nanoacoustics ii lecture #4 -...
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Nanoacoustics II
Lecture #4
Brillouin scattering
Dr. Ari Salmi
29.3.2018 1
Last lecture – key points
• Phonons propagate in liquids but not in gases
• Shear phonons have a minimum frequency
• Phononic microscope can be used for biological sample
characterization
• Coherent phonon propagation in transparent media
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STAMP
• Average interatomic distance different in different
phases
• For solids (metals) ~ 2.5 Å
• For liquids (water) ~ 4 Å
• For gases (air) ~ 60 nm
• What does this mean for the allowed frequencies?
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Coherent phononics in fluids
http://www.middleschoolchemistry.com/img/content/multimedia/chapter_2/lesson_5/states_of_matter_big.jpg
• Erohkin, J. Russ. Las. Res. 4, 2002
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Attenuation of phonons in water
α = ~1000 dB/cm = 0.1 dB/µm @ 1 GHz
λ = 1.5 µm
α = ~10000 dB/cm = 1 dB/µm @ 10 GHz
λ = 150 nm
α = ~70000 dB/cm = 7 dB/µm @ 100 GHz
λ = 15 nm
α = ~1000000 dB/cm = 100 dB/µm @ 1 THz
λ = 1.5 nm
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Scattering of light (from
mechanical movement)
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• Elastic vs. Inelastic scattering
• Elastic – Energy of the incident photon is conserved
• Example: Rayleigh scattering (from particles smaller
than the wavelength)
Scattering
Xkcd.com
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/imgatm/raymie.gif
• Inelastic scattering – Energy of the incident photon is
changed
• Raman scattering
• Photon inelastically scatters from the intramolecular
vibration
Scattering
• Brillouin scattering
• Photon scatters from a phonon (intermolecular
vibration)
Scattering
http://www.icmm.csic.es/brillouin/SBS-bulk.jpg
• Stokes and anti-Stokes events
• Stokes = photon loses energy and translates it to the
molecule (Raman) or phonon (Brillouin)
• Anti-Stokes = photon gains energy by absorbing a
phonon (Brillouin) / molecule vibrational energy
(Raman)
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Stokes and anti-Stokes events
• Electromagnetic wave
• Acoustic wave (phonon)
• The electromagnetic wave scatters into an angle
• Meng et al., Advances in optics and photonics
Brillouin scattering
• The frequency of the scattered light is increased or
decreased by the Brillouin shift (+ = Stokes, - = Anti-
Stokes)
• Depending on the attenuation, the Brillouin peaks
widen
(Spontaneous) Brillouin
scattering
• Thus, one can determine the entire complex
elasticity at hypersonic frequencies from the
frequency and peak width:
Brillouin scattering
Speziale et al., Rev. Min. & Geol., 2014
• This means that one can determine the mechanical
properties at a certain frequency of phonons by light
scattering
• Dependent on the color of the probing beam
Brillouin scattering
• Brillouin scattering does not require a pulsed laser
• Requirement: Very monochromatic laser beam (single
mode)
• Accurate spectrometer required
‒ This takes a lot of time, since one has to scan over the
frequencies
Brillouin scattering
Speziale et al., Rev. Min. & Geol., 2014
• In the recent days, VIPA has been used for fast
spectrometry combined with a CCD
• Shirasaki 1999
• A lot faster, measures the entire spectrum at once
Brillouin scattering
Speziale et al., Rev. Min. & Geol., 2014
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Applications of Brillouin
scattering
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• Koski et al., Nature Materials 2013
• Measured the full mechanical properties of spider
silk
• Anisotropic material different orientation of
measurements required
• Reveals different acoustic modes
Spider silk
• From the speeds of sound in different directions, the
full stiffness tensor
Spider silk
• Steelman et al., Journal of Biophotonics 2014
• Detected a difference in the elasticity of healthy and
diseased spinal fluid samples
Detecting the presence of
meningitis
• Reiss et al., IEEE TBME 2012
• Brillouin shift as a function of time after death in a
human eye lens
Studying the time of death
• Troyanova-Wood et al., SPIE BiOS 2016
• Studied ex vivo melanoma with Brillouin scattering
Studying the time of death
• Brillouin spectroscopy as a function of position (in
3D)
• Scarcelli et al., Nature Methods 2015
Brillouin microscopy
• Also longitudinal (time-wise) studies
• Scarcelli and Yun, Nature Photonics 2008
• Curing of a polymer
Brillouin microscopy
• Mouse eye mechanical characterization
Brillouin microscopy
• Cell mechanical property characterization
• Antonacci and Braakman, Sci. Rep. 2016
Brillouin microscopy
• Kim et al., Optics Express 2016
• Shear Brillouin microscopy
Brillouin microscopy
• Kim et al., Optics Express 2016
• Shear Brillouin microscopy
Brillouin microscopy
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Stimulated Brillouin
scattering
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• Brillouin scattering is a ’static’ process
• Stimulated Brillouin scattering (SBS) uses beat
frequency of two laser beams to generate phonons
• This beat frequency has to match the desired mode
Brillouin frequency
SBS
• This can also be shown in the omega-k space more
intuitively
SBS
• Massively increased SNR compared to Brillouin
scattering
• Only 1 in 107 photons is passively Brillouin scattered
• E.g. Ballmann et al., Sci. Rep. 2015
Uses of SBS
• Cooling using SBS
• Bahl et al., Nature Physics 2012
• Custom made silica resonator with one acoustic and
two optical whispering gallery modes
Uses of SBS
• Based on two optical modes
• Their beat frequency pumps the whispering gallery
acoustic mode
• The system amplifies the anti-Stokes process
resonantly
Uses of SBS
• Selectively either pump the Stokes (heating) or anti-
Stokes (cooling) process and observe the Brillouin
scattering from the acoustic mode
Uses of SBS
• Selectively either pump the Stokes (heating) or anti-
Stokes (cooling) process and observe the Brillouin
scattering from the acoustic mode
Uses of SBS
• Cooled down to 19K
Uses of SBS
• Storing light
• Zhu et al., Science 2007
• Based on data storage by stimulated Brillouin
scattering (Stokes and anti-Stokes)
Uses of SBS
• Stored light for up to 12 ns
Uses of SBS
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Take-home
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Take-home
• Brillouin scattering can be used to measure the entire stiffness
tensor at GHz+ frequencies
• Stimulated Brillouin scattering can be used to generate
interesting effects in materials (cooling, light storing, light diode)