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From: "Jonathan Post"
The shape of things, illuminated: Metamaterials, surface topology and
light-matter interactions
April 28, 2012 by Stuart Mason Dambrot
http://phys.org/news/2012-04-illuminated-metamaterials-surface-topology-light-matter.html
(Phys.org) -- Finding new connections between different disciplines
leads to new – and sometimes useful – ideas. That's exactly what
happened when scientists in the Department of Physics, Queens College,
City University of New York (CUNY), in collaboration with City College
of CUNY, Purdue University and University of Alberta, leveraged
mathematical topology to create an artificially nanostructured
anisotropic (exhibiting properties with different values when measured
along axes in different directions) metamaterial that can be switched
from a non-conductive dielectric state to a medium that behaves like
metal in one direction and like a dielectric another. The
metamaterial's optical properties was mapped onto a topological
transformation of an ellipsoidal surface into an hyperboloid – and
transitioning from one to the other dramatically increases the photon
density, resulting in dramatic increase in the light intensity inside
the material. The researchers state that by allowing
topologically-based manipulation of light-matter interactions, these
types of metamaterials could lead to a wide range of photonic
applications in solar cells, light emitting diodes, displays, and
quantum computing and communications.
Associate Professor Vinod M. Menon recalls that the project started
out with theoretical predication and computational modeling. "Our
subsequent experimental work was based on computational modeling of
the structures and the anticipated effects," he relates to Phys.org.
"At that point, the main challenge in describing the
ellipsoid-to-hyperboloid transition was the design of the structure
that will show the transition in the relevant spectral range.
Relatedly," Menon continues, "showing that this topological transition
manifests itself in increased rates of spontaneous emission of
emitters positioned near the metamaterial required the identification
of a suitable light emitting material. In our case, that material was
quantum dots." This critical choice of emissive material allowed the
researchers to study the enhancement in spontaneous emission in both
the elliptical and hyperbolic ranges.
Artist's interpretation of the optical topological transition
occurring in metamaterials. Here the transformation from an ellipsoid
to a hyperboloid (left) is associated with a huge increase in the
light intensity inside the metamaterial (right). Courtesy Vinod Menon
| Animation created by Vladimir Shuvayev / Queens College - CUNY.
Menon is equally to-the-point when describing the key insights,
innovations and techniques the team used to address the above
challenges. "In addition to the right photon emission source and a
suitable material system for metamaterial fabrication, it was
necessary to come up with an appropriate control sample to isolate the
effect that we were looking for."
Menon adds that the team's next steps are to reduce optical losses,
improve the quality of silver films, and look into new material
systems that will show similar effects. "Silver is the metallic
component in the metamaterial that allows us to realize the
anisotropy. Theoretically one could use any metal or even doped oxides
and semiconductors. In our case silver was used because of the lower
optical losses in the visible wavelength range, but the roughness of
silver layers used in the present structure is an issue. This will
have to be addressed in the next round of experiments," Menon
cautions. "Additionally, the optical losses in the material need to be
alleviated. Finally, approaches to enhance the transmission properties
need to be addressed for light emitting applications."
According to Menon, the team's findings impact the development of new
routes to manipulating light-matter interactions through using
metamaterials and controlling the topology of the iso-frequency
surface – that is, one having a constant frequency. "The structure
that we demonstrated shows a large increase in the light intensity
over a wide spectral range," he explains. "Such structures can help in
enhanced light harvesting, which could result in more efficient solar
cells. One could also envision using these to develop single photon
sources necessary for quantum communication protocols and quantum
computers. Finally, through engineering of transmission properties of
these systems, and by combining them with light emitters, one may also
realize super bright LEDs that would be useful for display
applications."
Venturing further afield, Menon descries more exotic possibilities.
"Ideas of light manipulation used here could be extended for control
of thermal properties as well. More esoteric are the proposed ideas of
realizing a table top optical black hole and manipulation of
space-time curvature – and in fact, these proposals have been recently
made1,2 by one of my co-authors, Evgenii Narimanov, and his
collaborators."
More information: Topological Transitions in Metamaterials, Science 13
April 2012: Vol. 336 no. 6078 pp. 205-209, doi:
10.1126/science.1219171
Related:
1Optical black hole: Broadband omnidirectional light absorber, Applied
Physics Letters 95, 041106 (2009), doi: 10.1063/1.3184594
2Metric Signature Transitions in Optical Metamaterials, Physics Review
Letters 105, 067402 (2010), doi: 10.1103/PhysRevLett.105.067402
