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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 10 © 2003 American Institute of Physics Downloaded 26 Nov 2008 to 130.225.54.2. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp 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. Downloaded 26 Nov 2008 to 130.225.54.2. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp 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. 1 J. Foresi, P. Villeneuve, J. Ferrera, E. Thoen, G. Steinmeyer, S. Fan, J. Joannopoulos, L. Kimerling, H. Smith, and E. Ippen, Nature 共London兲 390, 143 共1997兲. 2 M. Notomi, K. Yamada, A. Shinya, J. Takahashi, C. Takahashi, and I. Yokohama, Phys. Rev. Lett. 87, 253902 共2001兲. 3 J. Johnson, H. Yan, R. Schaller, L. Haber, R. 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