Chiral Metasurfaces

Abstract

Two dimensional chiral surfaces exhibit interesting polarization properties and can, by virtue of their additional symmetry-breaking, give rise to additional second harmonic generation signals. Here, we describe arrays of chiral nanostructures that form metasurfaces with dense sub-wavelength features, which can simultaneously exhibit multiple properties, including plasmonic enhancement and very strong optical activities. These chiral metasurfaces are based on a parallel fabrication scheme we have recently developed that allows shape-control at the nanoscale [1]. We have grown arrays of chiral helices that predominately contain silver, gold, copper, or oxides to name but a few of the materials. However, it is also possible to grow hybrid structures as well as alloys or magnetic materials. The scalability of the technique is unique as it permits the rapid structuring of nanophotonic elements at the wafer-scale. Chiroptical responses in transmission (optical rotation) and absorption (circular dichroism) are typically small effects. The chiroptical signal may be enhanced using amplification schemes, including weak value amplification [2], or by increasing the multipolar light-matter interaction itself. The latter is possible when the chiral object has a size that approaches the wavelength of light. Since we can fabricate structures that are of a similar length-scale as the wavelength of visible light, whilst being small enough to minimize scattering, large chiroptical signals can be recorded even in films that are only about a 100nm thick [1]. We show chiroptical measurements conducted on transparent substrates of wafer-scale metasurfaces [3–5]. In addition it is possible to study the same chiral nanostructures as a colloidal suspension, a “metafluid”. Dynamic alignment of the chiral nanocolloids permits spectral discrimination between those effects that arise in the chiral signal due to their orientation on a chiral metasurface and those that arise in the isotropic state. REFERENCES 1. Mark, A. G., J. G. Gibbs, T.-C. Lee, and P. Fischer, Nat. Mater., Vol. 12, 802, 2013. 2. Pfeifer, M. and P. Fischer, Opt. Express, Vol. 19, 16508, 2011. 3. Gibbs, J. G., A. G. Mark, T.-C. Lee, S. Eslami, D. Schamel, and P. Fischer, Nanoscale, Vol. 6, 9457, 2014. 4. Gibbs, J. G., A. G. Mark, S. Eslami, and P. Fischer, Appl. Phys. Lett., Vol. 103, 213101-1– 213101-4, 2013. 5. Eslami, S. J. G. Gibbs, Y. Rechkemmer, J. van Slageren, M. Alarc´on-Correa, T.-C. Lee, A. G. Mark, G. L. J. A. Rikken, and P. Fischer, ACS Photonics, Vol. 1, 1231–1236, 2014.