Metamaterials—artificial media with tailored subwavelength
structures—have now encompassed a broad range of novel properties that are
unavailable in nature. This field of research has stretched across different
wave platforms, leading to the discovery and demonstration of a wealth of
exotic wave phenomena. Most recently, metamaterial concepts have been extended
to the temporal domain, paving the way to completely new concepts for wave
control, such as nonreciprocal propagation, time-reversal, new forms of optical
gain and drag.
Meanwhile, the concept of designer matter has also inspired
significant research efforts in condensed matter physics, broadening the
horizon of known phases of matter. Of particular interest has been the recent
activity in Floquet matter, characterized by periodic modulations imposed, e.g.
via a strong optical pulse, on the energy landscape experienced by the
electrons in a system, thereby altering their steady-state dynamics
dramatically.
In a new Perspective paper published in eLight, a team of
scientists led by Professor Andrea Alù of the City University of New York
(CUNY) points out the window of opportunity offered at the confluence between
Floquet matter and metamaterials. Their Perspective paper highlights the
exciting opportunities emerging from their synergies.
One realm where Floquet physics has recently found fertile
ground is that of topological insulators, materials that host waves immune from
scattering off impurities or disorder in a material, and whose discovery led to
the 2016 Nobel Prize in Physics. Static topological insulators typically draw
their exotic properties from their specific spatial crystalline arrangement, or
on the application of a magnetic field. However, the periodic temporal
modulation in a Floquet systems can also produce a synthetic effective magnetic
field, which is not unique to electrons, but can thus be realized for
electromagnetic waves (photons), elastic vibrations in a solid material or air
(phonons), or even water waves, which do not normally experience the effects of
a physical magnetic field.
Optical implementations of Floquet systems have
traditionally been realized by replacing the temporal direction with a spatial
one. However, according to Noether's theorem, temporal inhomogeneities
intrinsically imply the presence of gain and loss in a system: The common
assumption of energy conservation does not generally hold in such a scenario,
whereby energy is exchanged with the external mechanism (which acts like an
energy bath) exerting the time-modulation. Owing to their intrinsic
non-equilibrium dynamics, Floquet topological systems can host unique features
not available within their static counterparts.
In parallel, metamaterials enable the tailoring of extreme
wave-matter interactions, and the temporal dimension has recently emerged as a
new degree of freedom to engineer exotic wave dynamics. This has included
time-reversal (namely the temporal analog of reflection at a boundary between
two media), nonreciprocity (direction-dependent wave propagation in a material)
and many other effects. Importantly, the metamaterial concept has now expanded
across most wave realms, offering an ideal platform where concepts which
originated in the Floquet physics community may flourish and find a rich
experimental playground.
However, the breadth of wave physics encompassed by
metamaterial concepts also brings its own exotic intricacies and wealth of
physical sophistication. For instance, most photonic systems feature an
intrinsic temporal retardation in their response to an impinging wave, which is
typically absent when solving the Schrodinger equation for matter waves such as
electrons. This effect, called dispersion (which lies behind the splitting of
white light into the rainbow colors by a prism, for example), introduces a rich
playground for designing new forms of material responses when the material
properties are switched in time at ultrafast speeds. These ultrafast (faster
than the wave period) changes in material properties mimic, in the temporal
domain, what in the metamaterials field are called meta-atoms: these are the
fundamental building blocks whose individual response and periodic arrangement,
give rise to the emergent properties of a metamaterial.
Hence, tailoring the specific temporal switching applied to
a meta-structure opens an unexplored avenue for the design of Floquet
metamaterials, structures where the synergy between the response of single
temporal meta-atoms and their emergent Floquet behavior can be leveraged for
the design of completely new forms of wave-matter interactions. Thus, this
confluence promises to enrich both fields with the development of novel
fundamental concepts, as well as a wealth of opportunities for experimental
implementations across all (classical) wave realms.
Reference:
Shixiong Yin et al, Floquet metamaterials, eLight (2022).
DOI: 10.1186/s43593-022-00015-1
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