2, Kiel University, Kiel, , Germany
4, Hamburg University of Technology, Hamburg, , Germany
3, University of Southern Denmark, Odense, , Denmark
Light interaction with metal nanoparticles (NPs) gives rise to a number of fascinating optical phenomena. One of the directions of research is the search for configurations that enable efficiently interconvert propagating and localised plasmon resonances and thereby promote the enhanced local fields. Therefore, there is a strong interest to the formation of nanoparticle arrays for practical applications in plasmonics, surface enhanced Raman spectroscopy and optical sensing.
To form arrays, cluster beam deposition is combined with electron beam lithography. The substrates are covered by thin (100 nm) layer of photoresist followed by thin poly(methyl methacrylate) PMMA layer using standard spin-coating. In the first series of experiments simple patterns representing 100 nm wide stripes with varying distance between the stripes (period) are written in PMMA by electron beam, thus, forming periodic structures of linear trenches down to the photoresist layer. Size-selected silver NPs (of about 18 nm in diameter in the current experiments) produced by the magnetron sputtering cluster apparatus are deposited in low-energy regime on the patterns reaching the coverage of a monolayer of NPs across the surface. After the deposition the samples are annealed at 180 C for 5 minutes to facilitate partial immersion of the deposited NPs into the polymer films. This treatment improves adhesion making the NPs resistant against wet chemical procedures. After that a standard lift-off is performed to remove the PMMA yielding sharp linear stripes of metal NPs slightly embedded into the photoresist layer. The samples demonstrate strong localised surface plasmon resonance bands at wavelength of ca. 400 nm which is good agreement with the simulations using Mie theory. Properties of propagating plasmon resonances of the stripes formed by individual NPs as well as possible optical coupling between the stripes are currently under the study.
In summary, by combining the electron beam lithography and cluster beam deposition techniques linear arrays (nanoscale stripes) of size-selected metal NPs are successfully obtained. The approach paves a way for formation of a variety of different geometrical arrays with the limitations in dimensions dictated only by lithography, thus, providing an excellent method for production of plasmonic systems with required configurations.