© 2013 Macmillan Publishers Limited. All rights reserved.
Real-time observation of ultrafast Rabi oscillations
between excitons and plasmons in metal
nanostructures with J-aggregates
Parinda Vasa
1,2
*
,WeiWang
1
,RobertPomraenke
1
, Melanie Lammers
1
, Margherita Maiuri
3
,
Cristian Manzoni
3
, Giulio Cerullo
3
and Christoph Lienau
1
*
Surface plasmon polaritons (SPPs), optical excitations at the
interface between a metal and a dielectric, carry significant
potential for guiding and manipulating light on the nanoscale
1–3
.
However, their weak optical nonlinearities hinder active
device fabrication, for example, for all-optical switching
4–7
or
information processing
8,9
. Recently, strong optical dipole coup-
ling has been demonstrated between SPPs and nonlinear
quantum emitters with normal mode splittings of up to
700 meV (refs 10–15). The predicted ultrafast energy transfer
between quantum emitters and SPP fields could be a crucial
microscopic mechanism for switching light by light on the
nanoscale. Here, we present the first real-time observation of
ultrafast Rabi oscillations in a J-aggregate/metal nanostruc-
ture, indicating coherent energy transfer between excitonic
quantum emitters and SPP fields. We demonstrate coherent
manipulation of the coupling energy by controlling the exciton
density on a 10 fs timescale, which represents a step towards
coherent, all-optical ultrafast plasmonic circuits and devices.
Metallic nanostructures supporting surface plasmon polaritons
(SPPs) guide light over ultrashort length and time scales and are
finding use in a growing range of applications. Currently, their
application is limited primarily by the intrinsically weak optical
nonlinearities and short propagation lengths of SPPs
16
. Coupling
SPPs to nonlinear optical resonances such as excitons (Xs) in mol-
ecular or semiconducting nanostructures can provide the missing
nonlinearity and gain. This is a key step towards realizing novel
active plasmonic devices such as all-optical switches
4,6,7
, single-
photon transistors
5
, and nanolasers
17–19
, combining the operational
bandwidth of photonics with the size scalability of electronics. In
appropriately designed nanostructures
10–12,15
, the coupling strength
between the optical dipole moment of the exciton and the vacuum
SPP field greatly exceeds their individual linewidth, resulting in the
formation of hybrid X–SPP modes with energy splittings approach-
ing the exciton resonance energy
13
. These hybrid excitations
combine the favourable light-localization properties of SPPs and
the enhanced nonlinearities of active quantum emitters. From a
dynamical perspective, the strong X–SPP coupling results in a
hitherto unexplored periodic and coherent energy exchange
between both systems. Here, we study these Rabi oscillations
between excitons and SPPs in a J-aggregate/metal hybrid nano-
structure in real time. We find that the period of these oscillations
(and thus the optical nonlinearity of this hybrid nanostructure) is
parametrically modulated on an ultrafast 10 fs timescale by the
exciton population, thus providing a new mechanism for ultrafast
all-optical switching in active plasmonic devices.
We studied the J-aggregate/metal hybrid nanostructure shown in
Fig. 1a, which comprises a J-aggregated cyanine dye embedded in a
50-nm-thick polymer film, and spin-coated onto periodic nanoslit
arrays in a gold film, with period a
0
ranging from 400 to 460 nm.
In such hybrid structures, J-aggregate excitons are strongly
coupled via their optical transition dipole moments m
X
to SPP
fields localized in and near the slits. The large oscillator strength of
the molecular J-aggregates and the local field enhancement results
in particularly large coupling energies
7,10,11,13
. This makes such struc-
tures emerging prototypes for investigating the dynamics of the
X–SPP hybrid modes
4,7,20,21
. For a single exciton, the strength of
this coupling hV
R
¼ m
X
.
E
V
(r) is given by the product of the dipole
moment and the amplitude of the SPP vacuum field, E
V
(r)atthe
exciton position r (refs 22–24). In our structures, a large number of
excitons (N
X
; see Supplementary Section S1) are coupled to a
quasi-continuum of SPP modes, so we are in the ‘classical’ limit of
a normal mode splitting between excitons and SPPs
25,26
.Inthis
limit, the linear optical properties of the hybrid system are essentially
understood as those of two dissipative oscillators (X ¼ i,SPP¼ j)
with complex frequencies ˜
v
i,j
u
()
=
v
i,j
u
()
− i
g
i,j
u
()
, coupled by a non-
dissipative interaction with strength hV
R
¼
p
(N
X
)m
X
.
E
V
. Here
v
i,j(
u
)
are the resonance angular frequencies,
g
i,j
(
u
) represents the
sum of radiative and nonradiative damping rates caused by coupling
to the environment, and
u
is the incidence angle. A strong coupling
regime is reached when the normal mode splitting V
NMS
≈2|V
R
|
exceeds all damping rates. This leads to the formation of coupled
upper (UP) and lower (LP) X–SPP polariton modes and a character-
istic anti-crossing in the dispersion relation (Fig. 1b). Evidence for
strong X–SPP coupling is given by recording angle-resolved,
p-polarized, steady-state reflectivity spectra R
0
(
u
,
v
), as shown in
Fig. 1c. The spectra reveal a significant bending of the X–SPP
modes exhibiting an anticrossing with hV
NMS
≈110 meV. The
spectral linewidths indicate lower limits for the dephasing times
of the probed ensemble of UP and LP modes of 20 and 40 fs,
respectively, given by the interplay between radiative damping and
exciton dephasing. In addition, a pronounced reflectivity dip is
seen at the bare exciton resonance
v
i
. By comparing its strength to
that of the UP and LP resonances at various angles
u
,weconclude
tha t only a small fra ction (0.01) of all excitons, namely those situated
inside or near the slits, are str ongly coupled to the SPP modes. The
majority of excitons remain unaffected by the presence of the SPP
field. Angle-dependent spectra simula ted based on the optical Bloch
equation model described in Supplementary Section S1 (Fig. 1d,e)
agree well with the experimental spectra and reveal information on
the polariton dispersion relations depicted in Fig. 1f.
1
Institut fu¨r Physik and Center of Interface Science, Carl von Ossietzky Universita
¨
t, 26129 Oldenburg, Germany,
2
Department of Physics, Indian Institute of
Technology Bombay, 400076 Mumbai, India,
3
IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, 20133 Milano, Italy.
*
e-mail: parinda.vasa@uni-oldenburg.de; christoph.lienau@uni-oldenburg.de
LETTERS
PUBLISHED ONLINE: 20 JANUARY 2013 | DOI: 10.1038/NPHOTON.2012.340
NATURE PHOTONICS | VOL 7 | FEBRUARY 2013 | www.nature.com/naturephotonics128
© 2013 Macmillan Publishers Limited. All rights reserved.
In accordance with previous work
7,10–12
, we suggest that a peri-
odic transfer of energy due to the emission and reabsorption of
photons is the physical cause for the normal mode splitting.
Because of the large value of V
NMS
, this exchange with a period
of T
R
≈2p/V
NMS
is expected to occur on a short timescale of
30 fs. To time-resolve these Rabi oscillations and to investigate
the coherent polariton dynamics, we performed angle-resolved
pump–probe spectroscopy on the hybrid system. Nearly collinearly
propagating, p-polarized visible pump and probe pulses with sub-
15 fs duration and centred at 1.8 eV are weakly focused onto the
sample at variable incidence angle
u
. The spectral bandwidth
covers both polariton branches, impulsively exciting polariton
wave packets whose dynamics are probed by monitoring the differ-
ential reflectivity spectrum (DR/R)(
v
pr
,
t
) as a function of probe
frequency
v
pr
and time delay
t
between pump and probe pulses.
The time evolution of DR/R is shown in Fig. 2a for a hybrid
nanostructure with a
0
¼ 430 nm and
u
¼ 398, and excited with a
fluence of 30 mJcm
22
. Here, the SPP mode is detuned by
75 meV to the red of the exciton resonance at 1.789 eV. The
data reveal a strong nonlinear signal near 1.789 eV and a weaker
signal at the LP resonance (1.65 eV). The DR signal near the LP res-
onance also exhibits pronounced temporal oscillations. The signal
near 1.789 eV is very similar to the nonlinear response of a bare
dye film deposited on a planar gold mirror
7,27
. It shows a dispersive
line shape reflecting the bleaching of the exciton resonance (DR .
0) and the resulting blueshifted exciton to biexciton (XX) absorp-
tion (DR , 0). Its dynamics mainly reflect the pump-induced
perturbation of the exciton free induction decay (
t
, 0, probe pre-
cedes pump) and the decay of the pump-induced exciton population
(
t
. 0) with a lifetime of 1 ps (ref. 7). The nonlinear spectra are
well understood in terms of a cascaded X–XX model
7,27
with a non-
linearity governed by the pump-induced saturation of the exciton
absorption (Supplementary Section S4).
We now discuss the nonlinear response at the LP resonance. Here,
we also find a dispersive spectral lineshape (Fig. 2b) with DR . 0at
lower probe energies. The signal shows temporal oscillations with a
period of T
R
¼ 27 fs both at positive (
t
. 0) and negative delays,
and has maxima at integer multiples of T
R
(Fig. 2c). These oscillations
persist for about +50 fs, suggesting that the radiative damping of the
SPP modes
28
limits the coherence time. Signal amplitude and oscil-
lation period increase when reducing the detuning between the
exciton and SPP (compare the trace at
u
¼ 318 showing mainly an
oscillation with period T
R
¼ 37 fs). All these observations indicate
that the transient oscillations seen on the LP resonance are a distinct
signature of coherent X–SPP Rabi oscillations.
To understand the origin of these oscillations, recall that the
linear spectra (Fig. 1) show that the absorption coefficient of the
SPP mode exceeds by a factor of 100 that of the small fraction
of those excitons that are coupled to SPP modes. Hence, the broad-
band pump pulse launches a coherent polariton wave packet that is
initially mainly localized on the SPP side. This initiates coherent
population oscillations between both states. Our data do not
support the assumption that exciton saturation is the dominant
nonlinearity. Exciton saturation would result in an absorptive line-
shape of the DR spectrum with a large amplitude whenever the wave
packet is localized on the exciton side, that is, at
t
¼ (2n þ 1)T
R
/2,
n [ Z, whereas we observe a dispersive DR spectrum with maxima
at integer multiples of the Rabi period,
t
¼ nT
R
. This indicates that a
1.6
1.7
1.8
1.9
2.0
LP
−π/a
0
PM[+1]
Angle θ (deg)
Energy (eV)
1.6
1.7
1.8
1.9
2.0
Energy (eV)
R
0
LP
UP
SPP
X
c
b
PM[−1]
X
ω
0
π/a
0
UP
Ω
NMS
a
k
x
20 30 40 20 30 40 1.6 1.7 1.8 1.9 2.0
d
UP
LP
X
SPP
Angle θ (deg)
20 30 40
Angle θ (deg)
0.1 0.6 1.0
x
y
z
0.0
0.5
1.0
Energy (eV)
e
X
UP
R
0
LP
θ = 30°
1.6
1.7
1.8
1.9
SPP
X
LP
f
Energy (eV)
UP
Reflected pulse
Incident pulse
a
0
Molecular J-aggregate
on a gold nanoslit array
−
+
−
+
−
+
−
+
−
+
−
+
Figure 1 | Strong exciton–SPP coupling in J-aggregate/metal hybrid nanostructures. a, Schematic of coherent ultrafast spectroscopy on a hybrid
nanostructure consisting of a 50-nm-thick film of J-aggrega te molecules in a polymer matrix coated onto a gold nanoslit arr a y with period a
0
. Strongly
localized SPP fields (in red) exist in and near the slits. Incident and reflected laser pulses and J-aggregate excitons are shown schematically. b,Schematic
dispersion relations of the exciton (X) and surface plasmon polariton (SPP) resonances at the polymer–metal interface (PM[+1]). The strong dipolar X –SPP
coupling results in the formation of upper (UP ) and low er (LP) exciton SPPs with normal mode splitting V
NMS
. c,Angle-resolved,p-polarized linear
reflectivity map R
0
(
u
,
v
)(
u
, incidence angle), showing a splitting of hV
NMS
≈110 meV for a grating with a
0
¼ 430 nm. The dispersionless feature at
1.789 eV arises from uncoupled J-aggregate molecules. d, Simulations of R
0
(
u
,
v
) based on an optical Bloch equation model. e, Experimental (solid line)
and simulated (dashed line) reflectivity spectra at
u
¼ 308. f, Measured (open circles) and simulated dispersion relations.
NATURE PHOTONICS DOI: 10.1038/NPHOTON.2012.340
LETTERS
NATURE PHOTONICS | VOL 7 | FEBRUARY 2013 | www.nature.com/naturephotonics 129
© 2013 Macmillan Publishers Limited. All rights reserved.
different microscopic mechanism governs the nonlinear response of
the hybrid system. Measurements with off-resonant pulsed excitation
have shown
7
that the creation of an exciton population n
X
(t) not only
bleaches the exciton resonance, but also transiently reduces the normal
mode splitting according to V
NMS
t()=V
0
NMS
1 − 2n
X
t()
(Fig. 2e).
The material polariza tion a v erages over the time-varying normal
mode splitting, and this gives rise to a transient blueshift of the
lower-fr equency LP resonance tha t is proportional to the time
average kV
NMS
(t)l, resulting in a dispersive LP nonlinearity
(Fig. 2b). The shift—and thus the amplitude of DR—is large whenev er
pump and probe pulses create in-phase wave packets at
t
¼ nT
R
,that
is, when the two wave packets are preferentially localized in the same
quantum sy s tem. In contrast, it is reduced when out-of-phase w a ve
pack ets are launched. Hence, a str ong pump-induced change in polar-
iton dynamics and thus a large optical nonlinearity is seen at integer
multiples of the Rabi period. This dynamical pictur e is strongly sup-
ported by optical Bloch equa tion simulations (Fig. 2d), reproducing
both the dispersive DR lineshape (Fig. 2b) and the oscillatory LP
dynamics (Fig. 2c). For this incidence angle, the LP oscillator strength
is much larger than that of the UP resonance (Fig. 1), so Rabi oscil-
lations are predominantly seen on the LP branch. When reducing
the detuning by changing the incidence angle to
u
¼ 318,theRabi
period and the magnitude of the nonlinearity incr ease in good agree-
ment with our model. The deduced exciton and SPP population, as
well as the V
NMS
dynamics, are depicted in Fig. 2e. Experiments per-
formed at several values of the detuning are consistent with the scen-
ario described abo v e and provide independent measur ements of the
Rabi period, with an accura cy limited by the ra ther short dephasing
times, and the LP dispersion rela tion (Fig. 2f).
These results suggest that the dominant mechanism gov erning the
polariton nonlinearity is a transient reduction in Rabi splitting due to
exciton sta te filling. In Fig. 2a, the decrease in Rabi splitting was ra ther
small (Fig. 2e) and probed by its effect on the lineshape and amplitude
of the DR spectrum. To enhance the effect, we increased the pump
fluence to 60 mJcm
22
and studied the response of the nanostructur e
in the limit of small detuning (a
0
¼ 412 nm,
u
¼ 338;Fig.3a).We
observe much stronger dispersiv e LP nonlinearities, again showing
clear signatures of Rabi oscillations (Fig. 3b, blue line). Importantly ,
the Rabi period decreases progr essiv ely with increasing delay. Under
these e xcita tion conditions the nonlinear response becomes mor e
comple x. The absorption coefficient of the UP br anch increases
(Fig. 1) and we also see out-of-phase Rabi oscillations on the UP
branch, spectrally overlapping with the response of the uncoupled J-
aggre ga te excitons. Measurements on gratings with differ ent periods
confirm the assignment of UP and LP resonances (Suppleme ntary
Section S6). The observed nonlinear response is again reasonably
we ll explained in terms of our Bloch equation model (Fig. 3c–e).
The impulsive excita tion launches a coherent SPP wave pack et, initiat-
ing a population transfer to the exciton sys tem. Consequently , the
exciton population rea ches a maximum after half a Rabi period and
this in turn transiently decreases the normal mode splitting. To
approxim a te the experimentally seen dynamics (Fig. 3b), a rather sub-
stantial decrease in normal mode splitting by more than 15% during
the first oscillation cycle (Fig. 3f) should be assumed. The rapid radia-
tive damping of the polariton population progr essiv ely re duces the
exciton s tate filling and restores the shorter Rabi period seen in
the absence of the pump-induced exciton popula tion (Fig. 3b,f).
The amplitude and frequency of this modulation can be controlle d
by varying the X–SPP mixing ra tio and/or pump fluence. The out-
of-phase oscillations observed on the UP branch reflect the inter-
ference between the phase-shifted fields emitted by the hybrid mode
and the uncoupled J-aggregate excitons.
In summary, our results present the first real-time observation of
coherent X–SPP Rabi oscillations in hybrid nanostructures. They
LP
LP
39°
39°
×3
×3
31°
Sim
X + UP
Exp
Sim
×0.05
ΔR/R
Ω
NMS
Ω
NMS
(meV)
26 fs
26 fs
0.02
0.01
−0.01
ΔR/R
θ = 39°
Angle θ (deg)
1.80
a
d
e
f
b
c
1.75
1.70
1.65
1.65
−0.01
0.00
0.01
0.02
ΔR/R
−0.02
0.00
0.02
0.04
1.70 1.75 1.80
Energy (eV)
Energy (eV)
1.6
1.7
1.8
UP
LP
Energy (eV)
1.80
1.75
1.70
1.65
Energy (eV)
−50−100 50 100
Delay τ (fs)
τ = 0 fs
0
−50 50
Delay τ (fs)
0
−50−100 50 100
Population (%)
0
1
2
3
4
Time in Rabi period (fs)
0
SPP
X
1234
25 30
Rabi period (fs)
20
30
40
35 40 45
107
108
109
110
Delay τ (fs)
0
0.02
0.01
−0.01
ΔR/R
X + UP
X + UP
×0.05
×0.05
LP
LP
Figure 2 | Coherent dynamics of X –SPP Rabi oscillations. a, Measur ed differential reflectivity map (DR/R)(
v
pr
,
t
) for a hybrid structure with a
0
¼ 430 nm,
recorded using two nearly collinearly propagating 15 fs pulses with time delay
t
at an incidence angle of
u
¼ 398. The pump fluence is set to 30
m
Jcm
22
.
Clear temporal Rabi oscillations are seen near the LP resonance at 1.65 eV. b, Comparison between measured (solid line) and simulated (dashed line) DR/R
spectra at
t
¼ 0. c, Time evolution of the DR/R signal near the LP resonance measured at two different angles
u
, exhibiting pronounced sub-40 fs Rabi
oscillations. The shorter oscillation period for
u
¼ 398 reflects the increased X–SPP detuning. Simulated dynamics for
u
¼ 398 are shown as a dashed line
(shifted vertically by 20.025). d,e Simulated (DR/R)(
v
pr
,
t
) map (d) and pump-induced SPP and exciton population dynamics at
u
¼ 398 (e). f, Comparison
between observed (open symbols) and calculated (solid lines, error bars taken as standard deviation of F ourier-tr ansformed DR
v
pr
,
t
traces) oscillation
periods and LP resonance energies as a function of
u
. The simulated dispersion relations are included.
LETTERS
NATURE PHOTONICS DOI: 10.1038/NPHOTON.2012.340
NATURE PHOTONICS | VOL 7 | FEBRUARY 2013 | www.nature.com/naturephotonics130
© 2013 Macmillan Publishers Limited. All rights reserved.
show that the optical dipole coupling between excitons and SPPs
drastically alters the optical response of the individual resonances.
Transient oscillations in exciton density induced by coherent
X–SPP population transfer give rise to a periodic modulation of
the normal mode splitting and thus optical nonlinearity on a 10 fs
timescale. This may be seen as a new approach to control the SPP
response of metallic nanostructures by coupling SPPs to active
quantum systems and providing an ultrafast switching functionality
to their otherwise linear response. We believe that this is a
promising route towards active all-optical nanophotonic circuits
and devices.
Methods
Sample fabrication. Nanoslit arrays with periods in the range 400–460 nm, depth of
30 nm, slit width of 45 nm and area of 150 mm × 150 mm were fabricated on an
optically thick gold film by focused ion beam milling (FEI Helios NanoLab 600i).
Such arrays give rise to significant field confinement inside the slits and near the
slit edges (Fig. 1a). Simulations of the electric field distribution are presented in
Supplementary Section S2. The cyanine dye 2,2
′
-dimethyl-8-phenyl-5,6,5
′
,6
′
-
dibenzothiacarbocyanine chloride (Hayashibara Bio-chemicals Laboratories) was
dissolved in a solution containing polyvinyl alcohol (PVA, 80% hydrolysed, Sigma
Aldrich, 26 mg), water (1 ml) and methanol (4 ml) and was spin-coated at
8,000 r.p.m. onto the nanoslit arrays to deposit a 50-nm-thick film. The
concentration of the dye in dry PVA was typically 0.5 mol dm
23
, resulting in an
absorption coefficient
7,11
of 2 ×10
5
cm
21
. The slit-array period a
0
of 400–460 nm
was chosen so that the first-order polymer–metal plasmon (PM[–1]) mode was in
resonance with the J-aggregate exciton at 1.789 eV. Although the coupling energy
is somewhat reduced
7,11,15
, we used thinner dye films and narrow, shallow slit arrays
to minimize radiative SPP damping
28
and to increase the polariton lifetime. The
geometric parameters were optimized such that the array acted as a plasmonic
resonator, allowing optical confinement of the far-field light predominantly within
the nanoslits. The optical properties of the optically thick nanoslit array were studied
in reflection geometry.
Experimental. All experiments were performed using an angle-resolved reflectivity
setup with an angular resolution of 0.28. The p-polarized linear reflectivity spectra
were recorded using a coherent white-light source (Fianium SC-450 -4), whereas
transient DR/R measurements were performed in a femtosecond pump–probe
spectrometer (Supplementary Section S3). The setup began with a regeneratively
amplified mode-locked Ti:sapphire laser system (Quantronix Integra C) delivering
pulses with a duration of 150 fs and energy of 500 mJ at a repetition rate of 1 kHz and
with 1.6 eV photon energy. The system drove a non-collinear optical parametric
amplifier generating broadband near-infrared pulses at 1.8 eV and with a spectrum
extending from 1.65 to 1.9 eV (ref. 29). These pulses were compressed to sub-15 fs
duration by multiple bounces on chirped mirrors (Venteon) and were split into a
pair of p-polarized pump and probe pulses. The pulses were shifted slightly
vertically, along the slit axis, and were nearly collinearly focused onto the sample to a
beam diameter of 100 mm at an incidence angle of 308 with respect to the sample
normal. The data in Fig. 2 and the low-power curve in Fig. 3c were obtained with
pump and probe pulse energies of 2.5 and 0.5 nJ, corresponding to fluences of 30
and 6 mJcm
22
, whereas the measurements in Fig. 3a were performed at fluences of
60 mJcm
22
and 30 mJcm
22
, respectively. One of the pump beam mirrors was
vibrated to suppress interference artefacts in the DR/R spectra close to zero delay. All
experiments were performed at room temperature under vacuum to minimize J-
aggregate photobleaching.
Theoretical modelling. As described in Supplementary Section S1, the hybrid
X–SPP polariton modes are given as a coherent superposition of exciton and SPP
wavefunctions, UP, LP
|
l = D + A
()
i
|
l + 2Dj
l
/B
+
. Their characteristic
eigenfrequencies are given as ˜
v
UP,LP
=(1/2)(˜
v
i
+ ˜
v
j(
u
)
+A), where
A =
D
2
+ 4CD
√
, D = ˜
v
i
− ˜
v
j
u
()
, C = V
R
− i
g
ij
and D = V
∗
R
− i
g
ij
. The
normalization constant B
+
=
D + A
||
2
+4 D
||
2
√
, and
g
ij
is a rate for incoherent
photon exchange giving rise to collective spontaneous emission of the coupled
system
30
. The measured linear and differential reflectivity spectra are simulated by
solving optical Bloch equations for the system of interacting excitons and SPPs using
the density matrix formalism. In the simulations, the exciton system is modelled as
an effective three-level system (ground state, X, XX), whereas the SPP mode is
described as a photon-like harmonic oscillator. The simulations include dipole
coupling between exciton and SPP
30
and a semiclassical coupling of both systems to an
external laser field. Radiative damping and pure dephasing of excitons is implemented
phenomenologically by following the Lindblad formalism
30
. In our samples, the SPP
fields are mainly confined inside or near the slits, so the Rabi splitting is strongly
reduced for those excitons located away from the slits. Contribution from such weakly
interacting ‘uncoupled’ dye molecules has also been taken into account. A detailed
description of the density matrix calculations together with estimates of the coupling
parameters are given in Supplementary Section S1.
Received 19 July 2012; accepted 4 December 2012;
published online 20 January 2013
1.85
1.80
0.00
1.81 eV
X + UP
1.81 eV
X + UP
1.76 eV
LP
1.76 eV
LP
SPP
−0.15
0.00
−0.15
−3
−2
−1
0.15
1.75
1.70
Energy (eV)
1.85
a
b
c
d
e
f
1.80
1.75
1.70
Energy (eV)
−100 100
LP
LP
LP
X + UP
X + UP
0.1
0
−0.1
Delay τ (fs)
0
−100 100
Delay τ (fs)
0
−100−200 100 200
Delay τ (fs)
0 −50−100 50 100
Delay τ (fs)
0
−100 100
Delay τ (fs)
ΔR/R
ΔR/R
ΔR/R (norm.)
ΔR/R
0
−100
0
20
40
Ω
NMS
Ω
NMS
(meV)
X
38 fs
40 fs
35 fs
43 fs
45 fs
45 fs
60
40
60
80
100
Exp
Sim
Population (%)
100
200
Time (fs)
0
0.1
0
−0.1
ΔR/R
−
Figure 3 | Transient manipulation of the Rabi energy. a, (DR/R)(
v
pr
,
t
) spectra at
u
¼ 338 for a J-aggregate/metal nanostructur e with a
0
¼ 412 nm, close to
the X–SPP crossing, recorded using sub-15 fs pump pulses with a fluence of 60
m
Jcm
22
.TheDR/R tra ces show coher ent oscillations with a delay-dependent
period. b, Time evolution of DR/R at the LP resonance (1.76 eV, blue line) and near the overlapping UP and bare J-aggregate resonances at 1.81 eV (red line).
c, LP dynamics at low (30
m
Jcm
22
, blue) and high (60
m
Jcm
22
, red) pump fluence showing an increased and delay-dependent Rabi period at high fluence
together with Bloch equation simulations (dashed lines). The traces are shifted vertically for clarity. d,e,SimulatedDR/R spectra (d) and cross-sections (e)
at the UP and LP resonances. f, Dynamics of the pump-induced SPP (blue line) and exciton (green line) population together with V
NMS
(t) (black line),
indicating that a transient modulation of V
NMS
is the micr oscopic origin of the optical nonlinearity of the J-aggr egate/metal nanostructure .
NATURE PHOTONICS DOI: 10.1038/NPHOTON.2012.340
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References
1. Maier, S. A. et al. Local detection of electromagnetic energy transport below the
diffraction limit in metal nanoparticle plasmon waveguides. Nature Mater. 2,
229–232 (2003).
2. Bozhevolnyi, S. I., Volkov, V. S., Devaux, E., Laluet, J. Y. & Ebbesen, T. W.
Channel plasmon subwavelength waveguide components including
interferometers and ring resonators. Nature 440, 508–511 (2006).
3. Lal, S., Link, S. & Halas, N. J. Nano-optics from sensing to waveguiding.
Nature Photon. 1, 641–648 (2007).
4. Dintinger, J., Robel, I., Kamat, P., Genet, C. & Ebbesen, T. Terahertz all-optical
molecule-plasmon modulation. Adv. Mater. 18, 1645–1648 (2006).
5. Chang, D. E., Sorensen, A. S., Demler, E. A. & Lukin, M. D. A single-photon
transistor using nanoscale surface plasmons. Nature Phys. 3, 807–812 (2007).
6. MacDonald, K. F., Samson, Z. L., Stockman, M. I. & Zheludev, N. I. Ultrafast
active plasmonics. Nature Photon. 3, 55–58 (2009).
7. Vasa, P. et al. Ultrafast manipulation of strong coupling in metal–molecular
aggregate hybrid nanostructures. ACS Nano 4, 7559–7565 (2010).
8. Engheta, N. Circuits with light at nanoscales: optical nanocircuits inspired by
metamaterials. Science 317, 1698–17 02 (2007).
9. Gonzalez-Tudela, A. et al. Entanglement of two qubits mediated by one-
dimensional plasmonic waveguides. Phys. Rev. Lett. 106, 020501 (2011).
10. Bellessa, J., Bonnand, C., Plenet, J. C. & Mugnier, J. Strong coupling between
surface plasmons and excitons in an organic semiconductor. Phys. Rev. Lett. 93,
036404 (2004).
11. Dintinger, J., Klein, S., Bustos, F., Barnes, W. L. & Ebbesen, T. W. Strong
coupling between surface plasmon-polaritons and organic molecules in
subwavelength hole arrays. Phys. Rev. B 71, 035424 (2005).
12. Hakala, T. K. et al. Vacuum Rabi splitting and strong-coupling dynamics for
surface-plasmon polaritons and rhodamine 6G molecules. Phys. Rev. Lett. 103,
053602 (2009).
13. Schwartz, T., Hutchison, J. A., Genet, C. & Ebbesen, T. W. Reversible switching
of ultrastrong light–molecule coupling. Phys. Rev. Lett. 106, 196405 (2011).
14. Guebrou, S. A. et al. Coherent emission from a disordered organic
semiconductor induced by strong coupling with surface plasmons. Phys. Rev.
Lett. 108, 066401 (2012).
15. Fofang, N. T. et al. Plexcitonic nanoparticles: plasmon–exciton coupling in
nanoshell–J-aggregate complexes. Nano Lett. 8, 3481–3487 (2008).
16. Stockman, M. I. Nanoplasmonics: past, present, and glimpse into future.
Opt. Express 19, 22029–22106 (2011).
17. Bergman, D. J. & Stockman, M. I. Surface plasmon amplification by stimulated
emission of radiation: quantum generation of coherent surface plasmons in
nanosystems. Phys. Rev. Lett. 90, 027402 (2003).
18. Noginov, M. A. et al. Demonstration of a spaser-based nanolaser.
Nature 460,
1110–1112 (2009).
19. Oulton, R. F. et al. Plasmon lasers at deep subwavelength scale. Nature 461,
629–632 (2009).
20. Zheludev, N. I., Prosvirnin, S. L., Papasimakis, N. & Fedotov, V. A. Lasing spaser.
Nature Photon. 2, 351–354 (2008).
21. Fofang, N. T., Grady, N. K., Fan, Z., Govorov, A. O. & Halas, N. J. Plexciton
dynamics: exciton–plasmon coupling in a J-aggregate–Au nanoshell complex
provides a mechanism for nonlinearity. Nano Lett. 11, 1556–1560 (2011).
22. Thompson, R. J., Rempe, G. & Kimble, H. J. Observation of normal-mode
splitting for an atom in an optical cavity. Phys. Rev. Lett. 68, 1132–1135 (1992).
23. Reithmaier, J. P. et al. Strong coupling in a single quantum dot–semiconduc tor
microcavity system. Nature 432, 197–200 (2004).
24. Yoshie, T. et al . Vacuum Rabi splitting with a single quantum dot in a photonic
crystal nanocavity. Nature 432, 200–203 (2004).
25. Weisbuch, C., Nishioka, M., Ishikawa, A. & Arakawa, Y. Observation of the
coupled exciton–photon mode splitting in a semiconductor quantum
microcavity. Phys. Rev. Lett. 69, 3314–3317 (1992).
26. Khitrova, G., Gibbs, H. M., Jahnke, F., Kira, M. & Koch, S. W. Nonlinear optics
of normal-mode-coupling semiconductor microcavities. Rev. Mod. Phys. 71,
1591–1639 (1999).
27. Fidder, H., Knoester, J. & Wiersma, D. A. Observation of the one-exciton to
two-exciton transition in a J aggregate. J. Chem. Phys. 98, 6564–6566 (1993).
28. Kim, D. S. et al. Microscopic origin of surface-plasmon radiation in plasmonic
band-gap nanostructures. Phys. Rev. Lett. 91, 143901 (2003).
29. Manzoni, C., Polli, D. & Cerullo, G. Two-color pump–probe system broadly
tunable over the visible and the near infrared with sub-30 fs temporal resolution.
Rev. Sci. Instrum. 77, 023103 (2006).
30. Akram, U., Ficek, Z. & Swain, S. Decoherence and coherent population transfer
between two coupled systems. Phys. Rev. A 62, 013413 (2000).
Acknowledgements
The authors thank the Deutsche Forschungsgemeinschaft (SPP 1391 and DFG-NSF
Materials World Network), Fondazione Cariplo (‘Engine ering of optical nonlinearities
in plasmonic metamaterials’), European Community (FP-7 INFRASTRUCTURES-2008-1,
‘Laserlab Europe II’, contract no. 228334 and FP-7 NMP ‘Cronos’) and the Korea
Foundation for International Cooperation of Science and Technology (Global Research
Laboratory project, K20815000003) for financial support. The authors also thank
E. Sommer for preparing some of the figures.
Author contributions
P.V., R.P., W.W. and C.L. designed and fabricated the hybrid nanostructures. All authors
participated in conducting the experiments. R.P., P.V., W.W. and C.L. contributed to the
theoretical modelling. All authors discussed the results and implications at all stages.
P.V., R.P., G.C. and C.L. wrote the paper.
Additional information
Supplementary information is available in the online version of the paper.
Reprints and
permission information is available online at http://www.nature.com/reprints. Correspondence
and requests for materials should be addressed to P.V. and C.L.
Competing financial interests
The authors declare no competing financial interests.
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