426
A. Pors et al.
Note that the reflection as a function of the analyzer angle is not perfectly flat when
Ω = 0 ◦ and Ω = 90 ◦ due to the commercial quarter-wave plate not providing a retardance of exactly one quarter at 780 nm. That said, it is clear from the measurements
that the nanobrick metamaterial functions as a quarter-wave plate in reflection.
11.4 Conclusion
The concept of detuned electrical dipoles (DED) have emerged from the plasmonic
realization of the dressed-state picture of the electromagnetically induced transparency (EIT) in atomic physics. The DED concept shows versatility in the sense
that it covers any plasmonic configuration consisting of two dipole resonances that
are oppositely detuned from the central frequency. Importantly, the detuning must
be weak so that the line shapes of the individual resonances overlap, hereby creating
scattering suppression at the central frequency. Furthermore, as the scattering suppression also occurs in the limit of noninteracting resonators, DED configurations
display robustness with respect to mutual positioning of individual scatterers. A fact
that amounts in less stringent fabrication requirements compared to the other EIT
realization mimicking the bare-state picture.
By employing gold nanorod antennas as dipolar scatterers, we have demonstrated
that two- and three-nanorod configurations constituting a unit cell in a DED metamaterial exhibit enhanced transmission (i.e., optical transparency) at the central frequency with the possibility of slowing down light. Specifically, the numerically studied two-nanorod system demonstrates slow down factors of ∼ 10 with propagation
loss of ∼ 2.5 dB per 300-nm-thick unit cell at the central wavelength ∼ 1150 nm,
whereas the three-nanorod system features a group refractive index of ∼ 8 with
loss ∼ 1 dB per 150-nm-thick cell at the central wavelength ∼ 860 nm. DED metamaterials and their associated effective refractive indexes have also been studied
experimentally, showing consistent trends with simulations.
Although the DED concept was presented in relation to slow-light metamaterials,
it has proven a much larger applicability. Here, we demonstrated that the scattering
asymmetry along the line connecting the two nanorods at the central wavelength
can be advantageously exploited for plasmonic sensing of the environment, both
as an individual subwavelength-sized sensor and as a unit cell of a periodic array.
Simulations of a single two-nanorod configuration revealed that the ratio of the
scattering asymmetry produces spectral peaks at the central wavelength ϕ = 941 nm
with a FWHM of ∼2 nm and a sensitivity of ∼600 nm/RIU resulting in a record
high figure of merit (FOM) of ∼300. Experimental proof-of-principle studies of
lithographically fabricated gold nanorod arrays demonstrate FWHM of ∼20 nm, a
value which is significantly smaller than the FWHM of plasmonic resonances of
individual nanorods.
Finally, we showed that a metasurface with unit cells consisting of perpendicular dipolar scatterers can be designed to function as a nanometer-thin quarter-wave
plate in reflection, meaning that the reflected light is circularly polarized for a lin-
A. Pors et al.
Note that the reflection as a function of the analyzer angle is not perfectly flat when
Ω = 0 ◦ and Ω = 90 ◦ due to the commercial quarter-wave plate not providing a retardance of exactly one quarter at 780 nm. That said, it is clear from the measurements
that the nanobrick metamaterial functions as a quarter-wave plate in reflection.
11.4 Conclusion
The concept of detuned electrical dipoles (DED) have emerged from the plasmonic
realization of the dressed-state picture of the electromagnetically induced transparency (EIT) in atomic physics. The DED concept shows versatility in the sense
that it covers any plasmonic configuration consisting of two dipole resonances that
are oppositely detuned from the central frequency. Importantly, the detuning must
be weak so that the line shapes of the individual resonances overlap, hereby creating
scattering suppression at the central frequency. Furthermore, as the scattering suppression also occurs in the limit of noninteracting resonators, DED configurations
display robustness with respect to mutual positioning of individual scatterers. A fact
that amounts in less stringent fabrication requirements compared to the other EIT
realization mimicking the bare-state picture.
By employing gold nanorod antennas as dipolar scatterers, we have demonstrated
that two- and three-nanorod configurations constituting a unit cell in a DED metamaterial exhibit enhanced transmission (i.e., optical transparency) at the central frequency with the possibility of slowing down light. Specifically, the numerically studied two-nanorod system demonstrates slow down factors of ∼ 10 with propagation
loss of ∼ 2.5 dB per 300-nm-thick unit cell at the central wavelength ∼ 1150 nm,
whereas the three-nanorod system features a group refractive index of ∼ 8 with
loss ∼ 1 dB per 150-nm-thick cell at the central wavelength ∼ 860 nm. DED metamaterials and their associated effective refractive indexes have also been studied
experimentally, showing consistent trends with simulations.
Although the DED concept was presented in relation to slow-light metamaterials,
it has proven a much larger applicability. Here, we demonstrated that the scattering
asymmetry along the line connecting the two nanorods at the central wavelength
can be advantageously exploited for plasmonic sensing of the environment, both
as an individual subwavelength-sized sensor and as a unit cell of a periodic array.
Simulations of a single two-nanorod configuration revealed that the ratio of the
scattering asymmetry produces spectral peaks at the central wavelength ϕ = 941 nm
with a FWHM of ∼2 nm and a sensitivity of ∼600 nm/RIU resulting in a record
high figure of merit (FOM) of ∼300. Experimental proof-of-principle studies of
lithographically fabricated gold nanorod arrays demonstrate FWHM of ∼20 nm, a
value which is significantly smaller than the FWHM of plasmonic resonances of
individual nanorods.
Finally, we showed that a metasurface with unit cells consisting of perpendicular dipolar scatterers can be designed to function as a nanometer-thin quarter-wave
plate in reflection, meaning that the reflected light is circularly polarized for a lin-