Oksana Kostiučenko

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.

Nanostructure induced changes in lifetime and enhanced second-harmonic response of organic-plasmonic hybrids

In this letter we show that the optical response of organic nanofibers, grown from functionalized para-quaterphenylene molecules, can be controlled by forming organic-plasmonic hybrid systems. The interaction between nanofibers and supporting regular arrays of nanostructures leads to a strongly enhanced second harmonic response. At the same time, the fluorescence lifetime of the nanofibers is reduced from 0.32 ns for unstructured gold films to 0.22 ns for gold nanosquare arrays, demonstrating efficient organic–plasmonic interaction. To study the origin of these effects, we applied two-photon laser scanning microscopy and fluorescence lifetime imaging microscopy. These findings provide an effective approach for plasmon-enhanced second-harmonic generation at the nanoscale, which is attractive for nanophotonic circuitry.

Local field enhanced second-harmonic response of organic nanofibers deposited on encapsulated plasmonic substrates

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.

Leakage radiation spectroscopy of organic/dielectric/metal systems: influence of SiO2 on exciton-surface plasmon polariton interaction

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.

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.

Field-enhanced nonlinear optical properties of organic nanofibers

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.

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.