Christian Maibohm

A 3D view on free-floating, space-fixed and surface-bound para-phenylene nanofibres

Nanofibres from para-hexaphenylene and functionalized quaterphenylene molecules are grown on mica surfaces and are thereafter transferred into solution, where they either freely rotate in water or are space-fixed in sucrose. From freely rotating aggregates highly anisotropic angular intensity distributions of emitted light for individual aggregates are determined. Luminescence is enhanced at the nanofibre tip as compared to the broad side by about an order of magnitude probably due to waveguiding along the long axis of the aggregates. For dense arrays of nanofibres on mica the increase of emitted intensity towards the substrate plane in the direction of the long axes of the nanofibres is smaller and it depends on the effective thickness of the nanofibre films. The difference between individual aggregates and aggregate arrays is interpreted in terms of light scattering at surface roughness inside the nanofibre film and on the border of the underlying mica substrate. Aggregates fixed in solution, with the help of femtosecond laser scanning microscopy, allow us to obtain two-photon absorption spectra of functionalized nanofibres between 720 and 900 nm as well as morphological features from three-dimensional optical images. The lateral resolution is about 400 nm.

Bleaching and coating of organic nanofibers

Degradation of nanofibers made from organic molecules such as para-hexaphenylene or functionalized quaterphenylene via photoexcitation or thermal irradiation is investigated by optical and morphological studies. Under ambient air conditions and in the limit of strong excitation, the degradation of luminescence intensity is accompanied by an increasing surface roughness of the aggregates and by material depletion. Whereas the luminescence intensity is decreasing exponentially with increasing illumination time, the material removal follows a linear relationship. Ablation can be stopped and bleaching can be slowed down by irradiating the nanofibers in vacuum or by coating them with a few hundred nanometers thick layer of silicon oxide (SiOx). Since the latter treatments do not completely stop the bleaching, it is concluded that bleaching of nanofibers involves at least three independent processes, namely, intramolecular configuration change, photo-oxidation, and material removal.

Laser ablation of polymer coatings allows for electromagnetic field enhancement mapping around nanostructures

Subdiffraction spatially resolved, quantitative mapping of strongly localized field intensity enhancement on gold nanostructures via laser ablation of polymer thin films is reported. Illumination using a femtosecond laser scanning microscope excites surface plasmons in the nanostructures. The accompanying field enhancement substantially lowers the ablation threshold of the polymer film and thus creates local ablation spots and corresponding topographic modifications of the polymer film. Such modifications are quantified straightforwardly via scanning electron microscopy and atomic force microscopy. Thickness variation in the polymer film enables the investigation of either the initial ablation phase or ablation induced by collective enhancement effects.

Mapping of gold nanostructure-enhanced near fields via laser scanning second-harmonic generation and ablation

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.

Periodic structures modified with silver nanoparticles for novel plasmonic application

Forming structures similar to or smaller than the optical wavelength offers a wide range of possibilities to modify the optical properties of materials. Tunable optical nanostructures can be applied as materials for surface-enhanced spectroscopy, optical filters, plasmonic devices, and sensors. In this work we present experimental results on technology and properties of periodical, polymer based optical structures modified by ordered adsorption of silver nanoparticles. These structures were formed combining UV hardening and dip coating from colloidal solutions. We have investigated the influence of silver nanoparticles assembly on the ambient conditions (deposition temperature and time) and surface features (periodicities and shape) of the template micro structures. Optical absorbance as well as morphology of the structures containing silver nanoparticles were investigated by UV-VIS spectroscopy, AFM, SEM and optical microscopy. The influence of silver nanoparticles on the optical properties of the structures was investigated by polarized light spectroscopy (Grating Light Reflection Spectroscopy - GLRS). From the results of this study we propose a low cost procedure for fabricating structures that could be potentially new type of plasmonic sensors exploiting surface enhanced plasmon resonance in silver nano structures.

Surface structure enhanced second harmonic generation in organic nanofibers

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.

Mapping of electromagnetic fields enhanced by gold nanostructures

Developments in lithography techniques have enabled the fabrication of metal structures with structural control down to the nanometer scale. The positioning of nanostructures on a metal film changes its optical surface response, resulting in unique optical properties with potential applications in diverse fields, such as surface-enhanced Raman and fluorescence spectroscopy, molecular sensing, photonic circuits, and optical waveguides. Most of these applications rely on the optical near-field distribution and its local enhancement on nanometer-length scales. As an alternative to conventional optical-scanning mapping techniques, we have developed a nondestructive and dry topographic modification method based on local laser ablation of an ‘imaging’ polymer layer deposited on the metal nanostructures.1, 2 Compared with other topographic methods,3, 4 our laser ablation technique has the advantage of avoiding any subsequent wet chemistry processing steps, which are otherwise necessary to remove the non-exposed material. Topographic modifications can be inspected immediately and recorded using high resolution imaging techniques, such as scanning electron or atomic force microscopy (SEM and AFM). In addition, the ablation threshold for the poly(methyl methacrylate) (PMMA) polymer used is relatively low compared with other ablation approaches, hence a less powerful laser system is needed. One of the most important advantages, however, is that the quasipermanent imprint obtained can be removed using a solvent, such as acetone, enabling a fully characterized sample (in terms of electromagnetic field enhancement) to be used for further applications without degrading the metallic surface. Figure 1. Scanning electron microscopy (SEM) images of surface modifications in a 80nm polymer coating obtained by (a) field-enhancement ablation and (b) nano-square and nano-triangle arrays, respectively; laser fluence 0.22J/cm2, 1000 pulses per spot, white arrow indicates polarization direction. (c) Combination of two mutually perpendicular polarizations, area illuminated twice. (d) The imprint after complete ablation of the gold nanostructure at high laser fluence of 0.5J/cm2.

Near-field mapping by laser ablation of PMMA coatings

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.

Light-induced processes on atoms and clusters confined in nanoporous silica and organic films

The study of light induced processes on atoms and nanoparticles confined in organic films or in dielectric structures is motivated both by fundamental interest and applications in optics and photonics. Depending on the light intensity and frequency and the kind of confinement, different processes can be activated. Among them photodesorption processes have a key role. Non thermal light induced atomic desorption has been observed from siloxane and paraffin films previously exposed to alkali vapors. This effect has been extensively investigated and used both to develop photo-atom sources and to load magneto-optical traps. Recently we observed huge photodesorption of alkali atoms embedded in nanoporous silica. In this case the atomic photodesorption causes, by properly tuning the light frequency, either formation or evaporation of clusters inside the silica matrix. Green-blue light desorbs isolated adatoms from the glass surface eventually producing clusters, whereas red-near infrared (NIR) light causes cluster evaporation due to direct excitation of surface plasmon oscillations. Green-blue light induces cluster formation taking advantage of the dense atomic vapor, which diffuses through the glass nano-cavities. Both processes are reversible and even visible to the naked eye. By alternatively illuminating the porous glass sample with blue-green and red-NIR light we demonstrate that the glass remembers the illumination sequences behaving as an effective rereadable and rewritable optical medium.