APPLIED PHYSICS LETTERS 91, 183110 共2007兲

Effects of molecular orientation on surface-plasmon-coupledemission patterns
Hoang Minh Hiep, Minoru Fujii, and Shinji Hayashia兲
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,
Rokko, Nada, Kobe 657-8501, Japan

共Received 10 September 2007; accepted 11 October 2007; published online 2 November 2007兲
We observed azimuthal distributions of surface-plasmon-coupled emission from thin layers of
randomly oriented and oriented sexithiophene molecules deposited on 50-nm-thick Ag films. For the
randomly oriented layer, we observed a uniform and isotropic cone of emission, showing that thesurface plasmons are excited isotropically in all directions on the Ag surface. The emission pattern
for the oriented layer exhibited a much stronger intensity along the direction of molecularorientation, showing that surface plasmons propagating parallel to the molecular orientation are
excited most effectively. These results demonstrate that surface-plasmon-coupled emission provides
a new simple approach to the determination of molecular orientation in thin films. © 2007 American
Institute of Physics. 关DOI: 10.1063/1.2804563兴
It is well known that excited fluorophores placed very
close to a metallic surface excite surface plasmon polaritons
共SPPs兲 via near-field interaction.1,2 The excited SPPs can beconverted into light by using an appropriate decoupling device such as a grating or a prism.3 Recently, the so-calledsurface-plasmon-coupled emission 共SPCE兲, which arises
from the combination of the above two processes, has attracted much interest because it offers a new method of fluorescence detection applicable to a variety of sensing devices,in particular, sensors of biomolecules.4–6 In SPCE, a thinmetal film, usually Ag or Au film, ⬃50 nm in thickness,
deposited on a glass substrate, is used and fluorophores areplaced on top of the metal surface. The sample is attached toa base of a glass prism. The fluorophores, excited by lightincident either from the air or prism side, excite SPPs andgenerate highly polarized directional emission at the prismside. The unique properties of the generated emission make
SPCE a potentially important tool for many applications.
SPCE depends on a lot of parameters, such as the thickness of the metal film, distance between fluorophores and themetal surface, orientation of fluorophores, refractive index ofprism, etc. Winter and Barnes7 studied the influence of themetal thickness using Ag films and reported that a maximum
SPCE intensity is obtained at the thickness of ⬃50 nm. Veryrecently, Ray et al.6 studied the effects of fluorophore-metal
distance for cyanine dye molecules. They reported a maximum SPCE intensity at the fluorophore-metal distance of⬃20 nm. In spite of great experimental efforts made so far,the effects of fluorophore orientation on SPCE properties are
not yet well explored. Theoretical analyses performed by
Calander8 predict that the azimuthal intensity variation of
SPCE from a dipole oriented parallel to the metal surface isanisotropic and shows a maximum SPCE intensity in the
direction of molecular orientation. To our knowledge, suchan anisotropic SPCE pattern has not yet been reported. Here,
we report on successful observation of anisotropic SPCE azimuthal patterns generated by molecules aligned along agiven direction on Ag thin films. Observed patterns are ingood agreement with the theoretical prediction.
a兲Electronic mail: hayashi@eedept.kobe-u.ac.jp

Thin Ag films 50 nm in thickness were vacuum deposited 共10−6 Torr兲 on glass substrates. 20-nm-thick films of
sexithiophene 共6T兲 were then deposited on the Ag surface
with a low deposition rate of 0.5– 1.0 nm/ min. The 6T molecule is known to have the main optical transition dipole
parallel to the long molecular axis.9 According to previous
studies of vapor deposited films of thiophene oligomers,9–11
the films are polycrystalline and show varying degrees oforder depending on preparation conditions. When a low
deposition rate is used, deposited films consist of grains, inwhich the long molecular axis is oriented close to normal to
the substrate 共within ⬃25°兲. In the direction parallel to thesubstrate, the grains are randomly oriented.10 To our knowledge, no report exists on 6T films deposited on vacuumdeposited silver films. However, as confirmed later by polarized photoluminescence 共PL兲 measurements, presently
deposited 6T films have molecular arrangements similar to
those previously reported for the films prepared with a low
deposition rate. In order to obtain layers of 6T molecules
lying horizontally on the Ag surface with their long axis
oriented in a preferential direction, we used a rubbing technique similar to that used for octithiophene11 andp-sexiphenyl12 molecules. A 6T prelayer of 10 nm was first
deposited on the silver surface. The 6T prelayer was thenrubbed in a given direction by using a wiping paper. To complete the sample, an additional 6T layer 10 nm in thickness
was deposited.
The polarized PL spectra of the nonrubbed and rubbed
6T samples were examined to confirm the 6T molecular orientation. The measurement configuration is shown in Fig.
1共a兲. Polarized excitation light with wavelength of 408 nm
from a laser diode was incident normal to the sample surface.
The emitted light was collected at an angle of 45° from thenormal by a camera lens and passed through a polarizer. The
polarized light was then focused onto the entrance slit of a
25 cm single monochromator equipped with a chargecoupled device. A polarization scrambler was inserted in
front of the entrance slit. Polarized emission intensities IHH
and IVV are defined as follows: IHH 共IVV兲 is the intensity of
the horizontally 共vertically兲 polarized emission produced bya horizontally 共vertically兲 polarized excitation light.

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91, 183110-1© 2007 American Institute of Physics
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Hiep, Fujii, and Hayashi

Appl. Phys. Lett. 91, 183110 共2007兲

FIG. 1. 共Color online兲 共a兲 Polarized PL measurement scheme. 共b兲 Reversed
ATR measurement scheme.

Figures 2共a兲 and 2共b兲 show PL spectra of the nonrubbed
and rubbed 6T films, respectively. In Fig. 2共a兲, we see thatboth the IHH and IVV spectra exhibit similar profiles with two
peaks at ⬃550 nm 共2.25 eV兲 and ⬃590 nm 共2.1 eV兲, and ashoulder at 645 nm 共1.92 eV兲. The PL band positions are inagreement with those previously reported for 6T films deposited by thermal evaporations.13,14 We see that the PL intensityof the nonrubbed sample is independent of the polarization
direction. This suggests that in the nonrubbed sample, thecomponents of 6T transition dipole moment parallel to thesample surface are randomly oriented. The PL intensity of
the nonrubbed sample is very weak compared to that of the
IHH spectrum of rubbed sample shown in Fig. 2共b兲. This is anindication that the transition dipole of the 6T molecule in thenonrubbed sample is oriented almost perpendicularly to thesample surface and, consequently, is not efficiently coupledwith the electric field of the exciting laser.
In contrast to the nonrubbed sample, the PL intensity of
the rubbed sample shows a strong dependence on the polarization direction, as presented in Fig. 2共b兲. The results presented in this figure were obtained by setting the rubbing
direction to be horizontal. In the figure, IVV spectrum is dis-

FIG. 3. 共Color online兲 共a兲 SPCE pattern from randomly oriented 6T thinfilm. 共b兲 SPCE pattern from oriented 6T thin film. The rubbing direction was
horizontal. 共c兲 Same as 共b兲 but the sample 45°rotated counter clockwise. The
arrows in 共b兲 and 共c兲 indicate the rubbing direction.

played after multiplying the raw intensity by a factor of 5.
The dichroic ratio IHH / IVV calculated directly from the maximum peak heights is about 45. When the rubbing direction of
the sample is rotated around the normal axis from the horizontal direction, the intensity of IHH 共IVV兲 spectrum wasfound to decrease 共increase兲 and take a minimum 共maximum兲 when the rubbing direction was vertical. As in thecases of analogous molecules,12,15 these results allow us toconclude that the long axis of 6T molecules is preferentiallyaligned parallel to the rubbing direction on a macroscopic
scale. The maximum PL intensity much higher than that of
the nonrubbed sample results from much stronger couplingwith the incident electric field.
We then observed SPCE patterns from the nonrubbed
and rubbed 6T films on Ag films to find out the effects ofmolecular orientation. The samples were attached to a hemisphere prism made of BK7 glass with index-matching fluid.
A reversed attenuated total reflection 共ATR兲 configuration
was used, as shown in Fig. 1共b兲. For excitation, we used the457.9 nm line of an Ar+-ion laser. The laser beam with thehorizontal polarization was incident normal to the sample
surface from the air side. We examined the azimuthal distribution of SPCE produced at the prism side by projecting it
onto a screen and taking its photograph. Scattered excitation
light at 457.9 nm was eliminated with a long-pass filter witha cutoff wavelength of 530 nm.
Figure 3共a兲 shows a photograph of a SPCE pattern fromthe nonrubbed 6T film. The pattern observed is the projectionof a cone of light emission that is symmetric around the

FIG. 2. 共Color online兲 共a兲 Polarized PL spectra for randomly oriented 6T
thin film. 共b兲 Polarized PL spectra for oriented 6T thin film.
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183110-3

Appl. Phys. Lett. 91, 183110 共2007兲

Hiep, Fujii, and Hayashi

normal axis. We note here that the emitted light is p polarized and the emission angle agrees well with the angle of
ATR dip observed only for p-polarized incident light for thesame sample. These facts confirm that the observed emission
arises from the excitation and decoupling of SPPs, i.e.,
SPCE. The symmetric pattern of the emission in Fig. 3共a兲
implies that SPPs are excited uniformly in all directions on
the Ag surface by the nonrubbed 6T film. Figures 3共b兲 and3共c兲 show the SPCE patterns of the rubbed 6T sample. We
observe an anisotropic emission pattern in which the lightintensity is the strongest along the molecular orientation andbecomes weaker as the azimuthal angle deviates from themolecular orientation. The anisotropic patterns in Figs. 3共b兲and 3共c兲 imply that SPPs propagating along the molecularorientation are excited most efficiently. We note here that inaccordance with the results of the polarized PL measurements, the intensity of the SPCE is considerably lower forthe nonrubbed sample. This is the consequence of the weakcoupling of the molecular dipoles with incident light. The
results presented in Fig. 3 demonstrate that the SPCE patternis directly linked to the orientation of the molecules.
The effects of molecular orientation on the SPCE pattern
can be qualitatively explained by considering the efficiencyof SPP excitation, which is determined by the magnitude ofenergy flux associated with transverse magnetic 共TM兲 electromagnetic fields generated by a fluorescent molecule. In
theoretical considerations, it is very common to assume an
oscillating point dipole embedded in a dielectric layer coating a metallic surface.1,2,8 To calculate the energy flux, wefollow the formulation of Enderlein16 and use his notations.
As given by Eq. 共6兲 of Ref. 16, the dipole field is represented
by a sum of plane waves propagating in various directions;each plane wave is characterized by a different in-plane
wavevector q. When the magnitude of the in-plane wavevector q exceeds that of the wavevector in the dielectric layer
kW, the component of the wavevector normal to the interface
lW becomes imaginary and, consequently, the corresponding
dipole fields are evanescent. Although the dipole fields contain p and s waves as explicitly expressed in Eq. 共6兲 of Ref.
16, only the p polarized or TM evanescent waves can excite
SPPs at the metal-dielectric interface. It follows that SPPs
are excited more efficiently when the intensities of energy
flux associated with the TM evanescent waves are higher,
resulting in stronger SPCE intensities. The dependence of theenergy flux on the direction of q determines the azimuthal
angular pattern of SPCE.
Using the expression of the electric field of p waves in
Eq. 共6兲 of Ref. 16, we can derive an expression for the corresponding magnetic fields and finally obtain the flux intensity 共the modulus of the Poynting vector兲 by a standard manner. The metal-dielectric interface is taken to be the x, yplane, z axis being normal to the interface. A dipole p making an angle ␤ with the z axis is located very close to theinterface and assumed to lie on the x, z plane. We can write
as p = 共p sin ␤ , 0 , p cos ␤兲, where p is the magnitude. For a
TM evanescent wave propagating with an in-plane wavevector q specified by an azimuthal angle ␾ relative to the x
direction, the flux intensity is given by
STM共␤, ␾兲 = S0 p2关u2 cos2 ␤ sin2 ␾ + 共u2 − sin2 ␤兲cos2 ␾兴, 共1兲
where S0 is a constant and u is the magnitude of in-plane
wavevector q normalized to n共␻ / c兲, with the refractive index

of the dielectric layer n, angular frequency ␻, and speed oflight in vacuum c.
For a dipole oriented parallel to the surface 共␤ = ␲ / 2兲,Eq. 共1兲 suggests that STM is proportional to cos2 ␾. This implies that the energy flux takes a maximum value for theevanescent wave propagating along the same direction as the
dipole orientation 共␾ = 0兲 and decreases as ␾ increases. Thisbehavior of STM explains fairly well the experimentally observed behavior of the SPCE patterns of the rubbed sample
shown in Fig. 3. For a dipole oriented perpendicular to thesurface 共␤ = 0兲, Eq. 共1兲 becomes independent of ␾, predicting
an isotropic SPCE pattern. By taking into account this fact,
an isotropic SPCE pattern obtained for the nonrubbed sample
关Fig. 3共a兲兴 can be well explained by the almost perpendicularorientation of the 6T molecular axis and the in-plane randomorientation of the grains. Although perpendicular dipoles are
expected to couple with SPPs more efficiently than parallel
dipoles,1,2 in the present nonrubbed samples, the influence ofthe small coupling with incident light is much larger, resulting in weak overall SPCE intensities. The above qualitative
considerations of SPCE azimuthal patterns are consistent
with results of numerical calculation reported by Calander.8
In conclusion, we prepared thin films consisting of randomly oriented and oriented 6T molecules on top of Ag films
and observed SPCE patterns. In contrast to a uniform isotropic cone of emission from the randomly oriented 6T layer,
the SPCE from oriented 6T layer exhibited an isotropic pattern with the strongest intensity at the azimuthal angle atwhich 6T molecules were oriented. This behavior of the
SPCE pattern can be well explained by considering the energy flux associated with TM evanescent fields generated bya point dipole. The results presented here demonstrate that
the SPCE pattern provides a new versatile analytical tool todetermine the molecular orientation with a very simple andlow cost instrumentation.
We thank Professor Hisao Yanagi for many helpful discussions about preparation of molecular layers.
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