Chirality and metamaterials

Abstract

Electromagnetic metamaterials are artificial media that derive novel properties from periodic structuring on the sub-wavelength scale. Here, the consequences of two-dimensional (2D) and three-dimensional (3D) chirality for the electromagnetic properties of metamaterials are investigated. The focus of this work is on new ways of achieving circular conversion dichroism, optical activity and negative refraction in highly symmetric structures.

In the theoretical part of this work, fundamental constraints on polarization effects in planar metamaterials are established based on symmetry and energy conservation considerations.

Through the experimental study of 2D chirality, I have first observed circular conversion dichroism (i) in non-chiral structures and (ii) due to 2D-chiral arrangement of non-chiral elements. (iii) I have first seen enantiomerically sensitive reflection, yielding the experimental demonstration that circular conversion dichroism results in simultaneous directional asymmetries in transmission, reflection and absorption. In particular, a tunable transmission asymmetry of up to 21 % has been observed when extrinsic 2D chirality was associated with oblique incidence onto a non-chiral meandering wire pattern. At normal incidence circular conversion dichroism was seen for non-chiral split ring elements assembled into a 2D-chiral double-periodic array. Simultaneous directional and enantiomeric asymmetries in transmission (16 %), reflection (16 %) and absorption (32 %) were observed for normal incidence onto a double-periodic array of 2D-chiral split rings.

Regarding 3D chirality, I have (i) realized the first material with a negative refractive index due to chirality and (ii) observed optical activity in the first stereometamaterial. (iii) I have discovered that optical activity can be observed in non-chiral metamaterials and (iv) I have demonstrated that optical activity in such structures is tunable and occurs in transmission and reflection. In particular polarization rotation reaching 81° and circular dichroism of up to 26 dB have been observed for non-chiral arrays of split rings, when an extrinsically 3D-chiral experimental arrangement was formed by metamaterial and direction of incidence. Based on a previously-studied meta-molecule consisting of mutually twisted metal patterns in parallel planes, microwave and photonic stereometamaterials with optical activity have been realized in this thesis and such a structure has been shown to have a negative refractive index of -1.7 for right-handed circularly polarized microwaves.

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Identifiers

ORCID for Eric Plum: orcid.org/0000-0002-1552-1840

ORCID for N.I. Zheludev: orcid.org/0000-0002-1013-6636

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