We report that normal incidence transmission of circularly polarized light through lossy anisotropic planar chiral meta-material is asymmetric for opposite directions. The new effect is fundamentally distinct from conventional gyrotropy of bulk chiral media and the Faraday Effect

Chen, Yifang

Fedotov, Vassili A.

Khardikov, Vyacheslav V.

Prosvirnin, Sergey L.

Schwanecke, Alexander S.

Zheludev, Nikolay I.

English

Southampton (e-Prints Soton)

Asymmetric Transmission of Light through a Planar Chiral Metamaterial  Alexander S. Schwanecke, Vassili A. Fedotov, and Nikolay I. Zheludev EPSRC NanoPhotonics Portfolio Centre, School of Physics and Astronomy, University of Southampton, SO17 1BJ, UK Email: A.S.Schwanecke@soton.ac.uk  Vyacheslav V. Khardikov and Sergey L. Prosvirnin Institute of Radio Astronomy and Kharkov National University, Kharkov, 61002, Ukraine  Yifang Chen Central Microstructure Facility, Rutherford Appleton Laboratory, Didcot, Oxon, OX11 0QX, UK  Abstract: We report that normal incidence transmission of circularly polarized light through lossy anisotropic  planar  chiral  meta-material  is  asymmetric  for  opposite  directions.  The  new  effect  is fundamentally distinct from conventional gyrotropy of bulk chiral media and the Faraday Effect.  In this paper, we show that a previously unknown fundamental phenomenon of electromagnetism can be observed in the  optical  part  of  the  spectrum.  It  is  a  polarization  sensitive  transmission  effect,  asymmetric  with  respect  to  the direction of wave propagation, and results from the interaction of an electromagnetic wave with a nano-structured planar chiral meta-material. The new effect partially resembles the famous non-reciprocity of the Faraday Effect in magnetized media, but crucially requires no magnetic field. Both in the Faraday Effect and in that produced by planar chirality, transmission and retardation of a circularly polarized wave are different in opposite directions.  There are also essential differences between the two  phenomena.  The  asymmetry  of  the  Faraday Effect applies to the transmission and retardation of the incident circularly polarized wave itself, whereas the planar chirality effect leads to partial conversion of the incident wave into one of opposite handedness, and  it  is  the  efficiency  of  latter  that  causes  the asymmetry.  The  predicted  effect  is  also  radically different  from  conventional  gyrotropy  in  3D-chiral media,  which  is  completely  symmetric  for  waves propagating in opposite directions. The  asymmetric  transmission  phenomenon  can only  be  observed  if  both  planar  chirality  and anisotropy  are  present  in  the  structure  of  a  meta-material. It is therefore forbidden in isotropic planar chiral arrays of high symmetry such as array of 4-fold gammadions. The new propagation phenomenon was initially discovered experimentally in the microwave part of the spectrum [1]. It was then found that it extends into the  visible  to  near-IR  parts  of  the  spectrum  using pseudo-spectral time domain (PSTD) modeling. The chiral 'fish-scale' is a continuous 2D pattern of tilted meanders periodic in both directions (see figure) existing in two enantiomeric forms interconnected by reflection across a mirror line in the plane of the structure. The composition of the modeled metamaterial included a 50 nm thick aluminium layer patterned on sub-wavelength scale (meander strip width also 50 nm) and a much thicker supporting silica substrate. The square unit cell of the double-periodic pattern of 440 x 440 nm² ensured that the nano-structured meta-material would not diffract, even in the visible spectrum. The predicted effect, expected for a nano-structured anisotropic planar chiral meta-material in the optical part of the spectrum, results from 2D chirality and anisotropy of the metallic nano-structure and is inherently linked to dissipation in the metal. The discovery may result in the development of new spectroscopic techniques and may find numerous optoelectronic applications. For instance, when placed into a ring resonator, it will enforce generation of left and right elliptically polarized waves in opposite directions of circulation.   [1]  V. A. Fedotov, P. L. Mladyonov, S. L. Prosvirnin, A. V. Rogacheva, Y. Chen, N. I. Zheludev, “Circular conversion dichroism in planar chiral metamaterials,” Phys. Rev. Lett., in press (2006) [arXiv: physics/0604234]. Fig.: The difference in transmission for forward (dashed line) and  backward  (solid)  propagation  (nano-wire  layer considered  the  front)  of  right  circularly  polarized  light incident on planar chiral fish-scale nanostructures is shown. The  difference  in  transmission  is  the  signature  of  the discovered  asymmetry.  The  inset  depicts  an  accordingly manufactured  Al  on  SiO2  nanostructure,  seen  through  a scanning electron microscope. 

Asymmetric transmission of light through a planar chiral metamaterial

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