Fluorescence in plasmonic nanocavities near the contact limit

Plasmonic nanocavities can produce enhancements of the optical near field intensity and the local density of photonic states (LODS) by a factor of 10-1000 in very confined volumes, in the gap of particle pairs, or between a smooth metal film and the tip of a scanning tunneling microscope, for example. If a fluorophore (e.g., a molecule or quantum crystal) is placed within such a region of field enhancement, its fluorescence lifetime gets strongly reduced by the intense electromagnetic interaction with its environment. If the shortening reaches a factor in the order of 100-1000, the fluorescence lifetime gets comparable to or even shorter than the duration of internal conversion processes. This not only would lead to dramatic changes in excitation and emission spectra of single molecules (hot luminescence and modification in the emission spectrum), but should also cause strong coupling of the fluorophore with the nanocavity. As emphasized by simulations, the consideration of electron spillout and quantummechanical effects in < 2 nm nanocavities enable a charge transfer between neighbouring plasmonic nanoparticles before the actual contact. The resulting current can have two effects: either it simply reduces the field enhancement and LDOS compared to the classical case. Or, since the current has to pass the fluorophore between the particles, it causes an interaction of nanoplasmonic and molecular electronic structure beyond the electrodynamic model with up to date unknown implications. To find an answer to these questions, we plan in this project at first to investigate and understand the optical response of particle pairs with gapwidth of 0-20nm. In combination with a defined shaping of the particles, this will then enable us to control the plasmonic resonances of the particle pairs in a way, that we can investigate the behavior of fluorophores experiencing the high LDOS in the gap region. The research strategy is based on lithographically defined shapes and -positioning of the plasmonic particles and fluorophores, respectively, in combination with an actively adjustable particle distance with the help of a scanning probe microscope. With this work on one hand we hope to push the understanding of fluorophore interaction with plasmonic systems to new physical domains. On the other hand, a deepened insight into the topic will have potential impact in close research fields as, for example, single molecule optoelectronics or for the clarification of the fluorescence contribution to the background signal observed in surface enhanced Raman scattering.

Plasmonic nano-resonators are tiny metal nanoparticles into which, due to their tininess, light can penetrate to excite the conduction electrons (which are responsible for the metals electrical and optical properties) to coherent oscillations, so-called plasmons. Particularly in a small volume in the gap between two illuminated plasmonic nano-resonators, the light intensity and thus the light interaction with e.g. molecules in that volume, can be enhanced by several orders of magnitude. Those molecules could be for example fluorophores, which can absorb light and subsequently emit it again after a very short internal conversion process at a different color. This process is usually that fast, that it can hardly be observed or influenced. The coupling to the plasmonic nano-resonators, however, can be sufficiently strong to influence the conversion and emission process, which could be observed by a changed emission color. To identify, fabricate and characterize suitable plasmonic structures with a nanometric gap was the subject of this project. It could be achieved on one hand by lithography and on the other hand with the help of a scanning probe microscope. As lithography of metal nanoparticles is limited to lateral resolutions of about 10 nm, which is too coarse, we developed special metal- insulator-metal structures, where the gap between the plasmonic metal particles is defined by a dielectric layer whose thickness can be controlled to sub-nanometer precision. For the scanning probe microscopic method, one metal particle was fixed to a substrate, whereas another one was glued to a fine glass fiber tip, which was then positioned to nanometer precision relative to the particle on the substrate by so-called piezo-actuators. Both methods finally allowed to realize the nanometric gaps, where a surprisingly high number of plasmonic oscillations modes and local light intensity enhancements of about 3 orders of magnitude could be observed. The expected strong interaction with the molecular fluorescence can thus be reached. The results are important for the fundamental study of fluorescence processes but also have implications for the development of plasmonic sensors applied in microbiology.