We present a theory of the microscopic origins of the surface-enhanced circular dichroism (CD) with nanostructures. Recently, nanostructures and metamaterials have been used for the enhancement of CD signals of chiral molecules. However, the complete description of microscopic origins of the surface-enhanced circular dichroism (CD) has never been achieved. We find the total CD signals of the nanostructure coupled to chiral molecules can be decomposed into two factors: the induced and inherent CD. The inherent CD comes from the molecular absorption which can be enhanced by the strongly localized optical helicity density of resonant near-fields near the nanostructure. The induced CD is originated from the asymmetric absorption of light inside the nanostructure perturbed by nearby chiral molecules upon two opposite circularly polarized light. The recent surge of interest in the surface-enhanced CD spectroscopy has been inspired by the inherent CD enhancement, but our work shows that the induced CD can significantly contribute to the total CD signal of the chiral molecule/nanostructure coupled system. Using an example of gold nanodisk arrays, we demonstrate that the inherent and induced CD can compete with each other in plasmonic nanostructures. In this presentation, we also provide design principles for CD sensor using nanostructures and metamaterials.
Due to the relatively weak birefringence of natural materials in terahertz regime, metasurfaces have been proposed for compact terahertz phase modulators since they show effectively strong birefringence only with ultrathin structures. However, previous designs of metasurface show limited phase modulation reaching only up to the quarter-wavelength phase, and there has been no single metasurface design that works for a terahertz half-waveplate. Here, we present a metasurface that modulates the phase variably up to 180 degrees. The phase modulation is achieved by a hyperbolic metasurface composed of periodically arrayed rectangular metal rings with different periods for horizontal and vertical axis. By controlling each period, we show that our hyperbolic metasurface can possess large positive and negative permittivity values for horizontal and vertical axis and the phase shift can reach up to the 180 degrees. To check the validity of our design, we fabricate reconfigurable metasurface films and demonstrate the phase modulation 90 to 180 degrees. All results show good agreement with numerical simulation results.