can act as an interconnected nanocircuit
supporting a resonant magnetic response at
optical frequencies. 4 The induced magnetic
moment, however, is generally hidden
behind a stronger electric response. To boost
optical magnetism, we and our UT Austin
colleagues controlled the nanocircuit
block arrangement, introducing controlled
symmetry breaking in the nanoring that
triggers coupling between the bright
electric resonance and the subradiant dark
magnetic mode of the system. 5 This concept
unveiled a magnetic-based Fano resonance,
highlighted in scattering measurements,
and a corresponding almost-pure magnetic
hot spot at the nanoring center. The results
show the possibilities of metamaterial
concepts for integrated nanophotonics.
The maturity of nanofabrication and the rise of metamaterials is providing a new
generation of ultrafast, deeply sub-wavelength,
integratable nanophotonic devices and systems. The size scales of electronics and photonics have been separated by orders of magnitude
for decades. Only recently have researchers
in plasmonics and metamaterials recognized
their potential as a bridge to link these two
realms. Optical nanocircuits have introduced
the necessary concepts and building blocks for
this new nanophotonic paradigm. 1 Building on
these concepts, we have pushed the boundaries
to control, manipulate and process light at the
nanoscale, extending the reach of nanophotonic devices based on metamaterials.
Taking inspiration from microwave
phased arrays, we put forward a composite
plasmonic metascreen able to impart an arbitrary pattern to the impinging wavefront. 2
The design is based on a planar symmetric
stack of three metasurfaces, the simplest
possible structure able to fully control
transmitted light, at the same time ensuring
maximal efficiency and minimized reflections. 3 The required impedance profile on
each metasurface was synthesized by arranging optical nanocircuit blocks based on the
suitable alternation of two CMOS-compatible
materials. The resulting sub-wavelength
metascreen enables full transmission phase
coverage and allows processing and molding
of the impinging optical beam in arbitrary
ways (e.g., deflection, focusing, negative
refraction), at the same time minimizing the
energy lost in reflection or unwanted diffraction and largely improving the efficiency
of existing nanophotonic designs.
Optical nanocircuit concepts have also
helped realize giant optical magnetism,
another missing piece in nanophotonic
systems. Researchers have shown that a
sub-wavelength symmetric ring of plasmonic
nanoparticles separated by insulating gaps
(a) Optical nanocircuit building blocks and composite metascreen. (b) Full phase
coverage can be obtained with different unit-cell geometries (left). Efficient light
deflection (right). (c) Effect of symmetry breaking on the response of a nanoring.
(d) Measured scattering spectrum (inset: AFM nanoring image) and calculated
magnetic-field distributions at the scattering dip (I) and peak (II). White arrows
represent induced electric dipole moments.
and Andrea Alù
University of Texas at
Austin, Texas, U.S.A.
1. N. Engheta et al.
Phys. Rev. Lett. 95,
2. F. Monticone et al.
Phys. Rev. Lett. 110,
3. A. Alù. Physics 6, 53
4. A. Alù et al. Opt.
Exp. 14, 1557
5. F. Shafiei et al. Nature Nanotechnol.
8, 95 (2013).
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