Brillouin-Light-Scattering Spectroscopy

Transcription

1 Brillouin-Light-Scattering Spectroscopy 20th 21th of July 2010

2 Content Spin waves Brillouin Light Scattering (BLS) Quantum mechanical picture Conventional experimental setup Applications Time-resolved BLS Phase-resolved BLS Wavevector-resolved Summary BLS

3 Spin waves

4 Spin waves Landau-Lifshitz and Gilbert equation masterformate durch Klicken bearbeitendm = γ ( M H eff ) + α G ( M dm ) dt Ms dt te Ebene ritte Ebene Vierte Ebene Fünfte Ebene λ q

5 Spin waves Distinction between different energy contributions Exchange energy (generated by twist of neighbored spins, short range interaction) Dipolar energy (generated by magnetic poles in long-wavelength spin waves, long range interaction) q H0 q H0 q H0 q d

6 Brillouin-Light-Scattering

7 Inelastically scattered light Raman-Scattering Brillouin-Light-Scattering Optical phonons Acoustic phonons Molecule vibrations Spin waves Frequencies up to several THz Grating spectrometer Frequencies below ~ 500 GHz Tandem Fabry-Pérot interferometer

8 Brillouin-Light-Scattering Stokes: Anti-Stokes: Energy conservation law: ωs = ωi - ω ωs = ωi + ω Momentum conservation law: ks = ki - k ks = ki - k

9 Fabry-Pérot interferometer (FPI) The functionality of a Fabry-Pérot interferometer is based on multi-beam interference on two plane-parallel surfaces. The phase difference: 2π ϕ = 2nl cos(θ ) λ

10 Fabry-Pérot interferometer Transmittance and reflectivity The transmittance function is given by: (1 R ) 2 1 T= =, R 2 R cos( ϕ ) 1 + F sin ( ϕ / 2) where R is the reflectivity and F= 4isRthe coefficient of the finesse F. (1 R) 2 Maximum transmission occurs when m = 1,2,3, 2nl cos(θ ) = mλ,

11 Fabry-Pérot interferometer The free spectral range (FSR) is given by : λ20 λ20 λ = 2nl cos θ + λ0 2nl cos θ The finesse F is given by: F = λ π = δλ 2 arcsin 1 / F F π F π R = 2 1 R ( ) Our setup:f 110

12 Fabry-Pérot interferometer λ Maximal transmission: l = m 2 optical bandpass freuquency measurement l λ ( l ) = λr 1 + lr Linear relation: c 1 ν ( l ) = ν R ν = 1 l λr 1+ l R = c l c l λr l R + l λr l R for lr= 0,3 mm: ν= 500 GHz

13 Operation of the TFPI Possibility to suppress higher orders.

14 Multi-pass tandem Fabry-Pérot interferometer (TFPI) High contrast: more than 1:1010 High spectral resolution in the Sub-GHz-Regime (up to 50 MHz) Accessible frequency range: 0,2 GHz 500 GHz J. Sandercock, H. Schultheiß,

15 BLS setup Frequency Analysis optical resolution: up to 250 nm 2D scanning stage controlling sample position while measuring frequency range: 200 MHz 1 THz spectral resolution: up to 50 MHz λ Active Stabilization + Positioning Sample Stage Viewing System accuracy: better than 20 nm high reproducibility TFPDAS 4 ( by H. Schultheiss

16 Applications

17 Applications 1) 2) 3) 1) Time resolution H. Schultheiss et al, J. Phys. D 41, (2008) Phase resolution A. A. Serga et al., APL 89, (2006) F. Fohr et al., RSI 80, (2009) Wavevector resolution C. Sandweg et al, Rev. Sci. Instrum. 81, (2010) 3) 2)

18 Applications: Time-resolved BLS

19 Time-resolved BLS Study the decay of spin waves and the dissipation processes H. Schultheiss et al, J. Phys. D 41, (2008)

20 Applications: Phase-resolved BLS

21 Phase-resolved BLS Inelastically scattered light contains phase information But: BLS signal is proportional to laser intensity I x 2 (t )= ( A e i (ωt +ϕ) ) 2 = A 2 Idea: interference! phase-information is lost! Superposition of the inelastic scattered light with a coherent reference beam with the same frequency.

22 Example for phase-resolved BLS: Tunneling running spin wave: wave has not reached the barrier yet phase resolved scan: Hext = 1846 Oe νrf = 7,125 GHz tpuls = 200 ns λ YIG-film (1,6 x 7,7 mm) barrier (cut, 20 μm) λ/2 standard position scan: T. Schneider: PhD Thesis standing spin wave: wave is reflected back and forms a standing wave

23 Applications: Wavevector-resolved BLS

24 Wavevector-resolved BLS No translational invariance in z direction Only k = k sin(θ) conserved Maximum wavevector: k < cm-1 (Backscattering geometry: k = 2 k )

25 Wavevector-resolved BLS Wavevector selection by rotating the sample C. Sandweg et al, Rev. Sci. Instrum. 81, (2010)

26 Summary Brillouin light scattering study spin waves wavevector resolved frequency resolved time resolved space resolved phase resolved