Optical antennas

Efficient interconversion of propagating light and localized, enhanced fields is instrumental for advances in optical characterization, manipulation and (quantum) optical information processing on the nanometer-scale. A resonant optical antenna (OA) might be an optimum structure that links propagating radiation and confined/enhanced optical fields. This thesis is concerned with the fabrication and investigation of optical antennas (OAs). We demonstrate that gold dipole and bow-tie antennas can be designed and fabricated to match optical wavelengths. For instance we fabricated slim gold dipole antennas with total lengths L in the half-wavelength range (L = 190 to 400 nm) on an ITO-coated glass cover slides. Micro-fabrication was performed in a two step process, applying a combination out of electron lithography and focused ion beam milling. For OA studies we built up a scanning confocal optical microscope (SCOM) with a polarization-controlled, picosecond pulsed light source. The SCOM design aimed on the excitation and detection of nonlinear effects like the two-photon photoluminescence of gold (TPPL) in individual nano structures. Using SCOM we analyzed dipole antennas and stripes of different length. We have identified specific antenna effects, like field-confinement and enhancement in the antenna feed gap. Upon illumination with picosecond laser pulses, white-light supercontinuum (WLSC) radiation is generated in the antenna feed gap in addition to twophoton photoluminescence (TPPL) in the antenna arms. The strength of emission and order of nonlinearity was used as a measure for the field enhancement at the position of an OA structure. On resonance strong field enhancement in the antenna feed gap drives even highly nonlinear phenomena like WLSC. The antenna length at resonance is considerably shorter than one half of the effective wavelength of the incident light. This is in contradiction to classical antenna theory, but in qualitative accordance with computer simulations that take into account the finite metallic conductivity at optical frequencies. Computer simulations revealed that an antenna resonance is also present for aluminium dipole antennas. The resonance length of a aluminium antenna is close to one half of the effective wavelength, in agreement with classical antenna theory. In contrast to gold, aluminum dipole antennas show a much broader resonance and four times less intensity enhancement at the wavelength investigated (830 nm). Surface plasmon resonances can be excluded for aluminium antennas at this wavelength and structural dimension. Therefore the strong enhancement and shift in resonance length of the gold dipole antenna can be explained with the excitation of a surface plasmon mode with strong field concentration in the antenna feed gap. This means, that the existence of surface plasmon resonances in suitably designed antennas can greatly enhance antenna performance in the optical wavelength range. The dimensions of the OA feed gap are far below the diffraction limit, and field distributions are only directly accessible by near-field microscopy techniques. The implementation of a scanning tunnelling optical microscope (STOM) was aimed at the direct detection of the optical near-field distribution around OAs. In a new design of the STOM scan head, fixation of the optical fiber is achieved by means of controlled pressure and elastic deformation. The avoidance of glued connections was found to improve the Q factor of the shear force sensor as well as to facilitate the replacement of the fiber probe. Illumination of the antenna structure was achieved under total internal reflection with s- and p-polarized light and three different wavelength (532 nm, 675 nm, 830 nm). A shear-force feedback system allowed for a direct comparison between optical and topographic image. STOM measurements on a single bow-tie structure (L = 300 nm) revealed a fieldcon finement in the antenna feed gap for a polarization parallel to the antenna long axis and an excitation wavelength of 830 nm, which was absent for the other wavelengths and polarizations. The observed field localization is in qualitative agreement with computer simulations. Future work in this field will concentrate on the exploration of OAs for high resolution SNOM imaging and on the investigation of the interaction of OAs with single-quantum systems

Loschmidt echo for a chaotic oscillator

Chaotic dynamics of a nonlinear oscillator is considered in the semiclassical approximation. The Loschmidt echo is calculated for a time scale which is of the power law in semiclassical parameter. It is shown that an exponential decay of the Loschmidt echo is due to a Lyapunov exponent and it has a pure classical nature.Comment: Submit to PR

Communication in Fungi

We will discuss fungal communication in the context of fundamental biological functions including mating, growth, morphogenesis, and the regulation of fungal virulence determinants. We will address intraspecies but also interkingdom signaling by systematically discussing the sender of the message, the molecular message, and receiver. Analyzing communication shows the close coevolution of fungi with organisms present in their environment giving insights into multispecies communication. A better understanding of the molecular mechanisms underlying microbial communication will promote our understanding of the “fungal communicome.

Thermodynamic properties in the normal and superconducting states of Na(x)CoO(2)*yH(2)O powder measured by heat capacity experiments

The heat capacity of superconducting Na(x)CoO(2)*yH(2)O was measured and the data are discussed based on three different models: The thermodynamic Ginzburg-Landau model, the BCS theory, and a model including the effects of line nodes in the superconducting gap function. The electronic heat capacity is separated from the lattice contribution in a thermodynamically consistent way maintaining the entropy balance of superconducting and normal states at the critical temperature. It is shown that for a fully gapped superconductor the data can only be explained by a reduced (about 55 %) superconducting volume fraction. The data are compatible with 100 % superconductivity in the case where line nodes are present in the superconducting gap function.Comment: Revised, 19 pages, 3 figure

Enhanced surface plasmon resonance absorption in metal-dielectric-metal layered microspheres

We present a theoretical study of the dispersion relation of surface plasmon resonances of mesoscopic metal-dielectric-metal microspheres. By analyzing the solutions to Maxwell's equations, we obtain a simple geometric condition for which the system exhibits a band of surface plasmon modes whose resonant frequencies are weakly dependent on the multipole number. Using a modified Mie calculation, we find that a large number of modes belonging to this flat-dispersion band can be excited simultaneously by a plane wave, thus enhancing the absorption cross-section. We demonstrate that the enhanced absorption peak of the sphere is geometrically tunable over the entire visible range.Comment: 4 pages, 3 figures, Accepted for publication, Optics Letters. Revisions upon final revie