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World Journal Of Engineering
ENHANCEMENT OF OPTICAL BIREFRINGENCE IN ANISOTROPIC
NANOCOMPOSITES MEDIATED BY LOCALIZED SURFACE PLASMON
RESONANCE
Shunsuke Murai, Takuya Tsujiguchi, Koji Fujita, and Katsuhisa Tanaka
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura Nishikyo-ku,
Kyoto 6158510, Japan
magneto-optical effect evaluation system (JASCO
K-250) was utilized at zero magnetic field for the
measurement. Linearly polarized light with an electric
field oscillating along the vertical axis was normally
incident on the sample surface and the polarization
rotation angle  of transmitted light was detected by
using a polarization modulation technique.
Introduction
Localized surface plasmon resonance (LSPR) of a
metal nanoparticle is sensitive to its environment, and
absorption and scattering properties change drastically
around the resonance frequency.
When the
nanoparticle is elongated in one direction, the resonance
frequency of LSPR varies with the polarization direction
of the incident light, which induces the variation in real
part of refractive index (i.e., optical birefringence) via
Kramers-Kronig relation [1]. The same phenomenon
can be induced when an isotropic metal sphere is
embedded in an anisotropic matrix [2]. Here we have
fabricated anisotropic nanostructures consisting of
isotropic metallic core and anisotropic shell, and observed
the enhancement of birefringence at the frequency of
LSPR. The composite shows the birefringence because of
Results and Discussion
Figure 1 shows a FE-SEM image of the sample. By
obliquely depositing an iron oxide layer (oblique angle =
60°), we obtained an elongated shell on the top of Ag
nanoparticles. XRD pattern confirms that the iron
oxide layer is amorphous. Optical birefringence was
evaluated by use of the polarization rotation of linearly
polarized light passing through the sample (Fig. 2).
The rotation was very small for the heat-treated Ag film
before the oblique deposition, while the sample shows a
notable optical rotation after the oblique deposition of
iron oxide. It is also noticed that the optical rotation is
wavelength-dependent ant it is largely enhanced around
the wavelength of LSPR.
the anisotropy in shell deposited obliquely. The difference
in refractive indices along two orthogonal principle axes
(n) shows the maximum at around the wavelength of
LSPR, and the value of n reaches to as large as 0.34.
Experimental
The core-shell structure was fabricated by obliquely
depositing an amorphous oxide on the top of Ag
nanoparticles that assembled beforehand on the substrate.
First, an Ag thin film was grown on SiO2 glass substrate
and was heat-treated to convert the Ag film into Ag
nanoparticles. Then, an amorphous iron oxide layer
was grown obliquely on the top of nanoparticles by using
a pulsed laser deposition.
The optical extinction spectra were measured with a
spectrophotometer (JASCO V-570). Field emission
scanning electron microscopy (FE-SEM; JEOL
JSM-6700F) was utilized to evaluate the nanostructure of
the samples. X-ray diffraction (XRD) measurement
was performed to identify crystalline phases or to
confirm that the sample was amorphous.
Fig. 1. A FE-SEM image of the sample. The arrow indicates the
direction of elongation of iron oxide. Inset shows a schematic
illustration of the cross section of the sample.
Optical birefringence was determined from the
polarization rotation of linearly polarized light. A
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World Journal Of Engineering
It is also shown that the optical rotation  is changed
with a variation of sample azimuth . Figure 2(b)
illustrates the variation of  at  = 650 nm as a function
of . The variation of  with  can be expressed by
the relation
  A sin{ 2(  B)} ,
(1)
where A and B are fitting parameters. The results of
fitting are superimposed on the data as a solid curve.
The agreement between experimental data and calculated
curve is reasonably good. Based on the fact that the
period of oscillation of  is 180˚, it is suggested that the
phenomenon comes from the birefringence.  becomes
zero at every 90°, where the polarization direction of
incident light is parallel to the one of the principle axes
of the film. The value of A corresponds to a maximum
angle of rotation, and is related to the value of n by 2A
= 2nd/ (d: sample thickness). The n reaches to as
large as 0.34 for a 650 nm light, which is larger than
twice of calcite, a typical birefringence crystal (0.14 for
visible light).
In the present sample, the form birefringence appears
because of the anisotropic structure of shells; the
refractive index for the light polarized parallel to the
elongation direction of ellipsoid is different from that for
the light polarized perpendicularly.
This causes
polarization-dependent response of LSPR, which induces
a difference in real part of refractive index through the
Kramers-Kronig relation.
Fig. 2. (a) Wavelength-dependence of the rotation angle of the
polarization plane of linearly polarized light, , for the sample
(rectangles) and that for the heat treated Ag film before the
deposition of an iron oxide layer (circles). (b) Dependence of 
at a wavelength of 650 nm on the sample azimuth . Solid curve
was drawn by fitting eq. (1) to the experimental data.
Summary
We have prepared the nanostructured materials
consisting of an isotropic core and an anisotropic shell
by using an oblique deposition of iron oxide on the top
of Ag nanoparticles, and observed the enhanced optical
birefringence at around the wavelength of LSPR.
Birefringence is induced via the anisotropic dielectric
shell and is enhanced by the LSPR of Ag nanoparticles.
We believe that the combination of an isotropic metallic
core with an elliptical shell is a good alternative to the
oriented arrays of metallic ellipsoids as form
birefringence materials showing a wavelength-selectivity,
taking into account the simple and robust fabrication
process.
References
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Sung, J., Sukharev, M., Hicks, E. M., Van Duyne,
R. P., Seideman, T., and Spears, K. G.,
Nanoparticle Spectroscopy: Birefringence in
Two-Dimensional Arrays of L-Shaped Silver
Nanoparticles, J. Phys. Chem. C 112 ,3252 (2008).
2 Murai, S., Hattori, R., Fujita, K., and Tanaka, K.,
Optical Birefringence in Tellurite Glass Containing
Silver Nanoparticles Precipitated through Thermal
Process, Appl. Phys. Express 2, 102001 (2009).