Nanoacoustics II Lecture #4 Brillouin scattering

Transcription

1 Nanoacoustics II Lecture #4 Brillouin scattering Dr. Ari Salmi

2 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

3 STAMP Coherent phonon propagation in transparent media

4 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?

5 Attenuation of phonons in water Erohkin, J. Russ. Las. Res. 4, 2002 α = ~1000 db/cm = GHz λ = 1.5 µm α = ~10000 db/cm = 1 10 GHz λ = 150 nm α = ~70000 db/cm = GHz λ = 15 nm α = ~ db/cm = THz λ = 1.5 nm

6 Scattering of light (from mechanical movement) Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi

7 Scattering Elastic vs. Inelastic scattering Elastic Energy of the incident photon is conserved Example: Rayleigh scattering (from particles smaller than the wavelength) Xkcd.com

8 Scattering Inelastic scattering Energy of the incident photon is changed Raman scattering Photon inelastically scatters from the intramolecular vibration

9 Scattering Brillouin scattering Photon scatters from a phonon (intermolecular vibration)

10 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)

11 Brillouin scattering Electromagnetic wave Acoustic wave (phonon) The electromagnetic wave scatters into an angle Meng et al., Advances in optics and photonics

12 (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

13 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

14 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

15 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

16 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

17 Applications of Brillouin scattering Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi

18 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

19 Spider silk From the speeds of sound in different directions, the full stiffness tensor

20 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

21 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

22 Studying the time of death Troyanova-Wood et al., SPIE BiOS 2016 Studied ex vivo melanoma with Brillouin scattering

23 Brillouin microscopy Brillouin spectroscopy as a function of position (in 3D) Scarcelli et al., Nature Methods 2015

24 Brillouin microscopy Also longitudinal (time-wise) studies Scarcelli and Yun, Nature Photonics 2008 Curing of a polymer

25 Brillouin microscopy Mouse eye mechanical characterization

26 Brillouin microscopy Cell mechanical property characterization Antonacci and Braakman, Sci. Rep. 2016

27 Brillouin microscopy Kim et al., Optics Express 2016 Shear Brillouin microscopy

28 Brillouin microscopy Kim et al., Optics Express 2016 Shear Brillouin microscopy

29 Stimulated Brillouin scattering Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi

30 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

31 SBS This can also be shown in the omega-k space more intuitively

32 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

33 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

34 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

35 Uses of SBS Selectively either pump the Stokes (heating) or anti- Stokes (cooling) process and observe the Brillouin scattering from the acoustic mode

36 Uses of SBS Selectively either pump the Stokes (heating) or anti- Stokes (cooling) process and observe the Brillouin scattering from the acoustic mode

37 Uses of SBS Cooled down to 19K

38 Uses of SBS Storing light Zhu et al., Science 2007 Based on data storage by stimulated Brillouin scattering (Stokes and anti-stokes)

39 Uses of SBS Stored light for up to 12 ns

40 Take-home Matemaattis-luonnontieteellinen tiedekunta / Henkilön nimi / Esityksen nimi

41 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)