Mapping plasmons of designed nanostructures with an e-beam

The lateral distribution of the optical field near to a metal nanoparticle gives detailed information about the interaction of light with the particle which is - besides other parameters - defined by the shape of the particle. In the last years it has been shown that laterally resolved electron energy loss spectroscopy (EELS) and energy filtered transmission electron microscopy (EFTEM) are techniques to map these near fields with lateral resolution on the nanoscale. However, up to now EELS and EFTEM measurement were performed at colloidal particles only which grow in a limited variety of shapes. It is the aim of this project to make particles of virtually any shape available for these measurement techniques by introducing electron beam lithography (EBL) a highly flexible technique for preparation of designed nanostructures. The thereby expected challenge is to get the EBL prepared nanostructures onto ultra thin (a few tenth of nanometers) substrates necessary for EELS/EFTEM. We want meet this challenge by developing a novel type of transfer process. The first designed structures to be investigated with EELS/EFTEM will be pairs of particles (particularly dipole and bowtie antennas) and nanorings. In the case of particle pairs we target to map the near field in the gap between the particles which is supposed to be dramatically enhanced with respect to an exciting light field, in the case of nanorings we aim to identify a localized particle plasmon polariton (LSPP) mode with a noteworthy magnetic dipole moment by comparison of calculated and experimental near field maps.

Conventional technologies for data transmission are based on light or electrons. Landline telephony, cable television and fiber-optic Internet benefit from the high bandwidth of transmission with light. Due to its wavelength light needs a lot of space and is therefore not suitable for the transmission of information at the nanoscale. Thus, in highly miniaturized applications such as computer chips or sensors electrons are used. Promising candidates in search of new technologies for information transmission at the nanoscale are plasmons, collectively oscillating electron clouds at metallic surfaces. They arise when a ray of light impinges on nanostructured metals. Plasmons pose an enormous potential because they combine the advantages of light and electrons: They can transfer large amounts of data in confined space with high bandwidth. Within the FWF-funded project Mapping plasmons of designed nanostructures with an e-beam scientists of the University of Graz developed a unified view of the oscillation behavior of plasmons, they cracked quasi the code of the choreography of the dancing electron clouds. Just like acoustic oscillations the mode patterns of plasmons follow a specific order", says Franz Schmidt, who was responsible for most of the experimental work. These findings are crucial for the design of efficient miniaturized components in various technological applications. Ernst Florens Friedrich Chladni described 1787 in his Entdeckungen über die Theorie des Klanges for the first time acoustic oscillation patterns now known as Chladni figures and thus attracted worldwide attention. Its not less fascinating to see what the Graz researchers discovered 230 years later: They found a universal formula for the mode patterns of plasmons depending on the geometry of a nanoparticle. These mode patterns could only be resolved experimentally by the use of an exceptionally well-equipped electron microscope. With the aid of a focused electron beam energy states of plasmons can be observed, and the lateral resolution of the microscope allows the reconstruction of the patterns.