PerspectiveApplied Physics

Structured Light Meets Structured Matter

See allHide authors and affiliations

Science  31 Aug 2012:
Vol. 337, Issue 6098, pp. 1054-1055
DOI: 10.1126/science.1226204

Metamaterials and singular optics are two fascinating branches of modern optics that until recently were rapidly developing in parallel yet independently. The former considers “simple” linearly or circularly polarized light or Gaussian beam propagation in “complex” materials with properties not found in nature. However, light can be a more complex phenomenon; in addition to conventional polarization states (spin), light beams can be radially or azimuthally polarized and carry orbital angular momentum (OAM). Structured light beams, containing phase or polarization singularities, enable properties and applications such as diffraction-free and self-healing propagation, single-molecule spectroscopy, nanoscale focusing, and even particle acceleration. A fascinating example of a beam carrying OAM is the optical vortex—a donut-shaped beam with a helical phase front (see the figure, panel A) (13).

The presence of a singularity can be observed by interfering a vortex beam with either a copropagating or tilted Gaussian beam, resulting in a spiral or fork-shaped interference pattern, respectively. Vortices play an important role in a number of physical processes, ranging from microscopic structures of superfluid helium to macroscopic structures of tornadoes.

When light propagates in a vacuum or a homogeneous, isotropic, nondispersive transparent medium, both spin and orbital angular momentum are independently conserved. However, if the medium is more complex, either anisotropic or inhomogeneous, the spin or angular momentum can change, which leads to spin-orbit interaction. Such a spin-orbit interaction leads to the mutual influence of the polarization and the trajectory of the beam propagation, as revealed in the geometrical Berry phase (4), the topological spin transport or intrinsic spin Hall effect (5), and new regimes of nonlinear optical processes (6).

Metamaterials enable unprecedented control over light propagation, opening new avenues for using spin and quantum optical phenomena, and design flexibility facilitating new linear and nonlinear optical properties and functionalities, including negative index of refraction, magnetism at optical frequencies, giant optical activity, subwavelength imaging, cloaking, dispersion engineering, and unique phase-matching conditions for nonlinear optical interactions. Provided that metamaterials can be engineered to realize nearly any imaginable optical properties, they are expected to change light-matter interactions of structural light. I will discuss examples of initial studies and outline directions where a synergy of the two fields may lead to a breakthrough.

The realization that the spin of photons provides an additional degree of freedom in nanoscale photonics led to the development of the field of spin optics (2). The possibility of using the plasmonic geometric phase was used to realize spin-dependent plasmonic focusing lenses (see the figure, panel B). Ultrathin metamaterial metasurfaces were also used to imprint abrupt discontinuities (vortices) on propagating light at mid-infrared frequencies (see the figure, panel A) (1). The recently demonstrated optical metasurfaces are likely to open new possibilities for the development of beam shaping and steering, plasmonic lenses, and other ultrathin components for optics on a chip (7).

In terms of signal processing, it was predicted that OAM could be used to encode information for quantum and classical systems (8). The combined use of different degrees of freedom of a single photon, such as spin and orbital angular momentum, enables the implementation of entirely new quantum information systems in a multidimensional space (9, 10). To date, most experimental studies, including field trials, were performed with wireless systems at radio frequencies. Can, then, OAM states be used for on-chip optoelectronic signal processing? This application requires the development of compact encoding and logic components, bringing us to the domain where metamaterials become indispensable.

So far, the discussion has focused on how metamaterials would modify light-matter interactions of structured light. However, singular optics can contribute to the development of complex metamaterial structures, as described in a recent demonstration of chiral metal nanoneedles formed by helicity transfer from vortex to metal (see the figure, panel C) (3). Chiral metamaterials enable nanoscale determination of the chirality and optical activity of molecules and chemical composites. Potential applications include nanoscale imaging, sensing, and optically active metamaterial surfaces.

Light in a spin.

(A) (Top) Calculated far-field intensity distribution of an optical vortex with topological charge one, spiral pattern created by the interference of the vortex beam and a copropagating Gaussian beam, and interference pattern with a dislocated fringe created by the interference of the vortex beam and a Gaussian beam tilted with respect to the vortex beam (1). (Bottom left) Scanning electron microscope image of a plasmonic metasurface that creates an optical vortex. (Bottom right) Helical wavefront [Courtesy of U. T. Schwarz]. (B) Spindependent plasmonic lens based on a geometric phase (2). (C) Optical vortex-controlled chirality of twisted metal nanostructures (3).

Another area where preshaping of optical beams using metamaterials may play a major role is light filamentation. Initial studies using vortex-preshaped femtosecond laser pulses indicate the possibility of achieving repeatable and predictable spatial and temporal distributions of the filaments (11). Metamaterials are expected to expand the capabilities of existing vortex optics for simultaneous control of intensity, polarization, dispersion, and phase properties of the beam for controlled formation of multiple filaments and filament arrays that would facilitate virtual waveguides for transporting and manipulating microwave radiation in air using filaments. Controlled filamentation of intense femtosecond pulses propagating in air may lead to various applications, such as remote sensing, light detection and ranging, and even lightning control.

Dispersion is one of the fundamental effects that describe the propagation of light in media, and it plays a key role in fiber-optic communication systems, nonlinear parametric interactions, supercontinuum generation, and solitons. The development of metamaterials with precisely tailored dispersion profiles will increase the efficiency of the spontaneous parametric down-conversion effect, an effect that was suggested as a source of high-dimensional states entangled in OAM (12).

Metamaterials are poised to bring new dimensions to the science and applications of complex light, including novel regimes of spin-orbit interaction, extraordinary possibilities for dispersion engineering, novel possibilities for nonlinear singular optics, trapping and optomechanical micromanipulation, as well as potential for applications in optical signal processing.

References and Notes

  1. Acknowledgments: The author appreciates discussions with M. Berry, A. Boardman, A. N. Cartwright, R. T. Hammond, M. Padgett, A. Pandey, M. Richardson, M. Segev, V. M. Shalaev, G. Swartzlander, J. P. Torres, X. Wang, and J. Zeng and acknowledges support of the U.S. Army Research Office under awards W911NF-11-1-0333 and Multidisciplinary University Research Initiative grant W911NF-11-0297.
View Abstract

Navigate This Article