11 Plasmonic Functionalities Based on Detuned Electrical Dipoles
415
the fact that the refractive index is directly related to the amplitude and phase of the
transmitted and reflected fields [33]. Importantly, for the case of the DED metamaterial consisting of three nanorod antennas, n decreases in the experiment from ∼1.9 to
∼1.7 in the wavelength range ∼750 to 850 nm, thereby indicating normal dispersion
in the transparency window. The group refractive index, which is estimated from
the slope of the linear fit to n(ϕ), is, however, only n g = 3.6 in the experiment and
n g = 2.7 in the simulations, i.e. the light slow-down is quite moderate. The imaginary part ∂ is approximately two times larger in the experiment compared to the
simulated results, which is reasonable due to the surface scattering and grain boundary effects leading to increased damping in the gold nanorod antennas compared to
bulk gold [35, 36].
11.3 Applications
Despite the fact that the DED concept follows naturally from the plasmonic realization of the dressed state picture of EIT, featuring DED metamaterials with wavelength
regions of slow light and enhanced transmission, the concept shows broader applicability. Based on DED configurations, we suggest in this section a new approach to
the design of a plasmonic sensor for refractive index sensing and a novel type of
ultrathin plasmonic quarter-wave plate.
11.3.1 Plasmonic Sensing
Recent developments have greatly improved the sensitivity of optical sensors that
are based on metal nanoparticles and the exploitation of their localized plasmon
resonances [37]. The resonance wavelenght is determined by the susceptibilities of
constituent materials as well as by the nanoparticle size and geometry. The former
feature provides the possibility of monitoring environmental changes by tracking
the resonance wavelength, whereas the latter allows one to adjust the LSPR position
across the visible and near-infrared spectral regions. The overall sensing performance
of a plasmonic nanostructure is typically characterized by the figure of merit (FOM)
FOM =
s
πϕ
,
(11.9)
where s = ψϕ res /ψn d is the sensitivity (measured in nm/RIU) of the resonance
wavelength ϕ res to changes in the environmental refractive index n d , and πϕ is the
resonance spectral width at half-maximum (FWHM) [38]. Both characteristics are
rather difficult to improve, especially for single nanoparticle configurations [37, 39].
For example, the resonance quality (Q) factor is problematic to increase over the
electrostatic limit imposed by the Ohmic losses in metals [19]. In fact, one can
reach this limit only with ultrasmall metal particles, whose scattering (radiation)
415
the fact that the refractive index is directly related to the amplitude and phase of the
transmitted and reflected fields [33]. Importantly, for the case of the DED metamaterial consisting of three nanorod antennas, n decreases in the experiment from ∼1.9 to
∼1.7 in the wavelength range ∼750 to 850 nm, thereby indicating normal dispersion
in the transparency window. The group refractive index, which is estimated from
the slope of the linear fit to n(ϕ), is, however, only n g = 3.6 in the experiment and
n g = 2.7 in the simulations, i.e. the light slow-down is quite moderate. The imaginary part ∂ is approximately two times larger in the experiment compared to the
simulated results, which is reasonable due to the surface scattering and grain boundary effects leading to increased damping in the gold nanorod antennas compared to
bulk gold [35, 36].
11.3 Applications
Despite the fact that the DED concept follows naturally from the plasmonic realization of the dressed state picture of EIT, featuring DED metamaterials with wavelength
regions of slow light and enhanced transmission, the concept shows broader applicability. Based on DED configurations, we suggest in this section a new approach to
the design of a plasmonic sensor for refractive index sensing and a novel type of
ultrathin plasmonic quarter-wave plate.
11.3.1 Plasmonic Sensing
Recent developments have greatly improved the sensitivity of optical sensors that
are based on metal nanoparticles and the exploitation of their localized plasmon
resonances [37]. The resonance wavelenght is determined by the susceptibilities of
constituent materials as well as by the nanoparticle size and geometry. The former
feature provides the possibility of monitoring environmental changes by tracking
the resonance wavelength, whereas the latter allows one to adjust the LSPR position
across the visible and near-infrared spectral regions. The overall sensing performance
of a plasmonic nanostructure is typically characterized by the figure of merit (FOM)
FOM =
s
πϕ
,
(11.9)
where s = ψϕ res /ψn d is the sensitivity (measured in nm/RIU) of the resonance
wavelength ϕ res to changes in the environmental refractive index n d , and πϕ is the
resonance spectral width at half-maximum (FWHM) [38]. Both characteristics are
rather difficult to improve, especially for single nanoparticle configurations [37, 39].
For example, the resonance quality (Q) factor is problematic to increase over the
electrostatic limit imposed by the Ohmic losses in metals [19]. In fact, one can
reach this limit only with ultrasmall metal particles, whose scattering (radiation)