A. Potts

Broken time reversal of light interaction with planar chiral nanostructures.

We report unambiguous experimental evidence of broken time-reversal symmetry for the interaction of light with an artificial nonmagnetic material. Polarized color images of planar chiral gold-on-silicon nanostructures consisting of arrays of gammadions show intriguing and unusual symmetry: structures, which are geometrically mirror images, lose their mirror symmetry in polarized light. The symmetry of images can be described only in terms of antisymmetry (black-and-white symmetry) appropriate to a time-odd process. The effect results from a transverse chiral nonlocal electromagnetic response of the structure and has some striking resemblance with the expected features of light scattering on anyon matter.

Giant optical activity in dielectric planar metamaterials with two- dimensional chirality

For the first time, all-dielectric planar chiral metamaterials consisting of arrays of silicon nitride gammadions on fused silica substrates have been fabricated, and shown to be capable of inducing large changes to the polarization states of transmitted light in a manner that is dependent on the two-dimensional chirality of the microstructured silicon nitride film. The polarization response is found to reverse for opposite enantiomers, and also for the same enantiomer when it is illuminated from opposite sides of the structure. In addition, the polarization states of the various diffracted beams are found to be non-reversible. These structures therefore appear to display elements of non-reciprocal behaviour. The polarization responses of complementary designs, different chiral geometries and various silicon nitride film thicknesses have also been studied. As a result we conclude that multiple reflections within the patterned silicon nitride layer play an important role in defining the mechanism by which these structures are able to modify the polarization states of diffracted light.

Intensity modulation and polarization rotation of visible light by dielectric planar chiral metamaterials

We have demonstrated how dielectric planar chiral surfaces can both modulate the intensity and change the polarization state of visible light diffracted from patterned surfaces. These effects are shown to be dependent on the sense of chirality of the surface and the input polarization state of the light. Individual diffracted beams can show variations of over 30% in their intensities for different input polarization states while opposite enantiomeric structures can exhibit differences of over 50%.The size of these effects could make these surfaces particularly promising candidates for the development of solid-state polarization-state detectors.

A new model of geometric chirality for two-dimensional continuous media and planar meta-materials

We have, for the first time, identified ten tenets of two-dimensional (2D) chirality that define and encapsulate the symmetry and scaling behaviour of planar objects and have used them to develop three new measures of geometric 2D chirality. All three models are based on the principle of overlap integrals and can be expressed as simple analytical functions of the two-dimensional surface density, ρ(r). In this paper we will compare the predicted behaviour of these models and show that two of them are fully integrable and scalable and can therefore be applied to both discrete and continuous 2D systems of any finite size, or any degree of complexity. The only significant difference in these two models appears in their behaviour at infinite length scales. Such differences could, however, have profound implications for the analysis of chirality in new generations of planar meta-materials, such as chiral arrays, fractals, quasi-periodic 2D crystals and Penrose tiled structures.

Planar chiral meta-materials for photonic devices

We report the first optical results due to the effects of two-dimensional (2D) chirality. Using electron beam lithography we have fabricated arrays of metallic planar chiral structures on dielectric substrates, with characteristic dimensions in the near infra-red, and observed the effects of their chirality on the polarization states of diffracted light. The results showed strong polarization effects, with the handedness of the individual elements having a direct effect on the sense and magnitude of rotation of the diffracted light. We believe such structures could, in the future, form the core of a new class of meta-materials with refractive indices engineered to suit individual optical applications.

Layered chiral metallic meta-materials

Using electron beam lithographic techniques we have manufactured left and right-handed forms of an artificial medium consisting of high densities of microscopic planar chiral metallic objects distributed regularly in a plane. In this artificial medium we have for the first time observed optical manifestations of planar chirality in the form of handedness-sensitive rotation of the polarization state and elliptization of visible light diffracted from the structure. Applications of such media in functional materials are discussed.

Optical properties of planar chiral meta-materials

The polarization state of visible light is found to be altered upon reflection from artificial two-dimensional chiral media. Arrays of metallic planar chiral structures were fabricated by electron beam lithography and ion beam milling. The characteristic dimensions on the chiral elements correspond to wavelengths in the near-IR. Our chiral media are found to induce strong polarization effects, with the handedness of individual elements having a direct effect on the sense and magnitude of rotation of the diffracted light.

Planar chiral meta-materials: controlling the polarization state of light in the far- and near-field

We have extended our research to the study of near-field polarization effects arising from the planar chirality. The interaction between the neighboring elements in a metallic structure can be visualized with near-field measurements. Coupling between metallic gammadions that gives rise to the scattered electromagnetic field with polarization components different from the polarization state of the incident light can be directly identified. We have also investigated far-field effects in two-dimensional gratings consisting of regular patterns of dielectric gammadions on a silicon substrate.