Recent research on hybrid plasmonic systems has shown the existence of a loss channel for energy transfer between
organic materials and plasmonic/metallic structured substrates. This work focuses on the exciton-plasmon coupling
between para-Hexaphenylene (p-6P) organic nanofibers (ONFs) and surface plasmon polaritons
(SPPs) in organic/dielectric/metal systems. We have transferred the organic p-6P nanofibers onto a thin silver film
covered with a dielectric (silicon dioxide) spacer layer with varying thicknesses. Coupling is investigated by two-photon
fluorescence-lifetime imaging microscopy (FLIM) and leakage radiation spectroscopy (LRS). Two-photon excitation
allows us to excite the ONFs with near-infrared light and simultaneously avoids direct SPP excitation on the metal layer.
We observe a strong dependence of fluorescence lifetime on the type of underlying substrate and on the morphology of
the fibers. The experimental findings are complemented via finite-difference time-domain (FDTD) modeling. The
presented results lead to a better understanding and control of hybrid-mode systems, which are crucial elements in future
low-loss energy transfer devices.
In this work, enhancement of the second harmonic response of organic nanofibers deposited on encapsulated and robust
plasmonic active substrate is experimentally demonstrated. Organic nanofibers grown from functionalized paraquaterphenylene
(CNHP4) molecules have been transferred on lithographically defined regular arrays of gold
nanostructures, which subsequently have been coated with thin films of diamond-like carbon with 25, 55 and 100 nm
thickness. Femtosecond laser scanning microscopy enables us to identify enhancement of the second harmonic response
of the fibers. This is facilitated by a preservation of the field enhancement effects, which appear on the nanostructures
and remain significant on top of the coating layer.
A promising method for improving light-absorption in thin-film devices is demonstrated via electrode structuring
using Anodic Alumina Oxide (AAO) templates. We present nano-scale concave Al structures of controlled dimensions,
formed after anodic oxidation of evaporated high purity aluminum (Al) films and alumina etching. We investigate both
experimentally and theoretically the field-enhancement supported by these concave nanostructures as a function of their
dimensions. For the experimental investigations, a thin layer of organic polymer coating allows the application of a nondestructive
laser ablation technique that reveals field-enhancement at the ridges of the Al nanostructures. The
experimental results are complemented by finite-difference time-domain (FDTD) simulations, to support and explain the
outcome of the laser ablation experiments. Our method is easily up-scalable and lithography-free and allows one to
generate nanostructured electrodes that potentially support field-enhancement in organic thin-film devices, e.g., for the
use in future energy harvesting applications.
Leakage radiation spectroscopy of organic para-Hexaphenylene (p-6P) molecules has been performed in the spectral
range 420-675 nm which overlaps with the p-6P photoluminescence band. The p-6P was deposited on 40 nm silver (Ag)
films on BK7 glass, covered with SiO2 layers. The SiO2 layer thickness was varied in the range 5-30 nm. Domains of
mutually parallelly oriented organic nanofibers were initially grown under high-vacuum conditions by molecular beam
epitaxy onto a cleaved muscovite mica substrate and afterwards transferred onto the sample by a soft transfer technique.
The sample placed on a flat side of a hemisphere fused silica prism with an index matching liquid was illuminated under
normal incidence by a He-Cd 325 nm laser. Two orthogonal linear polarizations were used both parallel and
perpendicular to the detection plane. Spectrally resolved leakage radiation was observed on the opposite side of the Ag
film (i.e. at the hemisphere prism) as a function of the scattering angle. Each spectrum contains a distinct peak at a
wavelength dependent angle above the critical angle. This way the dispersion curve was measured, originating from a
hybrid mode, i.e. the interaction between the p-6P excitons and surface plasmon polaritons (SPPs) of the metal/dielectric
boundary. The presence of the SiO2 layer considerably changes the dispersion curve in comparison to the one of the
Ag/p-6P/air system. However, the Ag/SiO2/p-6P/air stack forms a stable structure allowing construction of organic
plasmonic devices such as nano-lasers.
Second harmonic generation in nonlinearly optically active organic nanofibers, generated via self-assembled surface
growth from nonsymmetrically functionalized para-quarterphenylene (CNHP4) molecules, has been investigated. After
the growth on mica templates, nanofibers have been transferred onto lithographically defined regular arrays of metal and
dielectric nanostructures. Such hybrid systems were employed to correlate the second harmonic response to both
morphology of the fibers i.e. local field enhancement due to local changes in the fiber’s morphology and field
enhancement effects appearing on the nanostructures. With the help of femtosecond laser scanning microscopy two-dimensional
second-harmonic images of individual nanoaggregates were obtained and analyzed.
Leakage radiation spectroscopy of organic nanofibers composed of self-assembled organic molecules (para-Hexaphenylene,
p-6P) deposited on a thin (40-60 nm) Ag film has been performed in the spectral range 420-675
nm which overlaps with the nanofiber photoluminescence band. Using a soft transfer technqiue, domains of
mutually parallel oriented organic nanofibers were initially grown under high-vacuum conditions by molecularbeam
epitaxy onto a cleaved muscovite mica substrate and afterwards transferred onto a silver film prepared on
the glass carrier. The sample placed on a flat side of a hemisphere prism with an index matching liquid was
illuminated by either a He-Cd 325 nm laser or by white light from a bulb. In the case of laser excitation two
orthogonal linear polarizations and two different configurations of p-6P nanofibers were applied, both parallel and
perpendicular to the plane of detection. The leakage radiation was observed on the opposite side of the Ag film
at the phase matching angle. The spectrally resolved intensity of the scattered radiation has been measured as a
function of scattering angle at normally incident light. The spectrum contains a distinct peak at an wavelength
dependent angle above the critical angle. By analyzing this dispersion curve one can argue that it originates
from the interaction between the nanofiber excitons and surface plasmon polaritons of the metal film.
The optical near-field of metal films can be modified in a straightforward manner by incorporating nanostructures on the surface. The corresponding field enhancement, which may be due to the lightning rod effect as well as the excitation of plasmon modes, results in a local change of the optical surface response. A transparent thin film on top of the nanostructures can be partially ablated via illumination with near-infrared light. Local variations of the ablation rate due to field enhancement are readily mapped with subdiffractional resolution, as confirmed by a direct comparison to theoretical calculations. Variation of the thickness of the transparent film enables discrimination between localized enhancements at the sharp corners of the structures and collective enhancements at locations between the structures due to surface plasmon polariton modes. In addition, applying the same method to study the effect of nanostructure morphology on localized second-harmonic generation using arrays of rectangular as well as triangular structures, we observed a second-harmonic (SH) signal from both centrosymmetric and noncentrosymmetric nanostructure arrays, indicating that the SH excitation is not due to a collective phenomenon but originates locally from the individual structures.
Second-harmonic generation upon femto-second laser irradiation of nonlinearly optically active nanofibers grown from
nonsymmetrically functionalized para-quarterphenylene (CNHP4) molecules is investigated. Following growth on mica
templates, the nanofibers have been transferred onto lithography-defined regular arrays of gold square nanostructures.
These nanostructure arrays induce local field enhancement, which significantly lowers the threshold for second harmonic
generation in the nanofibers.
Surface plasmon polariton (SPP) excitation at a gold-vacuum interface via 800 nm light pulses mediated by a periodic
array of gold ridges is probed at high lateral resolution by means of photoemission electron microscopy (PEEM). We
directly monitor and quantify the coupling properties as a function of the number of grating ridges and compare the
PEEM results with analytic calculations. An increase in the coupling efficiency of ≈ 3 is observed when increasing the
number of ridges from 1 to 6. We observe, however, that a further addition of ridges is rather ineffective. This saturation
behavior is assigned to the grazing incidence excitation geometry intrinsic to a conventional PEEM scheme and the
limited propagation distance of the SPP modes at the gold-vacuum interface at the used wavelength.
KEYWORDS: Gold, Nanostructures, Near field, Laser ablation, Polymers, Polymethylmethacrylate, Near field optics, Scanning electron microscopy, Metals, Polarization
The optical near-field of lithography-defined gold nanostructures, arranged into regular arrays on a gold film, is
characterized via ablation of a polymer coating by laser illumination. The method utilizes femto-second laser pulses from
a laser scanning microscope which induces electrical field enhancements on and around the gold nanostructures. At the
positions of the enhancements, the ablation threshold of the polymer coating is significantly lowered creating subdiffractional
topographic modifications on the surface which are quantified via scanning electron microscopy and atomic force microscopy. The obtained experimental results for different polymer coating thicknesses and nanostructure geometries are in good agreement with theoretical calculations of the near field distribution for corresponding enhancement mechanisms. The developed method and its tunable experimental parameters show that the different stages
in the ablation process can be controlled and characterized making the technique suitable for characterizing optical near-fields
of metal nanostructures.
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