature Materials, Published Online, January 27, 2013, doi:10.1038/nmat3549.
  2. Andrew Myers, "Stanford Researcher Sheds New Light on the Mysteries of Spider Silk, Stanford University Press Release, February 4, 2013.
  3. Gaurav Bahl, Kyu Hyun Kim, Wonsuk Lee, Tal Carmon, Wonsuk Lee, Jing Liu and Xudong Fan, "Brillouin cavity optomechanics with microfluidic devices," Nature Communications, vol. 4, article no. 1994, doi:10.1038/ncomms2994, June 7, 2013.
  4. Rick Kubetz, "Whispering light hears liquids talk," University of Illinois Press Release, June 7, 2013.
  5. Gaurav Bahl, John Zehnpfennig, Matthew Tomes and Tal Carmon, "Stimulated optomechanical excitation of surface acoustic waves in a microdevice," Nature Communications, vol. 2, article no. 403, doi:10.1038/ncomms1412, July 26, 2011.
  6. Gaurav Bahl, Matthew Tomes, Florian Marquardt and Tal Carmon, "Observation of spontaneous Brillouin cooling," Bahl et al, Nature Physics, vol.8, no.3 (January 22, 2012), pp. 203-207.
  7. Ivan Favero, "Optomechanics: The stress of light cools vibration," Bahl et al, Nature Physics, vol.8, no.3 (January 22, 2012), pp. 180-181.

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