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Experimental Demonstration of the Microscopic Origin of Circular Dichroism in Two-Dimensional Metamaterials

January 17, 2017

Schnell, R. Hillenbrand, M. A. Belkin and G. Shvets

Key points

  • AFM-IR and s-SNOM have been used in combination to probe the role of chirality in the circular dichroism observed in nanoscale 2D metasurfaces
  • By accessing both the radiative (s-SNOM) and non-radiative (AFM-IR) information on plasmonics structures, unique and complementary plasmonic properties can be obtained.
  • s-SNOM is used to map the optical energy distribution of 2D metasurfaces.
  • AFM-IR is used to detect the Ohmic heating in the structure.
  • For the first time, it has been conclusively established the circular dichroism observed in 2D metasurfaces is attributed to handedness dependent Ohmic heating.


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.

Because chloride ions modify the perovskite lattice, thereby widening the bandgap, measuring the bandgap locally yields the local chloride content. After a mild annealing (60 min, 60°C) the films consist of Cl-rich (x < 0.3) and Cl-poor phases that, upon further annealing (110°C), evolve into a homogenous Cl-poorer (x < 0.06) phase, suggesting that methylammonium-chrloride is progressively expelled from the film.Despite the small chloride content, CH3NH3PbI3-xClx films show better thermal stability up to 140°C with respect CH3NH3PbI3 films fabricated with the same methodology.