Nanoacoustics II Lecture #4 Brillouin scattering Dr. Ari Salmi www.helsinki.fi/yliopisto 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
STAMP Coherent phonon propagation in transparent media 29.3.2018 3
Coherent phononics in fluids 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? http://www.middleschoolchemistry.com/img/content/multimedia/chapter_2/lesson_5/states_of_matter_big.jpg 29.3.2018 4
Attenuation of phonons in water Erohkin, J. Russ. Las. Res. 4, 2002 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 29.3.2018 5
Scattering of light (from mechanical movement) Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi www.helsinki.fi/yliopisto 29.3.2018 6
Scattering Elastic vs. Inelastic scattering Elastic Energy of the incident photon is conserved Example: Rayleigh scattering (from particles smaller than the wavelength) http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/imgatm/raymie.gif Xkcd.com
Scattering 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) http://www.icmm.csic.es/brillouin/sbs-bulk.jpg
Stokes and anti-stokes events 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
Brillouin scattering Electromagnetic wave Acoustic wave (phonon) The electromagnetic wave scatters into an angle Meng et al., Advances in optics and photonics
(Spontaneous) 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
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
Brillouin scattering 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 Speziale et al., Rev. Min. & Geol., 2014
Brillouin scattering 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 Speziale et al., Rev. Min. & Geol., 2014
Applications of Brillouin scattering Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi www.helsinki.fi/yliopisto 29.3.2018 17
Spider silk 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
Detecting the presence of meningitis Steelman et al., Journal of Biophotonics 2014 Detected a difference in the elasticity of healthy and diseased spinal fluid samples
Studying the time of death 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
Brillouin microscopy 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
Stimulated Brillouin scattering Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi www.helsinki.fi/yliopisto 29.3.2018 29
SBS 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
Uses of SBS Massively increased SNR compared to Brillouin scattering Only 1 in 10 7 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
Take-home Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi www.helsinki.fi/yliopisto 29.3.2018 40
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)