enhancement is a clear manifestation of
the quantum Purcell effect, and it can be
controlled by a metamaterial’s design.
This is an essential step towards the
development of metamaterial-enhanced
gain media and the “lasing spaser”:
a “flat” laser with emission fueled by
plasmonic excitations in an array of
coherently emitting metamolecules.
In contrast to conventional lasers that
operate at wavelengths of suitable natural
atomic or molecular transitions, the las-
ing spaser’s emission wavelength can be
controlled by metamolecule design.
tum interference devices
We envision that, in the future,
electrically pumped semiconductor gain
media will provide a practical solution
for metamaterial-based lasers at visible
and telecom frequencies, while quantum
cascade semiconductor amplifiers show
promise for tackling losses and providing gain in the infrared. Electrically and
optically pumped graphene is expected
to show strong plasmonic amplification
in the terahertz part of the spectrum.
Adding gain to metamaterials also
compensates for the joule losses that
damp plasmons in metal nanostructures.
Lowering losses is crucial for the performance of metamaterial-based negative-index devices, waveguides, spectral
filters, delay lines and, in fact, practically
any application of metamaterials.
Today’s photonic metamaterials are nanofabricated arrays of classical plasmonic
resonators that mimic and surpass the electromagnetic properties of natural solids.
Additional functionalities can be achieved by hybridizing these structures with nonlinear,
gain and switchable media. In the future, arrays of quantum interference devices will
form “quantum metamaterials” that provide a much closer analogy to natural crystals.
Chalcogenide glass layer
(a) MEMS actuators can reposition parts of the metamolecule (inset) in an array,
allowing the tuneability of infrared transmission and reflection spectra. (b) Reconfigu-
rable NEMS photonic metamaterial fabricated on bimorph membranes are tuneable by
temperature. (c) Switchable metamaterial with phase-change chalcogenide glass active
layer. (d) Optical luminescence of semiconductor quantum dots in a plasmonic array is
enhanced manifold. (e) Array of Josephson junction quantum interference devices is a
prototype quantum metamaterial. (f) Plasmonic metamaterial array provides an order-
of-magnitude enhancement of the ultrafast optical nonlinearity of carbon nanotubes.
Images courtesy of (clockwise from top left) Liu Ai Qun, N TU, Singapore; Bruce Ou, Zsolt Samson, Eric Plum,
Roger Buckingham, Andrei Nikolayenko, Univ. of Southampton
Switchable and tuneable metamaterials are other rapidly expanding areas of
research. Indeed, the development of
nanophotonic all-optical data processing circuits depends on the availability
of fast and highly responsive nonlinear
media that react to light by changing
their refractive index and absorption.
This is difficult to deliver in nanoscale-size devices using electronic or molecular
nonlinearities, where stronger responses
often come at the expense of longer reaction times and where the optical path
through the nonlinear medium is shorter
than the wavelength of light.
When high speed switching is not
the prime objective, metamaterials can
be reliably and reversibly controlled
by microelectromechanical (MEMS)
actuators that reposition parts of the
metamolecules. This has been convincingly demonstrated for terahertz and far-infrared metamaterials. Reconfigurable
optical metamaterials require moving
components on the scale of a few tens of
nanometers (NEMS actuators) to realize
a profound change in optical properties.
Metamaterials in which metal
nanostructures are hybridized with
nonlinear and switchable dielectrics or
semiconductors provide a way to achieve
changes faster than they can be achieved
by mechanical repositioning of parts.
32 | OPN Optics & Photonics News