Active magneto-plasmonics
in
hybrid
metal-ferromagnet structures
Vasily
V.
Temnov
a
,
Gaspar
Armelles
2
,
Ulrike
Woggon
3
,
Dmitry
Guzatov
4
,
Alfonso
Cebollada
2
,
Antonio
Garcia-Martin2,
Jose-Miguel
Garcia-Martin2,
Tim
Thomays,
Alfred
leitenstorfer
s
and
Rudolf
Bratschitsch
s
Surface-plasmon-mediated
confinement
of
optical
fields
holds
great
promise
for
on-chip
miniaturization
of
all-optical
circuits
1
-
4
•
Following
successful
demonstrations
of
passive
nanoplasmonic
devices
s
-
7
,
active
plasmonic
systems
have
been
designed
to
control
plasmon
propagation.
This
goal
has
been
achieved
either
by
coupling
plasmons
to
optically
active
materials
s
-
13
or
by
making
use
of transient
optical
nonlinearities
in
metals
via
strong
excitation
with
ultrashort
laser
pulses
14
-
17
•
Here,
we
present
a
new
concept
in
which
the
active
optical
component
is
a metal-ferromagnet-metal
structure.
It
is
based
on
active
magneto-plasmonic
microinterferometry,
where
the
surface
plasmon
wave
vector
in
a gold-ferromagnet-gold trilayer
system
is
controlled
using
a
weak
external
magnetic
field.
Application
of
this
new
technique
allows
measurement
of
the
electromagnetic
field
distribution
inside
a
metal
at
optical
fre-
quencies
and
with
nanometre
depth
resolution.
Significant
modulation
depth
combined
with
possible
all-optical
magnetiza-
tion
reversal
induced
by
femtosecond
light
pulses
1s
opens
a
route
to
ultrafast
magneto-plasmonic
switching.
Continuous improvements in nanofabrication and nanocharac-
terization capabilities have changed projections about the role that
metals could play in the development
of
new optical devices. Our
present ability to manipulate both the electromagnetic field localiz-
ation and the nanoscale coupling between light and surface plas-
mons leads to the observation
of
new optical phenomena such
as
enhanced optical transmission, sub-wavelength confinement
of
optical fields and sub-wavelength optical resolution.
A magnetic field can modify the properties
of
an electron plasma.
This effect has been analysed for surface plasmons at terahertz
fre-
quencies propagating at an interface between a semiconductor and a
dielectric
19
(also known
as
surface plasmon polariton, SPP). In par-
ticular, if the magnetic field
is
applied parallel to the interface and
perpendicular to the
SPP
wave
vector, it induces a modification
of
the SPP
wave
vector while keeping its transverse magnetic (TM)
character. Therefore, the magnetic
field
could
be
used to control
SPP propagation, opening the door for novel active plasmonic
devices. The technological drawback
is
that the magnetic fields
that enable noticeable changes in noble-metal-based plasmonic
structures in the visible spectral range are
of
the order of several
tesla and are therefore too large for realistic applications.
Ferromagnetic layers, however, exhibit a large magneto-optical
(MO) activity, but do not support propagating long-range surface
plasmons due to high ohmic losses. Combining magnetic and
plasmonic counterparts in a magneto-plasmonic system would
both enable high
MO activity and support propagating surface
plasmon modes. Orders of magnitude smaller external magnetic
fields might
be
sufficient to manipulate the optical properties
of
their
SPPS20.
This concept allows the design and tailOring
of
the
optical properties
of
complex composite magnetic materials
2I
-
24
towards particular potential applications, such
as
high-sensitivity
biosensing devices
25
.
Here,
we
use plasmonic
microinterferometry9,I2,I7,26,27
to demon-
strate active control
of
surface plasmons in a magneto-plasmonic
gold-cobalt-gold multilayer film by a weak external magnetic field
of
a
few
millitesla. For a gold-cobalt-gold multilayer
film
with a
cobalt thickness
of
a
few
nanometres, the magnetization along the
easy axes (in the plane
of
the film) may be saturated by applying a
very low magnetic field
of
the order
of
10
mT.
If the magnetic
field
is
applied along the x-axis (see geometry in
Fig.
1), the bulk
dielectric tensor
of
cobalt,
o
(1)
is
governed by the magnetization
Mx
in the same direction. Thus, in
the composite gold-cobalt-gold films the properties of surface
plas-
mons should not depend directly on the external magnetic field
B,
but on the magnetization
Mx
in a cobalt
layer.
The
wave
vector
of the gold-cobalt-gold
film
surface plasmon
ksp(Mx'
h)
=
k~p
+
!1k
m
/M
x
'
h) propagating in the y-direction depends on the com-
ponent of the magnetization
Mx
of
the cobalt layer and the depth h
of
the cobalt layer below the gold / air interface. Analytical thin-film
cal-
culations in gold-cobalt-gold multilayer
films
(see Supplementary
Information
for
a detailed discussion) in the approximation
of
an
infinitely thin cobalt layer demonstrate the possibility of magnetically
modulating the surface plasmon
wave
vector,
2hI
(kO"'AuEAir)2
i",yz
!1k
mp
(Mx
,
h)
'::::
( )( 2 _
2)
Mxexp(-h/askinl
(2)
"'Air + "'Au "'Air "'Au
"'xx
which scales linearly with
Mx
and exponentially decays
as
a function
of
h within a skin depth oflight a
skin
(hI
is
the cobalt layer thickness).
The maximum possible modulation
of
the surface plasmon
wave
vector
2!1k
mp
(h)
=
ksp(Mx
= +
1,
h)
-
k"}'.(Mx
=
-1,
h)
can be
achieved upon magnetization switching
~Mx
=
±1
for a saturated
sample).
Numerical calculations not only justify the validity of equation
(2) for
hI
«:
a
skill
' but also prove that the presence
of
a thin
cobalt-layer introduces only a minor perturbation in the electric
'Department of Chemistry, Massachusetts Institute
of
Technology, Cambridge, Massachusetts 02139,
USA,
'Instituto de Microelectr6nica de
Madrid-IMM
(CNM-CSIC),
28760
Tres
Cantos, Madrid,
Spain,
'Institut fur Optik und Atomare Physik, TU Berlin, Strasse des
17.
Juni
135,
10632 Berlin, Germany, 'Yanka
Kupala Grodno State University,
230023
Grodno, Belarus, 'Department
of
Physics and Center for Applied Photonics, University of Konstanz, D-78457
Konstanz, Germany, *e-mail: temnov@mit.edu
107
First publ. in: Nature Photonics 4 (2010), 2, pp. 107-111
doi:10.1038/nphoton.2009.265
Konstanzer Online-Publikations-System (KOPS)
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-130121
URL: http://kops.ub.uni-konstanz.de/volltexte/2011/13012/
Glass
substrate
Au
layer
Microscope
objective
~,~
,I
I'--l.
Au
I
~
yer
,:'
Electromagnet
B(t) = Bsin(w
t)
Figure
1 I Active magneto-plasmonic interferometry. A plasmonic
microinterferometer consisting of a tilt
ed
s
li
t-groove pair is milled
in
a
go
l
d-
cobalt-go
ld
multi layer film using a focu
sed
ion beam. Surface
pla
smons
are
launched by the groove, propagate towards the slit, and interfere with the
directly transmitted
li
ght to produce a periodic interference pattern along the
s
li
t
axes.
The osci
ll
ating magnetic field of
an
electromagnet
is
used to
periodically switch the magnetization
in
the thin cobalt
lay
er and thus
modify the wave vector of surface
plasmons. An imaging nano-optical set-up
is
used to record the magneto-plasmonic modulation signa
l.
field
distribution inside the metal
as
compared to bulk gold (see
Supplementary
Fig.
IS)
. These calculations suggest that the depen-
dence
of
the magnetic modulation
of
the surface plasmon
wave
vector
as
a function
of
cobalt layer position h may be used
to directly measure the skin depth
of
light in gold at optical
fre
-
quencies with a spatial resolution determined by the thickness
hi
of
the cobalt layer.
To explore the modulation
of
the surface plasmon
wave
vector,
the gold-cobalt-gold multilayer films were patterned with
plasmo-
nic microinterferometers (Fig.
1).
They consisted
of
a tilted slit-
groove pair milled
by
a focused ion beam in a gold-cobalt-gold
fi
lm grown
on
a gla
ss
substrate. The length
of
both the slit and
the groove
was
50
fLm.
The slit had a width
of
100 nm. Grooves
were
100
nm
deep and 200 nm wide. The slit-groove tilt angle 0
was
varied between 5° and
10
° and the minimum slit- groove dis-
tance
do
varied from 0 to
20
fLm.
Illumination
of
a microinterferom-
eter with a collimated p-polarized continuous-
wave
laser diode at
normal incidence
(lOO
mW, 808 nm, spot diameter
30
fLm
FWHM) excited surface plasmons, which were launched by the
groove. They propagated towards the slit, where they were
recon-
verted into free-space radiation, interfering with light directly trans-
mitted through the slit (Fig.
l).
Owing to the slit- groove tilt angle,
the geometrical path difference
d(x) =
do
+ x sin 0 acquired
by
surface plasmons
on
the
way
to the slit changes linearly along
the slit axis
x,
resulting in a sinusoidal interference pattern
lex,
B =
0)
=
El
(X)
2 + E
2
(x)2 +
2EI
(x)E
2
(x)cos(<fl(x)) with the two
electric fields
El
(x) and E
2
(x) and the phase
<flex)
= ks
pd(x)
+
4>0
'
An oscillating external magnetic
field
with amplitude B
was
applied in the x-direction to periodically switch the magnetization
of
the cobalt layer and modulate the surface plasmon
wave
vector.
In that
way,
the phase and the contrast
of
the plasmonic interfero-
gram changed. A lock-
in
based scanning imaging optical set-upl?
allowed the plasmonic interferogram
to
be
recorded at zero mag-
netic field lex, B =
0)
together with the magneto-plasmonic interfer-
ogram
lm/
x,
B)
=
lex,
+ B) -
lex,
-
B)
for a weak external magnetic
field
of
a
few
tens
of
millitesla. A normalization procedure
was
applied to compensate for the spatially inhomogeneous modulation
depth
of
le
x) (which
is
equal to 4E/(x)E
2
(x), marked
as
the grey
108
area in
Fig
.
2)
to
obtain the normalized magneto-plasmonic inter-
ference pattern
1~;rm
)
(X
,
B)
=
-
d(x)l~kmplcos(<fl(x)
+
4»
(3)
Interferograms for two different plasmonic microinterferometers
defined in a gold- cobalt-gold
(5
nm/6
nm/189
nm) multilayer
film on glass are shown in
Fig.
2.
The micro interferometers have
the same slit-groove tilt angle
0 = 5°,
but
differ in the initial dis-
tance
do
(do
= 0 and
do
=
10
fLm).
Both interference patterns lex)
and lmp(x) have the same period Asp/sin
0,
but are shifted with
respect to each other by a constant phase
4>
~
70
°. A change in
absorption
of
the surface plasmons only affects the contrast
of
the
plasmonic interference
lex); that
is,
lmp(
x)
is
in phase
(4)
=
0)
. A
change in the surface plasmon
wave
vector, however, results in a
fringe shift
of
lex); that
is,
lmp(x)
is
shifted
by
4>
=
71
/2
. The
observed phase shift
4>
~
70
°
is
close to
71
/2,
which clearly demon-
strates that the magneto-plasmonic Signal
is
dominated
by
the
change
of
the
real
part
of
the surface plasmon
wave
vector.
Figure 2g,h shows the normalized magneto-plasmonic signals
l~;:prm)(x)
described
by
equation (3) for an oscillating magnetic
field with an amplitude
of
20
mT.
As
expected, the amplitude
I~kmpld(x)
of
the normalized magneto-plasmonic interferogram
1~:~rl11
)
(x)
scales
linearly with surface plasmon propagation distance
d(x) =
do
+ x sin 0 between the slit and the groove, visualized
by
straight dashed lines. The intrinsica
ll
y small magneto-plasmonic
modulation
of
the surface plasmon
wave
vector
I~kmpl/ko
~
10
-
4
is
accumulated over the long propagation distance, resulting in
noticeable values
of
the magneto-plasmonic signal. In that
way,
a
substantial enhancement factor
of
kod
~
100
is
achieved. For the
central part
of
the microinterferometer in
Fig.
2b
with d =
12
fLm,
we
obtain a modulation depth
of
21~km'p
l
d
~
0.Ql
(the factor 2
accounts for magnetization switching between
Mx
=
-1
and
Mx = +
1)
. The experimental data for a microinterferometer with a
larger distance
d =
22
fLm
(interferograms not shown) provide a
value
21~kmpld
=
0.02,
which
is
promising for potential applications
such
as
an optical switch. The slit-groove spacing
of
a
few
tens
of
micrometres needed to achieve a significant modulation depth of
the magneto-plasmonic
Signal
limits the minimum device size. At
the same time, a long propagation distance
is
favourable for
sensing applications. A further increase
of
the slit-groove spacing
results in a reduced modulation depth
of
lex) due to larger surface
plasmon propagation losses.
To
exp
lore the physical mechanism that links the external mag-
netic field with the interference pattern, the magneto-plasmonic
signal
l~kmp(B)ld
is
plotted against magnetic
field
amplitude B in
Fig.
3. A gradual increase
of
the signal at small
fields
is
observed,
fo
ll
owed
by
a sudden jump at
12
mT and saturation behaviour
above
15
mT. This result indicates that the magneto-plasmonic
signal does not directly depend on the external magnetic
field
in a
linear
way,
but
is
governed
by
the cobalt layer magnetization. This
effect
is
demonstrated in the inset, where the magneto-plasmonic
hystereSiS
loop (reconstructed from the temporal dependence
of
~kmp(t)
and B(t))
is
depicted together with the transverse Kerr
effect hysteresis loop measured in an unpatterned sampl
e.
The
cycles have identical shapes, confirming the direct dependence
of
the magneto-plasmonic signal on the cobalt magnetization.
Up to this point,
we
have demonstrated control
of
the surface
plasmon
wave
vector by a small external magnetic
field.
We
now
show that this effect allows for local probing
of
the electromagnetic
fie
ld distribution in
si
de the
go
ld layer. For this purpose
we
have per-
formed magneto-plasmonic interferometry
in
multilayer structures
where the cobalt layer
is
located at different depth h below the
go
ld-
air interface. In
Fig.
4
we
compare the results
of
magneto-plasmonic
measurements obtained for
d =
22
fLm
with the theoretical formula
a
Groove
I d(x)
Slit
C
1,500
Tr===:::::;--------i
I-
I(
B=
O)
I
.,0
u " c .
.,
'"
~~
.,
'"
'tro
.,
c
E
.gf
e
2
.,0
u"
1
o
10
20
30
40
Position along the sl
it
x
(~m)
1
--
fmp
=
f(
+B)-
I(-B)1
~~
.,
'"
O
-+---~
'tro
.,
c
E'~
- 1
-2
B = 20
mT
o
10
20 30
40
Po
sition along the slit x
(
~m
)
g
0.
01
-0.01
B = 20
mT
o
10
20 30
40
Position along the slit x
(~m)
50
50
50
b
Groove
Slit
d
500
~======~--------~----1
400
I
--
I(
B=
O)
I - '-
rp
300
200
100
0
4-~
~.--r-'--'--r~r-,--,--
r-
~
10
.,0
u"
5
~~
.,
'"
0
't:m
.,
c
.E.~
-5
o
10 20
30
Position along the slit x
(~m)
-
10
B=20mT
h
.,0
u "
~
~
0.01
~
"*
0.
00
.,
c
:£
.
gf
-0.01
o
o
10 20 30
Position along the slit x
(
~
m)
B = 20
mT
10 20 30
Position along the slit x
(~m)
40
50
40
50
40
50
Figure
2 I Magneto-
plasmonic
interferograms
in
tilted
slit-groove
microinterferometers,
a,b,
Two
plasmonic microinterferomete
rs
with the same tilt-angle
of 0 = 5°, but different minimum slit-groove spacings: do = 0 (a) and
do
= 10
fLm
(b)
. c,d, The plasmonic
in
terference pattern I(x, B =
0)
at zero magnetic
field is due to surface plasmons propagating from the groove towards the slit.
e,f,
The periodic cycl
in
g of the magnetic
fi
eld between
+20
m T
and
-
20
m T
changes the surface
plasmon wave vector, shifts the fringes of I(x, B) and g
iv
es
rise to the magneto-plasmonic signal I",,,(x) = I(+ B) -
I(
- B).
g,h,
After
normali
za
tion to the spatia
ll
y inhomogeneous modulation depth of the plasmonic interferogram (grey shaded
area
in
c,d) the normali
ze
d pattern
I
~':~"")(x
)
shows the magneto-plasmonic modulation
lL'.kmpl
of
the surfa
ce
plasmon
wave
vector. The magnitude of the magneto-plasmonic signal
lL'.km
pld(x)
sca
l
es
lin
ea
rly with slit-groove distance
d(x)
.
given by equation (2), Excellent agrement between theory and
experiment
is
obtained without using any
fit
parameters, The
magneto-plasmonic signal decays exponentially within the skin
depth
of
13
nm, This quantitative agreement demonstrates the
ability
of
the new experimental method to directly measure the skin
depth
of
li
ght inside a metal
at
optical frequencies with nanometre
resolution,
It
also reveals the potential
of
equation (2) for the engin-
eering
of
magneto-plasmonic devices. For example, a strong enhance-
ment
of
magneto-plasmonic modulation could be achieved
by
increasing the dielectric constant
of
the upper dielectric material.
To explore the potential switching speed
of
our magneto-
plasmonic devices, local generation
of
magnetic
fie
lds by surface cur-
rents with integrated
on
-chip electric circuits may be applied, Using
magnetic pulses with appropriate characteristics, a preceSSion-driven
magnetic switching in the gigahertz range can be achieved
28
. The
ultimate speed limits
of
this technique could be explored by applying
currents driven with intense picosecond terahertz p
ul
ses and using
ultrafast coherent control
of
magnetization by femtosecond
l
asers29.
A possible route towards switching at terahertz rates has
been demonstrated recen
tl
y.
Stancit! et
al.
have shown that the
magnetization in GdFeCo can be reversed in a reproducible
manner
with a single circularly polarized
40
fs
laser pulse focused
to a
20
f..Lm
spot
l8
. The origin
of
this effect
is
still subject to debate
and may be governed by the interplay
of
different
non
-equilibrium
processes. Nevertheless, these
res
ults are promising for femtosecond
a
ll
-optical magnetization switching.
In summary,
we
have demonstrated magnetic field control
of
surface plasmons in a composite gold
-co
balt
-go
ld multilayer film
by magneto-plasmonic interferometry. Significant phase shifts are
achieved with external magnetic fields
as
sma
ll
as
a
few
millitesla.
Accurate measurements
of
the skin depth
of
visible
li
ght in gold in
the optical frequency range open the
way
for nanometre-resolved
measurements
of
optical fields and magneto-plasmonic properties
of
matter within the skin depth
of
nanostructured metal/ ferromagnet
composites. The modulation
of
plasmonic optical properties using
magnetization control in ferromagnetic materials
by
relatively weak
magnetic fields suggests a straightforward application
of
this system
as
opt
ical
switches. The magnitude
of
magneto-plasmonic effects
may be further enhanced through nanostructuring
of
metal compo-
sites, for exampl
e,
with sub-
wave
length hole
arrays30
. Motivated by
109
0.
006
1- I
Mmp
l
dl
0.004
""
~
-'<
~
0.002
d =
1
2
~m
Ken loop
"
1.O
~
S:
~
~
~ O.S
~
.
-
~
~
0.0
~
~
-
·~
-o.s
f
~
- 1
.0
'-'
- 30- 20
-10
0 10 20 30
Magnetic
fi
eld
(01T
)
0.000 -1-"'''''-,
----,------,----,------,---,-------,---,-------,---,-------,-----i
o 5
10
15 20
25
30
Magnetic
fie
ld
ampl
it
ude (mT)
Figure 3 I Magnetization switching of
the
magneto-plasmonic signal. The
magnitude
of
the
magneto-plasmonic
signal Itlk",pld sh
ows
a pronoun
ced
thr
es
ho
ld
behaviour
around
B = 12 m
T.
The
in
se
t
shows
how
the
magneto
-
plasmonic
hy
steres
is l
oop
(dots)
co
in
c
id
es
with
the
Kerr l
oop
measured
in
the
tran
sve
r
sa
l
co
nfi
gu
r
at
i
on
(line).
'"
"]
<I
N
0.01
0.
00
1
o
B=20mT
d =
22
~m
10
Au
• Experiment
---
Theory (equation (2))
20
30
40
50
Position
of
the cobalt layer
11
(nm)
Figure 4 I Probing
the
electromagnetic field inside
the
gold layer. The
measured
magnitude
of
the
normalized
magneto-plasmonic
si
gna
l
as
a
function of
the
coba
lt position h s h
ows
a
mono-exponential
d
ecay
within
the
sk
in
depth
of 13
nm
,
in
agreeme
nt
with
the
fit-free
th
eoretical
express
i
on
g
iv
en by
eq
u
at
i
on
(2)
.
The
10%
error
bars
s h
own
h
ere
are
related
to
un
ce
rtainti
es
in
the
groove-
t
o-s
lit
distance,
d,
and
in
the
numerical
calc
ul
a
ti
on of
modulation
depth.
recent
ex
periments with femtosecond light pulses IS,
we
believe that the
swi
tching speed
of
our
device could eventually reach the
terahertz regime.
Methods
Fabrication
of
magneto-plasmonic
microinterferometers.
Metal deposi
ti
on on
glass substrates was carried
out
by d.c. magnetron sputtering at 20 W power in an
ultrahigh-vac
uum
ch
amber
with a base pressure in the low
10
- "
mbar
range.
The
substrates were previously ult
rasound
-cleaned in successi
ve
baths
of
acetone,
methanol a
nd
deionized water. Once loaded
int
o the depositi
on
chamber. the
substrates
we
re outgassed for 30
min
at 150 °
C.
After cooling to room temperature.
a
ll
layers were deposited. First. a 2-
nm
-thick
chromium
buffer layer was grown to
improve adhesi
on
of
the successive layers. Then,
go
ld- cobalt- gold trilayers with a
total thickness
of
200
nm
were deposited.
The
positi
on
h
of
the 6-
nm
thin
cobalt
layer varied in
five
samp
les between B
nm
and
4B
nm
below the gold-air interface.
110
Argon deposition pressure
and
deposition rates were 1 x
10
-
3
mbar
and
12
nm
min
-
I
for gold. 3 x
10
-
3
mbar
and
2
nm
min
-
I
for
chromium.
and 6.6 x
10
-
3
mbar
and
2.B
nm
min - I for cobalt. Plasmonic microinterferometers were
m
ill
ed into the multilayers with a 30-kV Ga+ focused ion
beam
.
Meas
urem
e
nt
s. The magnetization in the cobalt layer was swit
ched
at a frequency
of
690
Hz
by
applying a periodic magnetic field with an
amp
litude
of
tens
of
millitesta
in a resonantly driven electromagnet
co
il
of
a transformer. which was embedded in a
seri
es
LC
-circuit (quality factor Q
'"
5. with peak currents
not
exceeding 2 A).
Technical details
of
th
e lock-in based sca
nnin
g imaging set-
up
used to record
plasmonic interferograms al
ong
the slit axes are given elsewh
ere"
.
Optical
constants
.
Thin
-film calculations in gold- cobalt
-go
ld m ult
il
ayers were
performed with experimenta
ll
y
ob
tained
va
lues
of
the dielectric susceptibi
li
ty for
gold
EA"
= - 24.8 +
1.
5i. for air
EA;,
= 1 and t
ensor
components
for cobalt
E
xx
= - 17
.1
+ 24.2i. Q =
iE)'
,!
E
X.<
= 0.0345 + O.
Oli
at A =
BOB
nm.
As
IEA
"I
» 1
th
e skin depth for the surface plasmon O,k;" =
(A
/
411')
Im
(J l +
EAu
/
EAu)
is
nearly id
en
tical to
that
for a plane electromagnetic wave
under
normal incidence
O,
k;"
=
(A/411')
Im
(1
/
JE
Au
).
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Acknowledgements
This work
was
supported by the European Network
of
Excellence PhOREMOST,
EU
(NMP3-SL-2008-214107-Nanomagma), Spanish MICINN ('FVNCOAT' CONSOLIDER
INGENIO
2010 CSD2008-00023
and
'MAGPLAS' MAT200S-06765-C02-0IjNAN) CM
('NANOMAGNET'
S-OS05/MAT /0194, 'MICROSERES' S-OS05/TIC/0191), The
German Research Foundation (DFG TE770/1).
We
also thank
J.L.
Costa-Kriimer for the
Transverse
Kerr
loop measurement
and
K.
Nelson
for
stimulating discussions.
Author
contributions
V.T.,
V.W.,
G.A.
and
A.C
wrote the proposal.
V.T.,
U.W., G.A., A.C, A.G.M. and
J.M.G.M.
conceived and designed the experiments.
G.A.,
A.C, A.G.M., J.M.G.M.,
T.T.,
A.L.
and
R.B.
prepared and characterized the samples. A.C, D.G., A.G.M., T.T.,
A.L.
and
R.B.
contributed
materials and analysis tools.
V.T.
and
G.A.
performed the measurements and analysed the
data. G.A., A.G.M.,
D.G.
and
V.T.
carried out theoretical calculations.
All
authors wrote
the manuscript.
Additional
information
The authors declare no competing financial interests. Supplementary information
accompanies
this
paper
at
www.nature.com/naturephotonics.
Reprints
and
permission
information
is
available
online
at
http://npg.nature.com/reprintsandpermissionsl.
Correspondence
and
requests
for
materials
should
be
addressed
to
V.V.T.
111