APPLIED PHYSICS LETTERS 95, 033303 共2009兲

Anisotropic propagation of surface plasmon polaritons causedby oriented molecular overlayer
Fadiah Adlina M. Ghazali, Minoru Fujii, and Shinji Hayashia兲
Department of Electrical and Electronic Engineering, Mesoscopic Materials Laboratory,Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan

共Received 5 June 2009; accepted 1 July 2009; published online 20 July 2009兲
We performed reverse attenuated total reflection measurements on 50-nm-thick Ag thin films coatedwith oriented sexithiophene molecular layers. The dispersion curves obtained clearly demonstratethat the wavevectors of surface plasmon polaritons propagating parallel to the molecular orientation
have larger magnitudes than those propagating perpendicular to the molecular orientation. Theresults provide us with an approach to generate anisotropic propagation of surface plasmon
polaritons simply controlled by the molecular orientation of an overlayer. © 2009 American
Institute of Physics. 关DOI: 10.1063/1.3184586兴
Surface plasmon polaritons 共SPPs兲 that propagate along
a metal-dielectric interface have been explored extensively
in the past decades due to their potential in a variety ofapplications, including surface-enhanced Raman scattering,1
surface-plasmon-resonance sensors,2 and subwavelength
optics.3 SPPs can be excited by excited fluorophores placed
very close to a metallic surface.4 When an appropriate decoupling device such as a prism and grating is used, theexcited SPPs can be converted into light,5 resulting in aphenomenon called surface-plasmon-coupled emission
共SPCE兲.6–8 The SPCE displays unique properties, strongly
directional emission and unique polarization, for instance,that can be useful in sensing applications.7,8
In general, SPPs propagate uniformly in all directions at
an interface between a metal and an isotropic dielectric medium. Gryczynski et al.7 reported a circular pattern of SPCE
from a fluorescent polyvinyl alcohol 共PVA兲 film evaporated
on a 50-nm-thick Ag film decoupled by a semisphere prismand projected onto a screen. The circular pattern implies thatthe wavevectors of SPPs propagating in all directions havethe same magnitude. Hiep et al.9 recently studied the effectsof fluorophore orientation on the SPCE pattern by usingsexithiophene 共6T兲 molecules deposited on 50-nm-thick Agfilms. For a randomly oriented 6T overlayer, they observed acircular SPCE pattern with a uniform intensity distribution,
which demonstrates uniform excitation of SPPs. In contrast,
the SPCE pattern observed for the oriented 6T layer exhibited a nonuniform intensity distribution with much stronger
intensities along the direction of the 6T molecular orientation. This result indicates that SPPs are excited most efficiently along the 6T molecular orientation. If we look moreclosely at the observed SPCE pattern for the oriented 6Tlayer 关Figs. 3共b兲 and 3共c兲 in Ref. 9兴, we find that the patternis an ellipsoid elongated along the molecular orientation. Theellipsoidal SPCE pattern suggests that the propagation of theSPPs is anisotropic. Namely, the magnitude of the SPP
wavevector depends on the direction of propagation and islarger along the molecular orientation. Although several efforts have been devoted to develop a theoretical analysis of
the anisotropic propagation of SPPs at an interface between aa兲

Electronic mail: hayashi@eedept.kobe-u.ac.jp.

0003-6951/2009/95共3兲/033303/3/$25.00

metal and an anisotropic dielectric medium,10,11 experimental
studies on such propagation are still lacking.
As an extension of the previous work by Hiep et al.,9 inthis study we perform reverse attenuated total reflection
共RATR兲 measurements for Ag films coated with oriented 6T
molecular overlayers. The RATR measurements allow us todetermine the dispersion curves for SPPs propagating parallel and perpendicular to the molecular orientation. From thedispersion curves, we obtain quantitative information about
the anisotropic SPP propagation.
Samples were prepared by a method similar to that described in our previous paper.9 Thin Ag films 50 nm in thickness were first vacuum-deposited 共10−6 Torr兲 on glass substrates. Then, 6T thin films with a thickness of 10 nm werevacuum-deposited on the Ag surface with a deposition rate of
0.5–1.0 nm/min. According to previous studies on thiopheneoligomers,12–14 vapor deposited films are polycrystalline andshow varying degrees of order, depending on preparation
conditions such as the deposition rate and substrate temperature. When deposited at a low rate, 6T molecules grow almost upright on the substrate 共within ⬃25° of the surface
normal兲 with no preferential in-plane orientation.13 In order
to align the molecules in a preferential direction, the 6T films
were rubbed several times in one direction using a wiping
paper. This rubbing method was previously used to alignoctithiophene14 and p-sexiphenyl15,16 molecules. Finally, anadditional 6T film 10 nm in thickness was vacuum deposited
at a rate of 10 nm/min to achieve a total thickness of 20 nm.
As a reference, a 20-nm-thick nonrubbed 6T film deposited
on a 50-nm-thick Ag film was also prepared. The reference
sample was produced in exactly the same manner as therubbed sample, apart from skipping the rubbing process.
Polarized photoluminescence 共PL兲 spectra were measured to confirm the orientation of the 6T molecules. The
sample was set in such a way that the normal to its surface
lay in the horizontal plane and the rubbing direction wasvertical. The plane of incidence was taken as the horizontal
plane and excitation light with a wavelength of 408 nm froma laser diode was directed on the sample surface with anincident angle of 45°. The polarization of the incident light
was set to be vertical. Emitted light was detected from thenormal to the sample surface and passed through a camera
lens and a polarizer. The polarized light was then focused on

95, 033303-1© 2009 American Institute of Physics

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033303-2

Appl. Phys. Lett. 95, 033303 共2009兲

Ghazali, Fujii, and Hayashi

Rubbingdirection

Light fromtungsten lampSamplePerpendicular
Parallelyxzθ
Decoupledlight
Monochromator

Hemi-cylinder
prism
Optical fiber

FIG. 2. RATR measurement scheme. Decoupled light is collected by an
optical fiber and guided to a monochromator.

FIG. 1. Polarized PL spectra for rubbed 6T sample. IVV 共IVH兲 is the intensityof vertically 共horizontally兲 polarized emission produced by vertically polarized excitation light. The rubbing direction is set vertical.

the entrance slit of a 25 cm single monochromator equippedwith a Si charge-coupled device. A polarization scrambler
was inserted in front of the entrance slit. The intensity of thehorizontal 共IVH兲 and vertical 共IVV兲 components of the emitted
light was analyzed.
Polarized PL spectra of the rubbed 6T film on Ag areshown in Fig. 1. Both IVV and IVH exhibit two peaks at⬃540 nm 共2.30 eV兲 and ⬃580 nm 共2.14 eV兲, and a shoulder at 635 nm 共1.95 eV兲. These PL band positions are ingood agreement with the literature,17,18 in which each bandcorresponds to 0–0, 0–1, and 0–2 vibronic progressions. ThePL intensity showed a strong dependence on the polarization
direction. While the polarization of the excitation light andthe rubbing direction were fixed to vertical, the PL intensity
was found to have a maximum 共minimum兲 value when thedirection of the polarization was vertical 共horizontal兲. The
dichroic ratio 共defined as IVV / IVH兲 calculated directly fromthe peak heights was about 10. This confirms that the transition dipole moment of the rubbed 6T sample was oriented
predominantly parallel to the rubbing direction. In contrast,
the nonrubbed 6T sample did not exhibit dichroism, which
confirms that the molecules in the nonrubbed sample wererandomly oriented. The prepared 6T films were also examined by an atomic-force microscope 共AFM兲. The AFM images obtained are very similar to those reported by Videlot
et al.,14 which is further evidence for the preferential orientation of 6T molecules along the rubbing direction.
To gain an insight into the anisotropic SPP propagation,
RATR measurements were carried out. The configuration
used for RATR measurements is shown in Fig. 2. The sample
was attached to a hemicylindrical prism made of BK7 glasswith index matching fluid. Light from a tungsten lamp wasincident on the sample surface after passing through an aperture and a lens. In such a configuration, SPPs are excitedwith the aid of surface roughness in the wide spectral rangeof the lamp.19 Decoupled light was collected by an opticalfiber cable with a solid angle of ⬃104 Sr−1 and sent to thesame monochromator as used for the PL measurements. Theobservation angle ␪ was varied from 45° to 59° with a step
size of 1°. It should be noted that in conventional ATR measurements, SPP excitation is detected as a dip in the reflectance spectrum, while in the present RATR measurements,

SPP excitation is detected as a pronounced peak in a spectrum of the decoupled light. In the configuration shown inFig. 2, SPPs propagating along the x-axis can be detected. Inorder to investigate the anisotropic SPP propagation for therubbed sample, we measured the spectra of the decoupledlight by setting the rubbing direction parallel and perpendicular to the x-axis, as depicted in Fig. 2. This allowed us to储
determine the SPP wavevectors parallel 共kSPP兲 and perpen⬜
dicular 共kSPP兲 to the rubbing direction.
RATR spectra obtained for the two configurations fromseveral observation angles are shown in Figs. 3共a兲 and 3共b兲,after being corrected by the raw spectrum of the tungsten
lamp. Pronounced peaks are seen which shift to shorter
wavelengths as the observation angle increases. For a given
observation angle, the peak for the parallel configuration

FIG. 3. 共Color online兲 RATR spectra for 共a兲 parallel and 共b兲 perpendicularconfiguration observed at different observation angles.

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033303-3

Appl. Phys. Lett. 95, 033303 共2009兲

Ghazali, Fujii, and Hayashi储⬜
FIG. 4. 共Color online兲 Dispersion curves for kSPPand kSPP
in the rubbed 6Tsample. Inset is an enlarged figure for a specific wavevector region.

关Fig. 3共a兲兴 is located at a longer wavelength than that for theperpendicular configuration 关Fig. 3共b兲兴. In contrast to therubbed sample, the RATR peak for the nonrubbed sample
was found to be located at the same wavelength for a givenobservation angle even when the sample was rotated aroundthe normal axis. When a RATR peak is located at a wavelength ␭ for an observation angle ␪, the magnitude of theSPP wavevector is given by kSPP = 共2␲ / ␭兲n p sin ␪, where n p
is the refractive index of the prism. Using this expression, we储⬜
in thecan obtain the dispersion relations for kSPP and kSPP
rubbed 6T sample. The results are plotted in Fig. 4. The inset
shows the dispersion curves enlarged in a region of wavevector from 1.03⫻ 10−2 to 1.28⫻ 10−2 nm−1. The interpolated
curves for each configuration are also shown. Figure 4 dem储
onstrates that in the whole measured wavelength region, kSPP⬜
is larger than kSPP, which clearly demonstrates that the thinoriented molecular overlayer induces anisotropy in the SPP
wavevector.
It is of interest to compare the dispersion relations obtained by the RATR spectra with those obtained by conventional ATR spectra. For the rubbed 6T sample, we performed
angle-scan ATR measurements for configurations perpen储⬜
兲 and parallel 共kSPP兲 to the rubbing direction usdicular 共kSPP
ing the 632.8-nm-line of a He–Ne laser. The ATR spectra
exhibited reflectivity dips at 47.0° ⫾ 0.1° and 49.1° ⫾ 0.1°
for perpendicular and parallel configurations, respectively.
⬜
= 1.10⫻ 10−2 ⫾ 0.01 nm−1 andThese dip positions yield kSPP储
−2−1kSPP = 1.14⫻ 10 ⫾ 0.01 nm at E = 1.96 eV 共632.8 nm兲.
⬜
= 1.11
These values are in good agreement with kSPP储
−2−1−2−1⫻ 10 nm and kSPP = 1.14⫻ 10 nm deduced from thedispersion curve shown in the inset of Fig. 4 by interpolating
the data to E = 1.96 eV.
The displacement of the dispersion curve seen in Fig. 4
can be attributed to the anisotropic optical properties of therubbed 6T film. In an anisotropic material, the dielectric constant is strongly dependent on the direction. The presentrubbed 6T film is thought to have three principle values of adielectric tensor ␧储, ␧⬜, and ␧z, corresponding to the directions parallel and perpendicular to the rubbing direction, andnormal to the film surface, respectively. The relationship储⬜
kSPP ⬎ kSPP
suggests that ␧储 ⬎ ␧⬜. Recent theoretical analysesof SPP propagation at an interface between a metal and an
anisotropic dielectric medium10,11 predict a displacement of
the dispersion curves that depends on the SPP propagation
direction. Although these analyses have been carried out fora uniaxial medium and differ from our biaxial 6T thin film,
the predicted displacement agrees qualitatively with thepresent observation. Further experimental and theoreticalstudies are required to fully account for the displacement of
the dispersion curve caused by the anisotropic overlayer.
In conclusion, we prepared oriented 6T layers on Ag thinfilms and observed anisotropic SPP propagation generated by
the oriented molecular overlayer. From RATR measurements, it was found that the SPP wavevector parallel to themolecular orientation takes a larger value than that perpendicular to the molecular orientation. The results presented
here may provide useful information for the development ofplasmonic devices based on the anisotropic SPP propagation
as suggested in recent theoretical papers.10,11
This work was supported by a Grant-in Aid for Scientific
Research on the Priority Area “Strong Photon-Molecule
Coupling Field for Chemical Reactions” 共Grant No. 470兲from the Ministry of Education, Culture, Sports, Science,and Technology of Japan.
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