In this abstract, I will discuss our efforts in realizing ultra-sharp nanoantennas with radius of curvature of 1 nm and 5 nm nanogap for extreme photon squeezing applications. Firstly, we propose an unconventional nanolithography technique called cascade domino lithography (CDL) to realize such sharp tip structures. By controlling collapse of micro/nanopillar photoresist structures, diverse plasmonic nanogap structures or single-digit nanometer scale structures can be realized, which are not obtainable with conventional nanofabrications. Secondly, we analyze the optical characteristics of sharp bowtie nanoantennas by using numerical simulations and near/far-field experiments. For instances, an extreme field localization and enhancement are verified by finite-difference time-domain (FDTD) simulations along with near-field scanning optical microscopy (NSOM) measurement, nonlinear spectroscopy and surface-enhanced Raman spectroscopy (SERS).
Light-matter interactions involving structured optical field (SOF) and meta-atoms have been recently investigated. SOF allows a large degree-of-freedom in the interaction, and includes vortex beams with helical wavefront, cylindrical vector beams with spatially inhomogeneous polarization, and non-diffracting Bessel beams. Meta-atoms even further extend the degree-of-freedom. Therefore, accurate and efficient description of both SOFs and arbitrary meta-atoms is required to study this problem, for which multipole approach can be a versatile framework. Here, we discuss description of SOFs and their implementation into multipole approach, and demonstrate dichroisms involving chiral meta-atoms and SOFs with spatially inhomogeneous phase and polarization distributions.
Mie scatterer resonantly scatters when wavelength of incident light is similar to the size of the scatterer. The scattering of Mie resonator can be analyzed using multipole decomposition; silicon nanostructure has multipole scattering modes in visible regime. When the Mie scatterers are arrayed, the scattering response can be greatly amplified. To properly design array of Mie scatterer, i.e. metasurface, the hybridization of radiation mode of scatterer and lattice effect, i.e. guided-mode resonance (GMR), must be understood. Herein, we would like to provide the scattering mechanisms behind the hybridization between individual scattering mode and lattice effect, and use them to realize gradient structural coloration by silicon-based metasurface. We believe that a solid understanding of the coupling between individual Mie resonators and the lattice resonances can be a strong basis for designing planar spectral filters.
Higher-order dynamic polarizability tensors are formulated using the irreducible Cartesian basis, which makes polarizability tensors and Green’s tensors to be symmetric. Basis transformation matrices are presented, which systematically transform Cartesian multipoles and local field quantities to spherical multipoles. These newly presented expressions allow systematic retrieval of higher-order dynamic polarizability tensors and formulation of coupled multipole method up to magnetic octupole. This multipole framework is (semi-)analytic and computationally efficient, allowing description of electromagnetically coupled meta-atoms. Meta-atoms in periodic or random lattices and those dispersed in three-dimensional random- and large-scale systems may be analyzed using this method.
A simple scheme for T-matrix retrieval with higher efficiency and accuracy by taking particle symmetries into account is presented. T-matrix is calculated using matrix inversion, where the matrix elements are numerically calculated. This proposed scheme additionally generates extra matrix elements using the particle symmetries and reciprocity. This process consumes little effort in terms of computations, while the retrieved T-matrix shows improved accuracy for geometric symmetries and reciprocity. We also study required symmetry operations depending on the symmetries of meta-atoms (point group), and the shapes of their T-matrices. This scheme allows efficient and accurate calculation of T-matrix of meta-atoms for further calculations and group-theoretical analysis.
Chiral metamaterials consisting of periodic asymmetric unit cells have a different complex refractive index depending on spin-states of impending light. These phenomena can be applied to chiral sensing applications, so enhanced chiral responses have been attracted to develop advanced spectroscopic devices. However, chiral devices, which made of metallic chiral metasurfaces, have weak circular dichroism due to ohmic loss of metal. Here, we proposed simulation results of chiral metasurfaces consist of helically located gold metallic nanodisc, called oligomers. The oligomers are located with C4 chirality, resulting in non-conversion efficiency for reducing noise when they are used for spectroscopy. Also, the oligomers have ultra-sharp circular dichroism that has been rarely reported in the near-infrared region. These results may have wide applications, including spectroscopy, thermal detectors, and biochemical distributors.
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