3876

LetterVol. 42, No. 19 / October 1 2017 / Optics Letters

Surface exciton polaritons supported by aJ-aggregate-dye/air interface at room temperature
KENTARO TAKATORI,1,2,3 TAKAYUKI OKAMOTO,1,4,* KOJI ISHIBASHI,1,4 AND RUGGERO MICHELETTO3
1

Advanced Device Laboratory, RIKEN, Hirosawa, Wako, Saitama 351-0198, Japan
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, Kanagawa 2268503, Japan
3Graduate School of Nanobioscience, Yokohama City University, Seto, Kanazawa-ku, Yokohama, Kanagawa 236-0027, Japan
4RIKEN Center for Emergent Matter Science, Hirosawa, Wako, Saitama 351-0198, Japan*Corresponding author: okamoto@riken.jp
2

Received 20 July 2017; revised 31 August 2017; accepted 1 September 2017; posted 6 September 2017 (Doc. ID 302978);published 25 September 2017

Surface exciton polaritons (SEPs) are very important for therealization of novel sensors and next-generation opticaldevices. Here we propose for the first time, to the best of
our knowledge, a Kretschmann–Raether device that is able
to induce SEPs propagating along the interface betweena J-aggregate cyanine dye and air at room temperature.
This configuration has the advantages of being straightforward to realize and easy to study because theKretschmann–Raether approach is the most simple andfundamental from the theoretical point of view. Here aJ-aggregate cyanine dye produces strong binding energy
due to Frenkel excitons, and this enables the observationof SEPs easily at room temperature. One of the advantagesof the use of the J-aggregate cyanine dye is the simple device
preparation. This is because the J-aggregate cyanine dye
can be easily deposited on any arbitrary substrates with aspincoating or dip-coating technique from its aqueous
solution in ambient condition. We observed SEPs at room
temperature, and the deepest resonant peak was obtained
for a 94 nm thick 5,6-dichloro-2-[[5,6-dichloro-1-ethyl-3(4-sulfobutyl)-benzimidazol-2-ylidene]-propenyl]-1-ethyl-3(4-sulfobutyl)-benzimidazolium hydroxide film at 532 nm
wavelength. Our results may pave the way for the realizationof novel SEP biosensors in a simple and straightforward way
at room temperature. © 2017 Optical Society of America
OCIS codes: (160.4890) Organic materials; (240.0240) Optics atsurfaces; (240.5420) Polaritons; (240.6680) Surface plasmons;(260.6970) Total internal reflection.
https://doi.org/10.1364/OL.42.003876

Surface polaritons are surface electromagnetic waves propagating along a surface of a medium whose permittivity is negative
due to plasmons, excitons, or phonons. Among them, surface
exciton polaritons (SEPs) have been experimentally observed ata surface of various inorganic and organic crystals in previous
works [1–5]. The common property of the excitons in these0146-9592/17/193876-04 Journal © 2017 Optical Society of America

media is their strong oscillation strength, which gives strong
coupling to the electric field. Extremely low temperature is required for supporting SEPs in inorganic crystals, because excitons in these media are Wannier–Mott excitons whose binding
energy (∼0.01 eV) is lower than the thermal energy at room
temperature (0.025 eV) [6]. On the other hand, excitons inorganic material are Frenkel excitons which have higher binding energy (0.1–1 eV) than the thermal energy at room temperature [6]. Actually, SEPs in organic crystals were observed atroom temperature [5]. However, SEPs in non-crystalline media
at room temperature have not been reported, to the best of ourknowledge. In this Letter, we observe SEPs at the surface of a
J-aggregate cyanine dye film as an organic non-crystalline
medium.
Cyanine dye easily forms a J-aggregate in polar solvent suchas water. In the J-aggregate, the binding energy of the Frenkel
excitons is quite high due to the dipole-dipole interaction between molecules. This results in a very narrow redshifted andstrong absorption band, called J-band, compared to that inmonomers [7–9]. This property realizes strong vacuum Rabi
splitting through coupling of the J-aggregate cyanine dyeand surface plasmon polaritons [10–12]. Bellessa et al. observed
strong coupling between surface plasmons and excitons at theinterface between silver and cyanine dye [13]. One of the advantages of J-aggregate cyanine dye to support SEPs is the simplicity of the device preparation. J-aggregate cyanine dye can beeasily deposited on any arbitrary substrates with spincoating or
dip-coating from its aqueous solution in ambient condition.
Figure 1(a) shows the chemical formula of a 5,6-dichloro-2[[5,6-dichloro-1-ethyl-3-(4-sulfobutyl)-benzimidazol-2-ylidene]propenyl]-1-ethyl-3-(4-sulfobutyl)-benzimidazolium hydroxide(TDBC) molecule, which is the cyanine dye we used.
Although TDBC is partially crystalline, it can be easily deposited
with spincoating or dip-coating. First, we measured the thickness
and the complex dielectric constant of a TDBC film. For thethickness measurement, we modified the surface of a 1 mm thickglass substrate by depositing a polyvinyl alcohol (PVA) and asilica film, whose total thickness was 25 nm [see Fig. 1(b)],because it is difficult to peel off the sticky TDBC film which

Vol. 42, No. 19 / October 1 2017 / Optics Letters

Letter3877

Table 1. Fitted Parametersjωpj (cm−1 )ω0j (cm−1 )γj (cm−1 )

12
3
4
5

4340
4383
3511
11830
1621

13,57015,33016,14016,96018,710

2409
1352
565.5
117.3
561.6appear at ∼530 nm [9]. We calculated the dielectric constantof the TDBC film by fitting a model consisting of five Lorentzianoscillators:ϵω  ϵ∞ Fig. 1. (a) Chemical formula of TDBC. (b) Measured (solid curve)
and fitted (circles) absorption spectra of a 60 nm thick TDBC film on
25 nm thick silica/PVA composite film and glass substrate at room temperature. The inset shows the structure of the sample. (c) Calculated
complex dielectric constant of the TDBC film via fitting.

is directly deposited on a glass substrate. This process is required
to measure the thickness of the film with an atomic force microscope (AFM). The PVA layer between the TDBC film and theglass substrate makes it easy to peel off the TDBC/silica/PVA
composite film from the glass substrate. The surface of the silica
layer gives the same condition as that of the glass substrate in thespincoating process. We spun a 30 mM TDBC aqueous solution
on this substrate, then peeled off a part of the film and measured
the step height by an AFM (VN-8010, Keyence). The obtained
thickness of the TDBC film was 60 nm. Figure 1(b) shows theabsorption spectrum of the same sample measured with aUV-Vis spectrophotometer (UV-2550, Shimadzu). The absorption peak at 588 nm represents the characteristics of theJ-aggregate, whereas the absorption peak for monomers may

5X

ω2pj;
ω2 − ω2 − iγ j ωj1 0j(1)

to the observed spectrum. In the fitting we ignored the silica/
PVA layers since they have almost the same refractive index
as the glass substrate in the measurement wavelength region.
Table 1 summarizes the fitted parameters. Figure 1(c) showsthe estimated real and imaginary part of the dielectric constantof the TDBC film, and the best fitting was given with ϵ∞  0.
To have SEPs on the TDBC film, the real part of the dielectric
constant should be less than −1. Thus, as shown in Fig. 1(c),the TDBC surface actually supports SEPs in the range from
463 to 589 nm.
Next, we observed SEPs using the Kretschmann–Raether
configuration [14], where the metal film was replaced with aTDBC film, as shown in Fig. 2. A glass substrate, of which
one surface was deposited with a TDBC film, was attachedto the bottom surface of a BK7 prism (n  1.52) using index
matching oil. We measured the reflection intensity as a function of the angle of incidence. A 532 nm wavelength laser beam(125 mW, GREEN-SLM LASER, Uniphase) with p-polarization was used as the incident light. All measurements were doneat room temperature. Figure 3(a) shows the measured reflection
curves for the TDBC films with several thicknesses. The reflection curves were normalized to the maximum intensity. Eachreflection curve exhibits a resonant dip derived from SEP resonance at an angle of ∼50°, which is larger than the criticalangle for the interface of the prism and air. The depth of
the resonance dip initially increases and then decreases withthe thickness of the TDBC film. The deepest dip was obtained
for a 94 nm thick TDBC film at 53.6°.

Fig. 2. Experimental arrangement for the attenuated total reflection(ATR) measurement.

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Vol. 42, No. 19 / October 1 2017 / Optics Letters

We calculated reflectance as a function of the angle ofincidence using the dielectric function shown in Fig. 1(c)for the same configuration as the experiment. Figure 3(b) shows
calculated reflection curves for TDBC films with several thicknesses. In this calculation, we used a fitted dielectric constantϵ  −2.59  0.375i. The deepest dip was obtained for a
60 nm thick TDBC film at 54.7°. The width of the calculated
dips was much narrower than that of the experimental results.
Two reasons can be considered for the broadening of the dip
width in the experiments. One is the surface roughness of theTDBC films, and the other is the thickness inhomogeneity inthose films. The measured root-mean-square roughness of theTDBC films was 6.4–9.9 nm. The surface roughness in theTDBC films causes the scattering of SEPs. This scattering
shortens the propagation length of SEPs, resulting in the broadening of the dip’s width [14]. The large thickness inhomogeneity also broadens the resonant dip width, since the resonant
angle depends on the thickness of the TDBC film in theKretschmann–Raether configuration. In this case, the dip becomes shallower, because the reflectance curve is given by theaverage of those that correspond to each thickness. However,the experimental spectrum for the 94 nm thick film showed

Fig. 3. (a) Experimental and (b) calculated ATR reflection of 41,60, 94, and 108 nm thick TDBC films.

LetterFig. 4. Calculated dispersion relation of an SEP at the interface
between a TDBC film and air in the Kretschmann–Raetherconfiguration.

almost zero reflectance at the dip. For this reason, we concluded
that the broadening of the dip width was mainly caused by thesurface roughness of the TDBC films.
Figure 4 shows the dispersion relation of the SEP in theKretschmann–Raether configuration shown in Fig. 2, calculated with the dielectric function of TDBC shown in Fig. 1(c).
The reflectance is mapped as a function of wavelength andthe angle of incidence. The assumed thickness of the TDBC
film was 60 nm. The locus of the reflection minimum inthe wavelength region shorter than 580 nm corresponds tothe dispersion relation of the SEP.
An interesting phenomenon can be seen in the behavior of
the resonance curves for SEPs, when it is compared to surface
plasmon polaritons in silver, gold, or aluminum in theKretschmann–Raether configuration. It is well known thatthe resonant angle shifts toward a smaller angle as the thicknessof the metal film increases [14]; in contrast, in our case, theresonant angle shifts toward larger angles with thickness. We
calculated reflectance spectra in the Kretschmann–Raether configuration with several combinations of the real part and theimaginary part of the dielectric constant of the thin film to investigate this phenomenon. We noticed that when the absolutevalue of the real part is relatively small, the resonant angle
shifted toward larger angles as the thickness of the film increases, whereas when the value is relatively large, the resonant
angle shifted toward smaller angles. We discovered that thedirection of the angular shift changed at jReϵj ∼ 5, when
the dielectric is air. The absolute value of the real part ofthe dielectric constant of TDBC films is jReϵj  2.59 at
532 nm, which is much smaller than that for silver, gold,
and aluminum.
In conclusion, we obtained for the first time, to the best of
our knowledge, a J-aggregate cyanine dye that is able to realizeSEPs at room temperature. In addition, we obtained the dielectric function of the TDBC film by fitting a Lorentzian model tothe absorption spectrum of the TDBC/silica/PVA film. From
this function, we were able to understand that SEPs can besupported by the TDBC films in the wavelength region startingfrom 463 to 589 nm. We used the Kretschmann–Raether configuration on the TDBC organic non-crystalline medium at

Letterroom temperature. We observed SEPs on this surface and obtained the deepest resonant dip for the 94 nm film thickness at
an incident angle of 53.6°, when excited at a wavelength of532 nm. The observed dip width was broader than the calculated one because of the scattering loss due to the surface roughness of the TDBC film. Nevertheless, the results demonstrate
the existence of SEPs at room temperature with our J-aggregate
cyanine dye system. This may have implications for the realization of novel SEP-based biosensors or optical devices. Thereare additional potential advantages of the use of organic materials, such as the possibility to synthesize novel molecules withless damping and the possibility to pattern such active materials
with more ease than using metals, with better versatility in thefunctionalization/linking of molecules. Moreover, organic excitonic materials have substantial optical nonlinearity, which
could be exploited for novel sensors or devices [15–18].
Acknowledgment. A part of this Letter was supportedby the RIKEN Junior Research Associate Program.
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