Nanoacoustics II Lecture #4 - electronics.physics.helsinki.fielectronics.physics.helsinki.fi/wp-content/uploads/2018/03/Lecture-4... · Nanoacoustics II Lecture #4 Brillouin scattering

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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

29.3.2018 3

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?

29.3.2018 4

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

29.3.2018 5

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


Scattering of light (from

mechanical movement)

29.3.2018

Matemaattis-luonnontieteellinen tiedekunta /

Henkilön nimi / Esityksen nimi 6

• 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)

29.3.2018 10

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


• 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:


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 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



• 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




Applications of Brillouin

scattering

29.3.2018



• 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


• Mouse eye mechanical characterization


• Cell mechanical property characterization

• Antonacci and Braakman, Sci. Rep. 2016


• Kim et al., Optics Express 2016

• Shear Brillouin microscopy


• Kim et al., Optics Express 2016

• Shear Brillouin microscopy



Stimulated Brillouin

scattering

29.3.2018



• 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


Take-home

29.3.2018



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)

Nanoacoustics II Lecture #4 - electronics.physics.helsinki.fielectronics.physics.helsinki.fi/wp-content/uploads/2018/03/Lecture-4... · Nanoacoustics II Lecture #4 Brillouin scattering

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