Optics & Photonics News

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.

Atoms
Plasmonic
resonators
Active/nonlinear
medium
Superconducting quan-
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.

Natural solid
Electromagnetic metamaterial
“Quantum” material
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.

(a)

Chalcogenide glass layer (c)(b)

360 nm

30 mm
Metamaterial
(gold)

850 nm
Si3N4
membrane
Quantum dots
(d)

(e)

(f)

400 nm

100 mm

500 nm
(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 metamaterials

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.