Chiral media
For example, the molecules of cholesteric liquid crystals are randomly positioned but macroscopically they exhibit a helicoidal orientational order.
Other examples of structurally chiral materials can be fabricated either as stacks of uniaxial laminas or using sculptured thin films.
For example, the propagation direction of a beam of light through an achiral crystal (or metamaterial) can form an experimental arrangement that is different from its mirror image.
[7] Bunn[8] predicted in 1945 that extrinsic 3d chirality would cause optical activity and the effect was later detected in liquid crystals.
Polarization of an electromagnetic wave is the property that describes the orientation, i.e., the time-varying direction and amplitude, of the electric field vector.
For example, the electric field vectors of left-handed or right-handed circularly polarized waves form helices of opposite handedness in space as illustrated by the adjacent animation.
Polarizations are described in terms of the figures traced by the electric field vector as a function of time at a fixed position in space.
The projection of the tip of the electric field vector upon any fixed plane intersecting, and normal to, the direction of propagation, describes a circle.
As a consequence, left- and right-handed circularly polarized waves accumulate different amounts of phase upon propagation through a chiral medium.
Extrinsic 3d chirality associated with oblique illumination of metasurfaces lacking two-fold rotational symmetry leads to large specular optical activity.
[28] At microwave frequencies, a 12 orders of magnitude stronger effect than in lithium iodate was observed for an intrinsically 3d-chiral structure.
2D-chiral materials, which are also anisotropic and lossy exhibit different total transmission (reflection and absorption) levels for the same circularly polarized wave incident on their front and back.
Like the twist of a 2d-chiral pattern appears reversed for opposite directions of observation, 2d-chiral materials have interchanged properties for left-handed and right-handed circularly polarized waves that are incident on their front and back.
In particular left-handed and right-handed circularly polarized waves experience opposite directional transmission (reflection and absorption) asymmetries.
[32] The concept has been exploited in holography to realize independent holograms for left-handed and right-handed circularly polarized electromagnetic waves.
[34] 3D chirality of anisotropic structures is associated with directionally asymmetric transmission (reflection and absorption) of linearly polarized electromagnetic waves.
[37] This article incorporates public domain material from websites or documents of the United States Department of Energy.
