Experimental demonstration of the microscopic origin of circular dichroism in two-dimensional metamaterials.

A B Khanikaev,Z Fan,M Schnell,F Lu,J Lee,G Shvets,R Hillenbrand,D Purtseladze,P Sarriugarte,N Arju,M A Belkin

doi:10.1038/ncomms12045

A B Khanikaev, Z Fan + Show 9 more

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https://doi.org/10.1038/ncomms12045

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Abstract

Optical activity and circular dichroism are fascinating physical phenomena originating from the interaction of light with chiral molecules or other nano objects lacking mirror symmetries in three-dimensional (3D) space. While chiral optical properties are weak in most of naturally occurring materials, they can be engineered and significantly enhanced in synthetic optical media known as chiral metamaterials, where the spatial symmetry of their building blocks is broken on a nanoscale. Although originally discovered in 3D structures, circular dichroism can also emerge in a two-dimensional (2D) metasurface. The origin of the resulting circular dichroism is rather subtle, and is related to non-radiative (Ohmic) dissipation of the constituent metamolecules. Because such dissipation occurs on a nanoscale, this effect has never been experimentally probed and visualized. Using a suite of recently developed nanoscale-measurement tools, we establish that the circular dichroism in a nanostructured metasurface occurs due to handedness-dependent Ohmic heating.

Highlights

Optical activity and circular dichroism are fascinating physical phenomena originating from the interaction of light with chiral molecules or other nano objects lacking mirror symmetries in three-dimensional (3D) space
It is not surprising that one of the earliest discovered manifestations of chirality belongs to optics: chiral materials are responsible for the phenomenon of optical activity that results in the rotation of the polarization state of light
Polarization rotation occurs because light of opposite handedness perceives different refractive indices and absorption coefficients when propagating through any homochiral material. These effects manifest themselves as unequal absorption rates and unequal phase advances for LCP and RCP light

Summary

Introduction

Optical activity and circular dichroism are fascinating physical phenomena originating from the interaction of light with chiral molecules or other nano objects lacking mirror symmetries in three-dimensional (3D) space. The most straightforward way of producing strong circular dichroism and birefringence is to mimic natural helical molecules, such as glucose, by synthesizing fully threedimensional (3D) chiral metamolecules that look like helices[5,6,7,8] Such artificial structures exhibit strong transmission difference between LCP and RCP polarization states of light, as well as handedness-dependent non-linear response[9]. In the absence of such losses on the nanoscale, the CDT defined as the difference DTCD 1⁄4 TR À TL between the transmission coefficients TR and TL of the RCP and LCP waves must identically vanish This surprising theoretical prediction has never been experimentally verified because of the challenge of measuring non-radiative loss on the nanoscale.

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