Brillouin-Light-Scattering Spectroscopykreisel/tr49/student_SS10/... · 2010. 8. 2. · Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010 Wavevector-resolved

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Brillouin-Light-Scattering Spectroscopy

Benjamin Jungfleisch

SFB/TR49 Student Seminar20th – 21th of July 2010

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Content

● Spin waves

● Brillouin Light Scattering (BLS)• Quantum mechanical picture• Conventional experimental setup

● Applications• Time-resolved BLS

• Phase-resolved BLS

• Wavevector-resolved

BLS● Summary

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Spin waves

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

)()(dt

MdM

MHM

dt

Md

s

Geff

×+×−= αγ

Spin waves

Textmasterformate durch Klicken bearbeitenZweite Ebene

● Dritte Ebene● Vierte Ebene

● Fünfte Ebene q

λ

Landau-Lifshitz and Gilbert equation

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

q

q

q

0H

0H

0H

dq ⋅||

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)

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Brillouin-Light-Scattering

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Inelastically scattered light

Brillouin-Light-Scattering

• Acoustic phonons

• Spin waves

Frequencies below ~ 500 GHz

Raman-Scattering• Optical phonons

• Molecule vibrations

Frequencies up to several THz

Tandem Fabry-Pérot interferometerGrating spectrometer

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Brillouin-Light-Scattering

Stokes: Anti-Stokes:

Energy conservation law: ωs = ωi - ω ωs = ωi + ω

Momentum conservation law: ks = ki - k ks = ki - k

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Fabry-Pérot interferometer (FPI)

The functionality of a Fabry-Pérot interferometer is based on multi-beam interference on two plane-parallel surfaces.

)cos(22 θλπϕ nl

=∆The phase difference:

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Fabry-Pérot interferometer

Transmittance and reflectivity

The transmittance function is given by:

where R is the reflectivity and is the coefficient of the finesse F.

Maximum transmission occurs when

m = 1,2,3,…

( ) ,2/sin1

1

)cos(21

)1(22

2

ϕϕ ∆+=

∆−+−=

FRR

RT

2)1(

4

R

RF

−=

,)cos(2 λθ mnl =

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Fabry-Pérot interferometer

θλ

λθλλ

cos2cos2

20

0

20

nlnl≈

+=∆

( )F/1arcsin2

πδλ

λ =∆=F

The free spectral range (FSR) is given by :

The finesse F is given by:

R

RF

−=≈

12

ππF

Our setup:F ≈ 110

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

RRRR

R

RR l

lc

ll

lc

ll

cl

∆≈∆+

∆=

∆+−=−=∆∆

λλλννν

1

11)(

Fabry-Pérot interferometer

Maximal transmission: 2

λml =

optical bandpassfreuquency measurement

∆+=∆R

R l

ll 1)( λλ

Linearrelation:

for lR= 0,3 mm: ∆ν= 500 GHz

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Operation of the TFPI

Possibility to suppress higher orders.

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

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, www.jrs-si.chH. Schultheiß, www.tfpdas.de

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

BLS setup

ActiveStabilization

+Positioning

• accuracy: better than 20 nm

• high reproducibility

TFPDAS 4 (www.tfpdas.de)by H. Schultheiss

Frequency Analysis

• frequency range:200 MHz – 1 THz

• spectral resolution:up to 50 MHz

Viewing System

• controlling sample position while measuring

Sample Stage

• optical resolution:up to 250 nm

• 2D scanning stage

λ

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Applications

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Applications

1) Time resolutionH. Schultheiss et al,J. Phys. D 41, 164017 (2008)

2) Phase resolutionA. A. Serga et al., APL 89, 063506 (2006)F. Fohr et al., RSI 80, 043903 (2009)

3) Wavevector resolutionC. Sandweg et al, Rev. Sci. Instrum. 81, 073902 (2010)

1)

2) 3)

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Applications:

Time-resolved BLS

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Time-resolved BLS

Study the decay of spin waves

and the dissipation processes

H. Schultheiss et al,J. Phys. D 41, 164017 (2008)

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Applications:

Phase-resolved BLS

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Superposition of the inelastic scattered light with a coherent reference beam with the same frequency.

Phase-resolved BLS

Inelastically scattered light contains phase information

22)(2 )()( AeAtxI ti =⋅=∝ +ϕω phase-information is lost!

But: BLS signal is proportional to laser intensity

Idea: interference!

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Example for phase-resolved BLS: Tunneling

running spin wave: wave has not reached the barrier yet

standing spin wave: wave is reflected back and forms a standing wave

YIG-film (1,6 x 7,7 mm)

barrier (cut, 20 μm)

Hext = 1846 OeνRF = 7,125 GHztpuls = 200 ns

standard position scan:

phase resolved scan:

λ/2

λ

T. Schneider: PhD Thesis

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Applications:

Wavevector-resolved BLS

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Wavevector-resolved BLS

No translational invariance in z direction

Only k|| = k sin(θ) conserved

Maximum wavevector: k < 2.10 5 cm-1(Backscattering geometry: ∆k = 2 k||)

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Wavevector-resolved BLS

Wavevector selection by rotating the sample

C. Sandweg et al, Rev. Sci. Instrum. 81, 073902 (2010)

Benjamin Jungfleisch SFB/TR49 Student Seminar 20th to 21th of July 2010

Summary

Brillouin light scattering

study spin waves…

…wavevector resolved

…time resolved

…phase resolved

…space resolved

…frequency resolved