412
A. Pors et al.
noting that both the two- and three-nanorod metamaterials show BDPs that are
favorably comparable to other EIT realizations using coupled split-ring resonators
[3] or waveguide-coupled plasmonic antennas [12].
11.2.3 Experimental Study of Detuned Electrical Dipole
Metamaterials
In this section we investigate the DED metamaterials comprised of two and three gold
nanorods experimentally. Specifically, the investigated samples consist of a silica
substrate with DEDs defined by electron-beam lithography (EBL) and fabricated by
lift-off of a 55 nm thick gold film, deposited by electron beam evaporation on top
of a 5 nm thick titanium adhesion layer. DED arrays comprised of two nanorods are
fabricated with a 300 nm period (Fig. 11.6a), whereas the DEDs comprised of three
nanorods are fabricated with a 400 nm period (Fig. 11.6d). Furthermore, in order to
ensure a nearly symmetric environment (phase matching) the sample is covered with
a ∼15 µm thick layer of polymethyl methacrylate (PMMA) by spin coating.
In order to measure the transmission spectra (Fig. 11.6b and e), light from a broad
band halogen light source with a fiber output, whose (weakly divergent) radiation
is directed (at normal incidence) through a polarizer, is incident upon the sample
with the electric field aligned to the axes of the gold nanorods in the array. After
transmission through the sample the transmitted light is collected by an objective with
60x magnification and 0.85 numerical aperture, sent through an analyzer (parallel
to the polarizer) and finally collected by an optical fiber connected to a VIS/NIR
spectrometer. Reflection spectra (Fig. 11.6b and e) of the DED arrays are measured
in a similar manner, where the incident polarized light is focused onto the sample
with the aforementioned objective, collected by the same objective, sent through
the analyzer and collected by the fiber connected to the VIS/NIR spectrometer. The
transmission spectra are afterwards normalized by the transmission measured outside
the DED arrays whereas the reflection spectra are normalized using the reflection
measurements from a 300 nm thick gold sample and afterwards weighted with the
reflectivity calculated from tabular values of the gold optical constants [25].
Along with the experimental investigations, the transmission and reflection
spectra for one layer of DED unit cells, with dimensions similar to those in the
experiment, are simulated. In the simulation, the gold nanorods are embedded in
a lossless dielectric with refractive index n d = 1.5, i.e. in a homogeneous dielectric with optical properties similar to those of PMMA. The choice of PMMA in the
simulations rather than silica (n d = 1.45) is due to the fact that in the experiment
the DEDs are situated on top of the silica and covered with PMMA. It is therefore expected that a larger part of the electric field of the plasmonic resonances are
concentrated at the high index side of the sample. The transmission and reflection
spectra (Fig. 11.6c and f), exhibit two resonances separated by a window of enhanced
transmission and, correspondingly, reduced reflection, i.e. EIT-like behavior due to
scattering suppression in between the resonances. In general, there is quite good
A. Pors et al.
noting that both the two- and three-nanorod metamaterials show BDPs that are
favorably comparable to other EIT realizations using coupled split-ring resonators
[3] or waveguide-coupled plasmonic antennas [12].
11.2.3 Experimental Study of Detuned Electrical Dipole
Metamaterials
In this section we investigate the DED metamaterials comprised of two and three gold
nanorods experimentally. Specifically, the investigated samples consist of a silica
substrate with DEDs defined by electron-beam lithography (EBL) and fabricated by
lift-off of a 55 nm thick gold film, deposited by electron beam evaporation on top
of a 5 nm thick titanium adhesion layer. DED arrays comprised of two nanorods are
fabricated with a 300 nm period (Fig. 11.6a), whereas the DEDs comprised of three
nanorods are fabricated with a 400 nm period (Fig. 11.6d). Furthermore, in order to
ensure a nearly symmetric environment (phase matching) the sample is covered with
a ∼15 µm thick layer of polymethyl methacrylate (PMMA) by spin coating.
In order to measure the transmission spectra (Fig. 11.6b and e), light from a broad
band halogen light source with a fiber output, whose (weakly divergent) radiation
is directed (at normal incidence) through a polarizer, is incident upon the sample
with the electric field aligned to the axes of the gold nanorods in the array. After
transmission through the sample the transmitted light is collected by an objective with
60x magnification and 0.85 numerical aperture, sent through an analyzer (parallel
to the polarizer) and finally collected by an optical fiber connected to a VIS/NIR
spectrometer. Reflection spectra (Fig. 11.6b and e) of the DED arrays are measured
in a similar manner, where the incident polarized light is focused onto the sample
with the aforementioned objective, collected by the same objective, sent through
the analyzer and collected by the fiber connected to the VIS/NIR spectrometer. The
transmission spectra are afterwards normalized by the transmission measured outside
the DED arrays whereas the reflection spectra are normalized using the reflection
measurements from a 300 nm thick gold sample and afterwards weighted with the
reflectivity calculated from tabular values of the gold optical constants [25].
Along with the experimental investigations, the transmission and reflection
spectra for one layer of DED unit cells, with dimensions similar to those in the
experiment, are simulated. In the simulation, the gold nanorods are embedded in
a lossless dielectric with refractive index n d = 1.5, i.e. in a homogeneous dielectric with optical properties similar to those of PMMA. The choice of PMMA in the
simulations rather than silica (n d = 1.45) is due to the fact that in the experiment
the DEDs are situated on top of the silica and covered with PMMA. It is therefore expected that a larger part of the electric field of the plasmonic resonances are
concentrated at the high index side of the sample. The transmission and reflection
spectra (Fig. 11.6c and f), exhibit two resonances separated by a window of enhanced
transmission and, correspondingly, reduced reflection, i.e. EIT-like behavior due to
scattering suppression in between the resonances. In general, there is quite good