587 research outputs found
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
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