Materials with mirror symmetry breaking exhibit differences in absorption for the two helicities of circular polarization, which is known as circular dichroism. It is a fundamental effect in electromagnetism, and yet it finds broad applications in analytical chemistry, molecular biology, and crystallography for identification and characterization of chiral molecules and solids. Giant CD at optical frequencies has been demonstrated in metamaterials, and such strong chirality effects are expected to open up novel opportunities in optics and photonics including negative index media.

We have unexpectedly discovered giant circular dichroism from individual air-suspended carbon nanotubes grown by chemical vapor deposition. In the figure below, photoluminescence spectra corresponding to excitation with right- and left-circularly polarized laser are shown. The difference in the emission intensity results from the difference in absorption for the two helicities. In our experiments, we have observed a polarization as high as 65% which is about 1000 times larger than a typical molecule.

Furthermore, we have investigated the dependence of the polarization on the nanotube angle as shown in the left figure below. The results are plotted in the right figure, and the bottom panel shows that the polarization is strongly dependent on the angle, even changing its sign.

It is quite surprising that such a giant response arises in a system as simple as a nanotube lying on a substrate. Our results demonstrate the feasibility of polarization manipulation with single nanomaterials. We hope that our finding will lead to new techniques for polarization manipulation at the nanoscale.

To learn more about this work, please refer to:
A. Yokoyama, M. Yoshida, A. Ishii, Y. K. Kato Giant circular dichroism in individual carbon nanotubes induced by extrinsic chirality Phys. Rev. X 4, 011005 (2014).