| Optical antennas have recently received enormous interest because of their enormous potential for various applications in nanophotonics such as nonlinear optics, Surface Enhanced Raman Scattering (SERS), fluorescence enhancement, and optical sensing. Various types of antenna structures have been under investigation, like line-antennas, bowtie antennas, and Yagi-Uda arrays. We have investigated a class of plasmonic antennas consisting of coupled nanorod dimers.
Figure 1 SEM image of a lithographically fabricated nanoantenna array
with varied antenna arm lengths (vertical) and antenna gaps
(horizontal), with detailed images of several single nanorods and
dimer antennas.
Individual gold nanoantennas have been fabricated using high-resolution
electron-beam lithography at Philips Research. The nanoantenna structures have a thickness of 20 nm. Scanning electron microscopy (SEM)
images of an array of nanoantennas are shown in
Figure 1, with magnified images of individual
nanorods and dimer antennas numbered 1-4. In the array, the antenna
arm length L is varied in the vertical direction, while the
antenna gap width D is varied in the horizontal direction.
The first column on the left contains single nanorods, as shown in
images 1 and 2. The second column contains structures of two
nanorods which are overlapping because the designed particle
separation was too narrow for the liftoff process to succeed (image
3). The antennas with the smallest gaps have a separation of 20 nm, as shown in image 4.
 
Figure 2 (left) Experimental darkfield configuration for detecting the spectrum of scattered light. (right) Optical scattering spectrum of a single gold nanorod of 100x70x20 nm3 for polarizations of the detected light parallel (black circles) and perpendicular (red diamonds) to the nanorod axes.
The plasmonic modes of individual antennas are characterized by
scattering spectroscopy using scanning confocal microscope in
darkfield configuration as shown schematically in Figure 2(left). A typical scattering spectrum of a single gold nanorod is shown on the right-hand side of Figure 2. By means of a polarization filter in front of the detector, the longitudinal and transverse plasmon modes of the structure can be clearly separated.
Figure 3 Color density graph of calculated
longitudinal antenna mode positions as a function of both antenna
arm length L and antenna gap width D. (open circles, red)
Experimentally obtained and (red line) calculated resonance position at a wavelength of 730 nm, and (black squares & lines) same at 660 nm.
We have performed model calculations predicting the position of the spectral resonances of the coupled antenna modes for the antenna parameters under study. The resulting positions of the longitudinal plasmon resonance are shown in Figure 3, together with two measurements at fixed wavelengths of 660 nm and 730 nm. We obtain good agreement between the experimental and calculated resonance positions.
Figure 4 (a) Near field intensity at the
longitudinal resonance for an antenna with strongly coupled arms. (b) Far-field scattering pattern of
the antenna (solid line, red), together with the emission patterns
of a point dipole (dash-dotted line, blue), and a half-wave antenna
(dashed line, black).
The calculated near-field and far-field scattering properties of a coupled dimer antenna at resonance are shown in Figure 4. The upper image shows the near-field intensity profile around the antenna, predicting up to 100-fold enhancement of the local intensity. The far-field emission pattern is shown in
the lower part of Figure 4. For comparison we
plotted radiation patterns of a point dipole (dash-dotted line,
blue), and a half-wave antenna (dashed line, black). Clearly, the
antenna pattern corresponds to that of a half-wave dipole antenna,
which has a more directional emission than a point dipole due to
interference of the radiation emitted over the total antenna length.
Reference:
O. L. Muskens, V. Giannini, J. A. Sánchez Gil, J. Gómez Rivas, Optical scattering resonances of single and coupled dimer plasmonic nanoantennas, Opt. Express 15(26), 17736:46 (2007)
|