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A. Pors et al.
nanostructured heterogeneous (composite) materials to homogeneous metamaterials,
whose effective parameters exhibit new optical properties that can even be inaccessible with natural materials. An interesting application of metamaterial dispersion
engineering, is the imitation of electromagnetically induced transparency (EIT),
which has attracted much attention [1–5] due to the possibility of slowing light down,
enhancing nonlinear interactions and improving the sensitivity of metamaterialbased nanosensors. Similarly, the classical realization of electromagnetically induced
absorption, which is the complementary phenomenon of EIT, has also received some
attention [6], though the less apparent usage within applications has limited the general interest.
The phenomenon of EIT is based on the laser induced coherence of atomic states
leading to quantum interference between the excitation pathways controlling the
optical response, which is thereby modified exhibiting enhanced transmission [7, 8].
The strongly enhanced transmission within the absorption band results in strong dispersion and consequently in a significant reduction in the group velocity (see e.g. [8]
and references therein). Slowing down the propagation of light pulses enhances lightmatter interactions, allowing one, for example, to boost optical nonlinearities and to
tackle the tantalizing problem of an all-optical buffer [9].
The phenomenon of EIT can be considered in two alternative ways: as resulting
from the destructive interference between two pathways involving either the bare,
dipole-allowed and metastable states or, equivalently, the doublet of dressed states
(created by strong pump radiation) representing two closely spaced resonances decaying to the same continuum [7, 8]. While these two physical pictures are equivalent
when dealing with EIT in atomic systems, their realization with plasmonic nanostructures, whose responses are determined by their configurations and not electromagnetically induced as in EIT, depends on the EIT mechanism that is imitated. The
first picture suggests employing radiative and subradiant (dark) plasmonic elements
that are strongly coupled by being closely placed and appropriately oriented [1–3, 5].
Note that the strong-coupling condition imposes rather stringent requirements on the
fabrication accuracy for the plasmonic structures to be operated at optical wavelengths [1]. Alternatively viewed, EIT is achieved due to the cancellation of opposite
contributions from two resonances, which are equally spaced but with opposite signs
of detuning from the probe frequency, due to the Fano-like interference of the decay
channels. Fundamentally, the dressed-state picture of EIT is equivalent to the case of
interference between two closely spaced lifetime broadened resonances decaying to
the same continuum [7]. The underlying physics of the cancellation of absorption in
EIT is also similar to that involved in the phenomenon of coherent population trapping [8]. Metamaterials utilizing trapped-mode resonances and featuring the EIT-like
transmission spectra have been realized in the cm-wavelength range using fish-scale
patterns [2] and concentric ring resonators [10]. In both configurations, electrical currents induced (at the trapped-mode resonance frequency) in different parts of a unit
cell oscillate with opposite phases, resulting in scattering suppression and enhanced
transmission.
It should be emphasized that the EIT realization with plasmonic nanostructures is
fundamentally different from EIT in atomic systems with respect to the linewidths of
A. Pors et al.
nanostructured heterogeneous (composite) materials to homogeneous metamaterials,
whose effective parameters exhibit new optical properties that can even be inaccessible with natural materials. An interesting application of metamaterial dispersion
engineering, is the imitation of electromagnetically induced transparency (EIT),
which has attracted much attention [1–5] due to the possibility of slowing light down,
enhancing nonlinear interactions and improving the sensitivity of metamaterialbased nanosensors. Similarly, the classical realization of electromagnetically induced
absorption, which is the complementary phenomenon of EIT, has also received some
attention [6], though the less apparent usage within applications has limited the general interest.
The phenomenon of EIT is based on the laser induced coherence of atomic states
leading to quantum interference between the excitation pathways controlling the
optical response, which is thereby modified exhibiting enhanced transmission [7, 8].
The strongly enhanced transmission within the absorption band results in strong dispersion and consequently in a significant reduction in the group velocity (see e.g. [8]
and references therein). Slowing down the propagation of light pulses enhances lightmatter interactions, allowing one, for example, to boost optical nonlinearities and to
tackle the tantalizing problem of an all-optical buffer [9].
The phenomenon of EIT can be considered in two alternative ways: as resulting
from the destructive interference between two pathways involving either the bare,
dipole-allowed and metastable states or, equivalently, the doublet of dressed states
(created by strong pump radiation) representing two closely spaced resonances decaying to the same continuum [7, 8]. While these two physical pictures are equivalent
when dealing with EIT in atomic systems, their realization with plasmonic nanostructures, whose responses are determined by their configurations and not electromagnetically induced as in EIT, depends on the EIT mechanism that is imitated. The
first picture suggests employing radiative and subradiant (dark) plasmonic elements
that are strongly coupled by being closely placed and appropriately oriented [1–3, 5].
Note that the strong-coupling condition imposes rather stringent requirements on the
fabrication accuracy for the plasmonic structures to be operated at optical wavelengths [1]. Alternatively viewed, EIT is achieved due to the cancellation of opposite
contributions from two resonances, which are equally spaced but with opposite signs
of detuning from the probe frequency, due to the Fano-like interference of the decay
channels. Fundamentally, the dressed-state picture of EIT is equivalent to the case of
interference between two closely spaced lifetime broadened resonances decaying to
the same continuum [7]. The underlying physics of the cancellation of absorption in
EIT is also similar to that involved in the phenomenon of coherent population trapping [8]. Metamaterials utilizing trapped-mode resonances and featuring the EIT-like
transmission spectra have been realized in the cm-wavelength range using fish-scale
patterns [2] and concentric ring resonators [10]. In both configurations, electrical currents induced (at the trapped-mode resonance frequency) in different parts of a unit
cell oscillate with opposite phases, resulting in scattering suppression and enhanced
transmission.
It should be emphasized that the EIT realization with plasmonic nanostructures is
fundamentally different from EIT in atomic systems with respect to the linewidths of