Wensheng Gao

Circular dichroism in double-layer metallic crossed-gratings

We report on the fabrication of double-layer gold crossed-gratings consisting of a convoluted top gold grating superposed on a bottom gold grating by using an e-beam direct write technique together with a lift-off process. The crossed-gratings exhibit, in the visible range, strong circular dichroism which is dependent upon the incident direction due to the convoluted top gold grating and also the substrate. Resonance dips in the transmittance of circularly polarized light are also observed. The experimental results are explained qualitatively by simulations using a finite-integration technique. The simulations confirm that the dips in the transmittance are electromagnetic resonances corresponding to parallel and anti-parallel current flows in the crossed-gratings.

Circular dichroism in single-layered gold sawtooth gratings

We report on the fabrication of single-layered gold sawtooth gratings consisting of И, V, and N basic units by using an e-beam direct write technique together with a lift-off process. The И- and N-type sawtooth gratings are 2D chiral but not the V-type sawtooth grating. Large circular dichroism (CD) in the visible range is observed for the chiral sawtooth gratings while there is none for the V-type sawtooth grating. The experimental results are modelled qualitatively by simulations using a finite integration technique. The simulations show that the CD observed is due to the difference in the absorption in the metal for circularly polarized incidence light of different handedness. The sawtooth gratings are simple and can be easily mass produced for possible applications in waveplates and circular polarizers.

Optical activities in complementary double layers of six-armed metallic gammadion structures

We report comprehensive measurements of optical activities, polarization shift and circular dichroism, in the visible range for double-layered structures consisting of a thin top air-hole on a metal layer and a bottom complementary metal layer of six-armed gammadion arrays. Large polarization shift and normalized circular dichroism are observed for gammadions with arms jointed together at gammadion centres. The activities are smaller for disjointed-arm gammadions. In contrast, no activities are observed for single-layered gammadions obtained from the double-layered structures by a lift-off process, indicating that the three-dimensional nature of the complementary gammadion structures is essential in producing large optical activities. Furthermore, the double-layered complementary gammadion structures obey the reciprocal property despite the asymmetric nature of the complementary samples.

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.

Multipolar Effects in the Optical Active Second Harmonic Generation from Sawtooth Chiral Metamaterials.

Based on the facts that chiral molecules response differently to left- and right-handed circular polarized light, chiroptical effects are widely employed for determining structure chirality, detecting enantiomeric excess, or controlling chemical reactions of molecules. Compared to those in natural materials, chiroptical behaviors can be significantly amplified in chiral plasmonic metamaterials due to the concentrated local fields in the structure. The on-going research towards giant chiroptical effects in metamaterial generally focus on optimizing the field-enhancement effects. However, the observed chiroptical effects in metamaterials rely on more complicated factors and various possibilities towards giant chiroptical effects remains unexplored. Here we study the optical-active second harmonic generation (SHG) behaviors in a pair of planar sawtooth gratings with mirror-imaged patterns. Significant multipolar effects were observed in the polarization-dependent SHG curves. We show that the chirality of the nanostructure not only give rise to nonzero chiral susceptibility tensor components within the electric-dipole approximation, but also lead to different levels of multipolar interactions for the two orthogonal circular polarizations that further enhance the nonlinear optical activity of the material. Our results thus indicate novel ways to optimize nonlinear plasmonic structures and achieve giant chiroptical response via multipolar interactions.

Controlling interface states in 1D photonic crystals by tuning bulk geometric phases

There is no known simple rule that assures the existence of interface states in photonic crystals (PCs). We show here that one can control the existence or absence of interface states in 1D PCs through engineering the bulk geometrical phase such that interface states can be guaranteed in some or all photonic bandgaps. We verify experimentally the interface state design paradigm in 1D multilayered PCs fabricated by electron beam vapor deposition. We obtain the geometrical phases by measuring the reflection phases at the bandgaps of the PCs and achieve good agreement with the theory. Our approach could provide a platform for generating a PC interface state for various applications.