Download Isolated hexaphenyl nanofibers as optical waveguides

APPLIED PHYSICS LETTERS
VOLUME 82, NUMBER 1
6 JANUARY 2003
Isolated hexaphenyl nanofibers as optical waveguides
F. Balzer,a) V. G. Bordo,b) A. C. Simonsen, and H.-G. Rubahnc)
Fysisk Institut, Odense Universitet, DK-5230 Odense M, Denmark
共Received 1 July 2002; accepted 5 November 2002兲
Laser-supported, dipole-assisted self-assembly results in blue-light guiding nanostructures, namely
single-crystalline nanofibers of hexaphenyl molecules. The nanofibers are up to 1 mm long,
extremely well-aligned to each other and their cross sections can be tuned to span the range from
nonguiding to guiding single optical modes at ␭⫽425.5 nm. An analytical theory for such organic
waveguides can reproduce quantitatively the experimentally observed behavior. From the measured
damping of propagating, vibrationally dressed excitons the imaginary part of the dielectric function
of isolated nanoscaled organic aggregates is determined. © 2003 American Institute of Physics.
关DOI: 10.1063/1.1533845兴
The ongoing rapid microminiaturization of optoelectronics has led to an increased need for the generation, characterization and interconnection of optoelectronic elements
with characteristic dimensions in the submicrometer length
scale regime. For this, organic materials with delocalized ␲
electrons, due to their easy optical tunability, their strong
luminescence efficiency and their preference for natural selfassembly, can serve as very effective flexible media to generate or propagate light in a predefined way. An important
building block for such organic optoelectronics is a dielectric
waveguiding element, which especially should allow
waveguiding not only in the infrared regime, but also in the
visible and blue spectral regimes. A lithographic, albeit complex, approach for obtaining these elements is the generation
of photonic band gap structures in inorganic dielectric
materials.1,2 Recently, waveguiding in the blue spectral regime has also been reported in vapor phase grown zinc oxide
nanowires.3
Here, we present a controlled self-assembly of
waveguiding aggregates 共nanofibers兲 of para-hexaphenyl
( p-6P) molecules. The fibers are grown via vacuum sublimation onto a freshly cleaved mica substrate, which is locally heated by a focused argon ion laser beam. Following
cleavage, the mica surface exhibits strong parallel surface
dipoles, which are oriented in large domains rotated 120°
with respect to each other. The surface dipoles induce a dipole moment in the organic molecules. The resulting interaction produces self-assembly of the molecules into large
areas of very long, parallel oriented fibers.4 As deduced from
low energy electron diffraction measurements, the individual
molecules are oriented at an angle of 76° with respect to the
long axis of the fibers.
It should be emphasized that the fibers are subjected to
straightforward manipulation via laser irradiation: A change
in the orientation of the surface dipoles via pulsed UV laser
irradiation5 facilitates manipulation of the direction of the
fibers, whereas a local change in surface temperature by the
a兲
Permanent address: Humboldt-Universität zu Berlin, Institut für Physik,
D-10115 Berlin, Germany.
b兲
Permanent address: Institute of General Physics, Russian Academy of Sciences, 119991 Moscow, Russia.
c兲
Electronic mail: [email protected]
heating laser induces local growth of the needle-shaped
aggregates.6 A combination of appropriate laser irradiation,
deposition rates and times then allows us to modify characteristic dimensions 共width, height, length兲 as well as the relative distance between the fibers. Growing them across dipole
domain boundaries even allows the fabrication of bent fibers.
Figure 1 shows fluorescence and atomic force microscope 共AFM兲 images of selected fibers, that demonstrate that
the appropriate combination of self-assembly and laserinduced growth manipulation allows one to select isolated
fibers 关Fig. 1共a兲兴, arrays of monodisperse, widely separated
aggregates 关Fig. 1共b兲兴 or dense arrays of parallel oriented
nanofibers 关Fig. 1共c兲兴. The heights and widths of the fibers
are rather uniform with typical dimensions of 350 nm width
and 100 nm height.
Due to a difference in the indices of refraction of the
underlying substrate ( 冑⑀ s ⫽1.58) and the nanofiber ( 冑⑀ iso
⫽1.7) 7 the fiber acts as a waveguide if a critical width is
overcome. In the absence of scattering from surface irregularities and for wavelengths large enough that absorption can
be neglected 共for para-hexaphenyl absorption goes to zero at
FIG. 1. Fluorescence images of two long, isolated nanofibers: 共a兲 322
⫻322 ␮ m2 , a bunch of nearly monodisperse short fibers; 共b兲 115
⫻115 ␮ m2 and densely packed long fibers; 共c兲 0.5⫻0.5 mm2 . 共d兲 Force
microscopy of the isolated fiber on the left-hand side of 共a兲 (21
⫻21 ␮ m2 ). The outcoupling region is shown in high resolution (1.6
⫻1.6 ␮ m2 ) in the inset.
0003-6951/2003/82(1)/10/3/$20.00
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© 2003 American Institute of Physics
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Balzer et al.
Appl. Phys. Lett., Vol. 82, No. 1, 6 January 2003
11
extraordinary length, straightness and polarization-dependent
reflectivity of the fibers.
Quantitative understanding and improved control of the
waveguiding properties require an analytical theory for optical waveguiding in nanometer-scaled aggregates. Motivated
by the force microscopy images, we assume the nanofiber to
have a rectangular cross section and to be an optically
uniaxial medium with the dielectric tensor component ⑀ 储
along the large molecular axis. The other two components
are equal to ⑀⬜ . In local approximation electromagnetic
waves can propagate in such a waveguide only as transverse
magnetic 共TM兲 modes.9 The cutoff wavelength for the
guided modes is
FIG. 2. Local spectrum of luminescence intensity that is waveguided
through a nanoscaled fiber, obtained at the tip of the fiber, 100 ␮m from a
break 共black dots兲. For comparison, a spectrum of a homogeneous p-6P film
is also shown 共gray dots, from Ref. 8兲. The solid lines are Gaussian fits to
the measured curves. The triangles denote the positions of the 0-1, 0-2 and
0-3 vibronic bands.
wavelengths larger than 500 nm兲, propagation losses in the
fiber are very small. Here, we investigate the waveguiding
behavior for a wavelength that is close to the lower limit for
low-loss guiding, namely, ␭⫽425.5 nm. At this wavelength
some losses are expected due to reabsorption of light.
Inside a fluorescence microscope 365 nm UV light is
focused onto a selected fiber 共focal radius 15 ␮m兲. The UV
light transfers population into the S 1 electronically excited
state, from which it relaxes predominantly into the first vibrationally excited level of the electronic ground state S 0 . A
typical spectrum of the resulting luminescence at a transition
wavelength of 425.5 nm measured at the tip of an individual
fiber is shown in Fig. 2. Due to the homogeneity of the
single-crystalline fiber the spectral linewidth is significantly
less as compared to the linewidths in the spectrum of a polycrystalline thin film.8 Note also that relaxation into the higher
vibronic levels 共0-2 and 0-3兲 is suppressed in the fiber.
Following initial excitation, the 425.5 nm light propagates via transfer of excitation between neighboring p-6P
molecules, i.e., it acts as a propagating molecular exciton. In
order to study the propagation losses we have deliberately
induced breaks in the fiber by thermal stress in the course of
the cooling process following growth of the aggregates. The
widths of the breaks are less than 150 nm which is much
narrower than the wavelength of the propagating light. The
breaks do not affect the straightness of the nanometric waveguide. Breaks are visible in the fluorescence microscope images as bright spots and are characterized morphologically
by AFM 关Fig. 1共d兲兴. Both the luminescence intensity induced
at the point of excitation and the luminescence outcoupled at
the position of a break are determined quantitatively with the
help of a charge coupled device 共CCD兲 camera. The distance
between the excitation and outcoupling points is varied by a
movable focusing lens and by moving the sample with micrometer precision. We point out that optical measurements
on selected individual nanofibers have been correlated with
tapping-mode AFM images obtained on the same fibers. The
correct positioning for AFM was achieved via optical microscopy of the sample and the cantilever through a transparent sample holder. This procedure became possible due to the
␭ c⫽
2 冑⑀⬜ a
,
m
共1兲
and the number of possible modes, m⫽1,2,3,..., is restricted
by the condition
m⬍
2a
␭
冑⑀⑀ 冑⑀
⬜
储
储
⫺ ⑀ s,
共2兲
with ␭ the wavelength, ⑀ s the dielectric constant of the substrate and a the fiber width.
In the present case we have ⑀ 储 / ⑀⬜ ⫽2.5. 10 From the
value of the refractive index for an isotropic p-6P film for
␭⫽425 nm, 7 one finds ⑀⬜ ⫽1.9 and ⑀ 储 ⫽4.8. With the spectroscopically determined propagation wavelength ␭
⫽425.5 nm we obtain a 1 ⫽222 nm for the minimum fiber
width for which at least one mode can propagate. The second
mode appears for widths larger than a 2 ⫽444 nm. This finding agrees quantitatively with experimental results that show
waveguiding for fibers with 400 nm diameter, but not for
fibers with diameters smaller than 222 nm.9 The cutoff wavelengths for nanofibers of different widths range from ␭ c
⫽1103 nm for a⫽400 nm to ␭ c ⫽689 nm for a⫽250 nm.
Since there is no coupling to the underlying substrate
and since the fluorescence microscope images suggest that
radiative losses can be neglected, damping of the intensity of
the propagating light is dominated by the imaginary part of
the waveguides’ dielectric tensor. The reabsorption of light
by 6P molecules takes place if the electric field vector of the
light is directed along the molecular axes. Hence only the
component ⑀ ⬙储 has to be considered. Its value can be determined from a comparison of measured and calculated losses
of luminescence intensity as a function of the distance along
the fiber axis.
Figure 3 shows measured distance dependencies of outcoupled luminescence intensity for a fiber on mica 共open
symbols兲 and on NaCl 共closed circles兲, respectively. The
measured characteristic widths are 400 nm 共fiber on mica兲
and 300 nm 共fiber on NaCl兲. From a variety of measurements
on selected individual fibers we obtain a value of ⑀ ⬙储
⫽0.012⫾0.002 for the imaginary part of the dielectric function of nanoscaled fibers. From density functional
calculations11 for the hexaphenyl bulk and from the ratio of
the spectral linewidths of the fiber and the crystal, provided
by the fits in Fig. 2, we estimate9 for a 6P fiber ⑀ ⬙储
⬃0.0119, in very good agreement with the value determined
from the attenuation curves.
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12
Balzer et al.
Appl. Phys. Lett., Vol. 82, No. 1, 6 January 2003
the fibers. Recent local spectroscopy at isolated nanofibers
has shown that a change in the morphology can also lead to
a change of the wavelength of the propagating light: this
allows one to tune the light in the nanofibers from one Raman band to another. In addition, we are able to generate
either isolated entities at well localized spots on the surface
or dense arrays of mutually parallel oriented waveguides.
The well-defined orientations of individual molecules within
the fibers give rise to strongly dichroic light emission and
absorption. Outcoupling of light at breaks of the waveguides
is very simple, whereas incoupling seems to be more difficult. However, since the nanofibers are easily grown on various substrates they might also be grown on micron-scaled
structures that facilitate the incoupling of light. Also, there is
no fundamental reason why the method should be limited to
conjugated organic molecules which often suffer from
impurity-induced absorption in the band gap.
FIG. 3. Outcoupled luminescence intensity as a function of distance from
the excitation point of an individual, 400 nm wide fiber on mica 共open
symbols兲 and for a 300 nm wide fiber on NaCl 共closed circles兲. The dotted
line represents the spatial excitation profile. Theoretical predictions are
given by dashed and continuous solid lines. In the upper part of the picture
we show fluorescence images along the fiber on mica for two distances
between the excitation and outcoupling region.
In summary, we have presented fully morphologically
characterized nanoscaled dielectric waveguides that allow
single mode waveguiding of blue light. A classical electromagnetic theory is able to describe the waveguiding properties quantitatively, which makes an evaluation of the imaginary part of the dielectric function of a nanoscaled fiber
aggregate possible. The length and orientation of the waveguide can be manipulated via a change of laser irradiation
and growth conditions.
Among the advantages of our approach is good control
of the waveguiding properties by tuning the dimensions of
The authors thank the Danish Research Agency SNF for
funding this work. One of the authors 共A.C.S.兲 is grateful to
the Danish National Research Foundation for support by a
grant to the MEMPHYS Center for Biomembrane Physcis.
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