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California Team Reports Breakthrough for Transformation Optics

by Editor1 last modified July 15, 2010 - 14:38

California researchers have developed computer simulations showing it may be possible to transform ‘optical space’ marrying transformation optics with a related field of electronics known as plasmonics.

California Team Reports Breakthrough for Transformation Optics

Yongmin Liu (left) Xiang Zhang and Thomas Zentgraf used sophisticated compuer modeling to develop a “transformational plasmon optics” technique. that may open the door to practical integrated, compact optical data-processing chips. (credit: Roy Kaltschm

The approach could enable advances in optics on several fronts, including design and production of beam splitters and shifters; directional light emitters; and integrated, compact optical data-processing chips, researchers said

The idea of “transformation optics’ is to transform the physical space through which light travels, sometimes referred to as “optical space.” The principals are similar to the way outer space is transformed by the presence of a massive object under Einstein’s relativity theory.

With transformation optics, light waves can be controlled at all lengths of scale through metamaterials, complex composites typically made from metals and dielectrics – insulator materials with a high polarizability, or that become polarized in the presence of an electromagnetic field.

But, scientists have found it’s difficult to modify the physical properties of metamaterials at the nanoscale (or subwavelength scale). So, researchers have been looking for alternatives.

DoE, UC Team’s Work Transforms
Transformation Optics with Plasmons
A team of scientists from the Department of Energy’s Lawrence Berkeley National Laboratory and the University of California-Berkeley, modeled an approach they call “transformational plasmon optics,” which focused on manipulation of the dielectric material adjacent to a metal -- but not the metal itself.

The simulations show the promise of allowing SPPs to travel across uneven and curved surfaces over a broad range of wavelengths without suffering significant scattering losses. The team is led by Xiang Zhang, director of UC Berkeley’s Nano-scale Science and Engineering Center (SINAM)

The approach, focused on dielectric interface, is to bypass the metals aspect of metamaterials altogether, making only moderate changes to the metamaterial’s dielectric component could render measurable transformation optics results.

Surface plasmons (SPs) are coherent electron oscillations that exist at the interface between any two materials where the real part of the dielectric function changes sign across the interface (e.g. a metal-dielectric interface, such as a metal sheet in air). When SPs couple with a photon, the resulting hybridised excitation creates an surface plasmon polariton (SPP).

“Since a significant portion of SPP energy is carried in the evanescent field outside the metal, that is, in the adjacent dielectric medium, we proposed to control SPPs by keeping the metal property fixed and only modifying the dielectric material based on the transformation optics technique,” Zhang said.

A Practical Way to
Route Light at Nanoscale
“Since the metal properties in our metamaterials are completely unaltered, our transformational plasmon optics methodology provides a practical way for routing light at very small scales,” Zhang says. “Our findings reveal the power of the transformation optics technique to manipulate near-field optical waves, and we expect that many other intriguing plasmonic devices will be realized based on the methodology we have introduced.”

The key to the success of this dielectric approach is in manipulation of a plasmon, the electronic surface wave that rolls through conduction electrons on a metal. The Zhang team’s approach is based on these findings:

  1. Just as the energy in waves of light is carried in photons, quantized particle-like units, plasmonic energy is carried in quasi-particles called plasmons.
  2. Plasmons interact strongly with photons at the interface of a metamaterial’s metal and dielectric surface to form a different type of quasi-particle, called a surface plasmon polariton (SPP).
  3. The intensity of SPPs is maximal at the interface between a metal and a dielectric medium and exponentially decays away from the interface.
  4. Manipulating these SPPs, rather than the metals, lies at the heart achieving transformation optics because scattering is dramatically suppressed when the optical space around the protrusion is transformed.

The team’s effort to develop full-wave simulations of different transformed designs proved that the proposed methodology was correct. The team also demonstrated by using a “prudent transformational plasmon optics scheme” the transformed dielectric materials can be isotropic and nonmagnetic,” which the team found boosts the practicality of this approach even further.

Use of a 180-degree plasmonic bend with almost perfect transmission was especially significant, according to researchers.