1) Brillouin scattering of whispering gallery mode light by magnetic quasivortices or Walker modes in a yttrium iron garnet sphere was investigated.
2) The orbital angular momentum of magnons and photons results in non-trivial reciprocal and nonreciprocal Brillouin scattering.
3) Cavity enhancement of the Brillouin scattering was observed, with enhancement of around 20 times measured.
2. 2
Optomagnonics
T. Satoh et al. Nature Photon. 6, 662 (2012)
T. Satoh et al. Nature Photon. 9, 25 (2014)
S. O. Demokritov et al. Nature 443, 430 (2006) R. Hisatomi, AO et. al., PRB (2016)
3. 3
Optomagnonics
T. Satoh et al. Nature Photon. 6, 662 (2012)
T. Satoh et al. Nature Photon. 9, 25 (2014)
S. O. Demokritov et al. Nature 443, 430 (2006) R. Hisatomi, AO et. al., PRB (2016)
Interaction is weak (spin-orbit coupling)
Enhancement by an optical cavity
Cavity optomagnonics
4. 4
Possible applications
Y. Tabuchi et. al., Science (2015)
SC qubit magnon
10 GHz
microwave
200 THz
light
Microwave-to-optical photon converter
I. Zutic and H. Dery Nature Materials 10, 647
Opto-spintronics Chiral photonics
I. Sollner et al.,Nature Nanotech. 10, 775
5. 5
Whispering gallery modes
(WGMs)
Al2O3 rod
1 mm
Yttrium iron garnet (YIG) sphere
Transparent
for 1.5 µm lightFerrimagnetic
Walker modes
A. Gloppe et al., arXiv:1809.09785
13. 13
Nonreciprocal Brillouin scattering
0 2 0 1
Different OAM of magnon reciprocal/non-.
Interplay with spin-Hall effect of WGM!
[AO et al., PRL 120, 133602 (2018)]
[AO et al., NJP 20, 103018 (2018)]
15. 15
Magnetic field dependence
Higher magnetic field
Larger signal
Coupling with other modes
(Fano interference)
Fit and extract (green curves)
Cavity enhancement ~ x20
16. 16
Magnetic field dependence
Higher magnetic field
Larger signal
Coupling with other modes
(Fano interference)
Fit and extract (green curves)
Similar behavior was observed by Cambridge group
[J. A. Haigh et. al., PRL 117, 133602 (2016)]
Cavity enhancement ~ x20
17. 17
Cavity optomagnonics activities
X. Zhang, N. Zhu, C.-L. Zou, and H. X. Tang, Phys. Rev. Lett. 117, 123605
(2016)
J. A. Haigh, A. Nunnenkamp, A. J. Ramsay, and A. J. Ferguson, Phys. Rev.
Lett. 117, 133602 (2016)
S. V. Kusminskiy, H. X. Tang, and F. Marquardt, Phys. Rev. A 94, 033821
(2016)
T. Liu, X. Zhang, H. X. Tang, and M. E. Flatte, Phys. Rev. B 94, 060405(R)
(2016)
S. Sharma, Y. M. Blanter, and G. E. W. Bauer, Phys. Rev. B 96, 094412
(2017)
…and more and more!
18. 18
Recent activities in Usami’s group
Induction tomography
of
Walker modes
A. Gloppe et al.,
arXiv:1809.09785
• Two-magnon process
• “Bizarre” Brillouin
scattering
- Umklapp process joins
- Looks like spin non-
conserving!
R. Hisatomi et
al.,
arXiv:1905.0401
8
Magnonic
crystal
S. Baba et al.,
arXiv:1905.0468
3
19. 19
Summary
• Brillouin scatterings of WGM light by Walker
modes were investigated
• OAM of magnon and photon result in
nontrivial reciprocal/nonreciprocal Brillouin
scattering
• Cavity enhancement of the Brillouin scattering
was examined K. Usami Y. Nakamura
Great thanks to
all the collaborators!
21. Whispering gallery mode (WGM)
• Resonance: 2𝜋𝑅 ⋅ 𝑛r = 𝑚𝜆
(𝑚 ∈ ℤ)
• Geometrical birefringence
𝑓TE
𝑚
< 𝑓TM
𝑚
21
𝑚 − 1 𝑚 𝑚 𝑚 + 1 𝑚 + 1
Red = TM Blue = TE
Mode index
Frequency
Transverse
Magnetic
Transverse
Electric
𝑓TM
𝑚
− 𝑓TE
𝑚
22. 22
Polarizations of WGMs
• Shift of the rays of the two spin components
Spin Hall effect of light [M. Onoda et. al. PRL 93, 083901 (2004)]
𝒌 ⋅ 𝑬 = 0 ⟹ spin-orbit coupling of light
TE TM (CCW) TM (CW)
±𝑚 𝜋 𝑚 − 1 𝜎−
+ 𝑚 + 1 𝜎+
−(𝑚 − 1) 𝜎+
+ −(𝑚 + 1) 𝜎−
23. Observation of WGMs in YIG
23
Q ~ 1×105
YIG sphere
(diameter 1mm)
Q = frequency / linewidth
Light beam
24. Walker mode (frequencies)
• 𝜔Kittel = 𝛾m 𝐻appl = 2𝜋 × 28 × 𝐻appl GHz
• Other Walker modes generally nonlinear
24
Thick solid: (m, m, 0)
Thin solid: (m+2, m, 0)
Thin dashed: (m+2, m, 1)
(n, m, s) mode
m ∝ 𝑒 𝑖𝑚𝜑
n-m 𝜃-distribution
s 𝑟-distribution
25. 25
Further improvements
• Removing “excess fat”
x 90
• Improvement of the quality factor of WGM
x 3500
𝑔(theory)
= 𝒱𝑐
1
𝑛spin 𝑉
Improved conversion efficiency
4 x 10-4
27. Observation of Walker modes
• Experiment using cylinder magnets
• Normal mode splitting
Finite inhomogeneity of applied magnetic field
27
28. Observation of Walker modes
• Experiment using cylinder magnets
• Normal mode splitting
Finite inhomogeneity of applied magnetic field
28
(1, 1, 0), Kittel
29. Observation of Walker modes
• Experiment using ring magnets
• Normal mode splitting & Frequency shift
Inhomogeneity of applied magnetic field
29
Editor's Notes
Thank you for the kind introduction and I am grateful to the organizers who invited me o such a celebrated, awesome conference. Here I will talk about the cavity optomagnonics with quasivortices which was one of the topics I dealt with in my PhD study.
Well we should first note that there are a numerous number of works aiming at controlling spin waves or its quantum, magnon, by electromagnetic waves. By microwave it’s been done from long ago, and current topic is to address or detect magnons by optical means, to access the k-space information held by the magnon. However, it’s quite hard. Quite hard because the light-magnon interaction is inevitably mediated by the spin-orbit interaction of the electron in the material. Our idea was to enhance this inherently weak interaction using the optical cavity. That’s the idea of cavity optomagnonics, which has the same spirit as the cavity optomechanics.
Well we should first note that there are a numerous number of works aiming at controlling spin waves or its quantum, magnon, by electromagnetic waves. By microwave it’s been done from long ago, and current topic is to address or detect magnons by optical means, to access the k-space information held by the magnon. However, it’s quite hard. Quite hard because the light-magnon interaction is inevitably mediated by the spin-orbit interaction of the electron in the material. Our idea was to enhance this inherently weak interaction using the optical cavity. That’s the idea of cavity optomagnonics, which has the same spirit as the cavity optomechanics.
The system stimulates us to apply it to the Microwave-to-optical photon converter that can possibly be used to the quantum interface, or some device made up of the combination of spintronic and optical technologies. Another interesting thing is the chiral photonic devices useful for the nanophotonic circuits.
Okay, then given such an idea, we want nice magneto-optical material that makes it feasible. And in most cavity optomagnonics activities the yttrium iron garnet sphere is adopted. It is ferrimagnetic on one hand to exhibit magnetostatic modes of rich spin textures. On the other hand YIG is highly transparent at telecom wavelength and the sphere supports whispering gallery modes. The light can circulate the periphery to form the optical resonance. Here’s the situation we want. Walker mode magnons can be addressed by the optical resonator!
Okay, let’s move on to the experiment. We have a YIG sphere of diameter 1mm and the magnons are excited by the microwave through the loop coil. The excited magnons are in this experiment detected by the WGM light injected evanescently through this prism. Since we are interested in creating or annihilating magnons by light, the Brillouin scattering is concerned here, namely the energy and the angular momenta of the magnon should be transferred to or retracted from the WGM. These facts requires the scattered light to possess the frequency shifted by magnon frequency, about 7 GHz, and the polarization rotated. So here we detect the fluctuation of the output light polarization by the fast photodetector to get the sideband signal at 7 GHz. The optical sideband is beaten down to the microwave regime by taking heterodyne signal. In the network analyzer the simplest, uniform magnetostatic mode, the Kittel mode, is observed as the dip in the reflection signal of the loop coil and the Brillouin-scattered light is detected like this yellow signal.
Okay, let’s move on to the experiment. We have a YIG sphere of diameter 1mm and the magnons are excited by the microwave through the loop coil. The excited magnons are in this experiment detected by the WGM light injected evanescently through this prism. Since we are interested in creating or annihilating magnons by light, the Brillouin scattering is concerned here, namely the energy and the angular momenta of the magnon should be transferred to or retracted from the WGM. These facts requires the scattered light to possess the frequency shifted by magnon frequency, about 7 GHz, and the polarization rotated. So here we detect the fluctuation of the output light polarization by the fast photodetector to get the sideband signal at 7 GHz. The optical sideband is beaten down to the microwave regime by taking heterodyne signal. In the network analyzer the simplest, uniform magnetostatic mode, the Kittel mode, is observed as the dip in the reflection signal of the loop coil and the Brillouin-scattered light is detected like this yellow signal.
What is astonishing is that when the direction of the WGM light is clockwise, indicated by blue, the Brillouin scattering can be observed, however, when counterclockwise, indicated by red, the signal disappears! The phenomenon is nonreciprocal. This is not as usual as simple Faraday effect because the Brillouin scattering is dynamical effect. This nonreciprocity was revealed to be due to the interplay among energy and spin angular momentum conservation, and the spin-orbit coupled nature of the WGM. We do not dive into the great detail in this talk. Let us see further mystery we encountered in the experiment. That is, when you see the higher-order magnons located at the higher frequency, you see the nonreciprocal or reciprocal nature is strongly dependent on those modes.
What is astonishing is that when the direction of the WGM light is clockwise, indicated by blue, the Brillouin scattering can be observed, however, when counterclockwise, indicated by red, the signal disappears! The phenomenon is nonreciprocal. This is not as usual as simple Faraday effect because the Brillouin scattering is dynamical effect. This nonreciprocity was revealed to be due to the interplay among energy and spin angular momentum conservation, and the spin-orbit coupled nature of the WGM. We do not dive into the great detail in this talk. Let us see further mystery we encountered in the experiment. That is, when you see the higher-order magnons located at the higher frequency, you see the nonreciprocal or reciprocal nature is strongly dependent on those modes.
Let’s take a look at these magnon modes. The leftmost is the Kittel mode, the uniform mode. And others can be identified by checking their frequencies for variable magnetic fields. These are the transverse magnetization distributions and you can see various textures exhibited by these, for example the 401 mode possesses the spiral texture at some instance. What makes these textures different from each other is resolved by examining the winding number of these vector fields. In order to do so we shall track the transverse magnetization in the vicinity of the perimeter.
First, the Kittel mode and 311 mode the transverse magnetization points the same direction all along the circumference, therefore the winding number reads 0.
For the 401 mode, it is up here, rotates by 180 degree on the other side and goes back. The total amount of rotation is 2pi, which yields the winding number of 1.
And for 31bar1 mode, from its pattern we see the magnetization is up here, down here, up, down, up, and rotated all the way by 4pi in total. Thus the winding number is 2.
Since such rotations are the spatial variation of the phase, we can interpret them as the orbital angular momenta.
Okay, then let’s have a look back. Now the orbital angular momenta are assigned to the Walker modes. And we notice that for magnons with vanishing OAM, the clockwise cases are more prominent than the counterclockwise cases. If the magnon have OAM of 1, the Brillouin scattering appears to be reciprocal. If 2, in this case the counterclockwise scattering is superior to the clockwise one. These results are in a quite nice agreement with the theory that take in to account the energy and angular momenta conservation.
Okay, then let’s have a look back. Now the orbital angular momenta are assigned to the Walker modes. And we notice that for magnons with vanishing OAM, the clockwise cases are more prominent than the counterclockwise cases. If the magnon have OAM of 1, the Brillouin scattering appears to be reciprocal. If 2, in this case the counterclockwise scattering is superior to the clockwise one. These results are in a quite nice agreement with the theory that take in to account the energy and angular momenta conservation.
In the last part I will talk about the effect of the presence of the cavity.