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.