Exceptional point-based plasmonic metasurfaces for vortex beam generation.

An exceptional point occurring in a tailor-made lossy optical system has been recently found to alter optical properties in counter-intuitive ways. In the context of tunable plasmonic devices, exceptional points can be useful as a driving mechanism to enhance tunability. Here, we experimentally demonstrate how a plasmonic exceptional point can be incorporated in metasurface Q-plates to have the generated vortex beam tuned through a change of structural parameter. We have observed an orbital rotation in the far-field by 45 degrees in crossing the exceptional point. We expect a new generation of tunable plasmonic devices in polarization control, beam structuring and holograms, which can take advantage of the huge sensitivity from exceptional points.

Measurement of reflection phase using thick-gap Fabry-Perot etalon.

We report measurement of the reflection phase of a dielectric (glass)/titanium (Ti) surface in the visible wavelength using a thick-gap Fabry-Perot (FP) interferometry technique. Using a two-beam interference model for the reflection peaks and troughs of the FP etalon, we obtain the air-gap spacing of the etalon and, more importantly, the reflection phase of the etalon substrate. We find systematic dependence of the as-measured reflection phase on the air-gap spacing due to the numerical aperture effect of the measuring objective. However, the relative reflection phase of Ti with respect to glass is independent of the air-gap spacing. As a demonstration of our approach in the optical characterization of small metamaterial samples, we also measure the reflection phase of a micron-sized 2D Au sawtooth nanoarray. The experiment is in good agreement with the model simulation.

Manipulation of the polarization of Terahertz wave in subwavelength regime.

By generalizing the concept of spoof surface Plasmons (Science 305, 847), we analytically demonstrate that subwavelength quarter-wave and half-wave plates can be realized in a metal hole array (MHA) sandwiched by two thin-layer materials, whose optical responses can be characterized by their optical conductivities. These abilities of polarization conversion can be attributed to the novel eigenstates induced by the hybridization of the spoof surface plamsons with the current generated in the thin-layer. Due to this mechanism, the robustness of the system is promised. The analytic predictions are verified numerically by modeling the thin-layer material as an experimentally feasible topological-insulator/SiO2 multilayer. Moreover, the possibility of extending the principle to a broad range of materials is dicussed.

Giant plasmonic circular dichroism in Ag staircase nanostructures.

We demonstrate large circular dichroism (CD) in the visible range resulting from electromagnetic couplings in three-dimensional Ag staircase nanostructures. Analytical calculations using effective constitutive parameters show that the CD originates from chiral resonances of the staircase in which the induced magnetic dipole moment has components parallel or antiparallel to the induced electric dipole moment. The strength of the coupling as well as the CD can be tuned by varying the configuration (e.g. the strip width) of staircase nanostructure. More importantly we are able to realize such chiral resonances with large CD in the visible range in topologically similar chiral nanostructures fabricated using a simple shadowing vapor deposition method. Our simple staircase model demonstrates the effect of couplings between electric and magnetic dipole moments in producing large chiral responses in 3D nanostructures and can enhance the understanding of hybrid chiral optical systems.