Chiya Saeidi

Wideband plasmonic focusing metasurfaces

We present a technique for designing wideband focusing metasurfaces. The proposed metasurface consists of unit cells of nanoparticle-based spatial phase shifters distributed over a planar surface. The topology of each spatial phase shifter is based on the design of plasmonic frequency selective surfaces. A true ab-initio design procedure for the proposed reflectarray is also proposed for the desired bandwidth and center frequency. A reflectarray for focusing the entire red spectrum (75 nm) is designed, with full-wave simulation results demonstrating desired focusing.

Nanoparticle array based optical frequency selective surfaces: theory and design

We demonstrate a synthesis procedure for designing a bandstop optical frequency selective surface (FSS) composed of nanoparticle (NP) elements. The proposed FSS uses two-dimensional (2-D) periodic arrays of NPs with subwavelength unit-cell dimensions. We derive equivalent circuit for a nanoparticle array (NPA) using the closed-form solution for a 2-D NPA excited by a plane wave in the limit of the dipole approximation, which includes contribution from both individual and collective plasmon modes. Using the extracted equivalent circuit, we demonstrate synthesis of an optical FSS using cascaded NPA layers as coupled resonators, which we validate with both circuit model and full-wave simulation for a third-order Butterworth bandstop prototype.

Synthesizing frequency selective metasurfaces with nanodisks

We present a comprehensive method for synthesizing frequency selective surfaces (FSSs) composed of arrays of periodic nanodisks. The synthesis is based on the recently derived equivalent circuit of the nanoparticle arrays, whose element values are related to the physical, geometrical, and electrical parameters of the FSS, and can be determined from the desired frequency response. We validate the synthesis procedure with design examples of FSSs with second-order Chebyshev and third-order Butterworth bandstop responses.

Bandwidth-tunable optical spatial filters with nanoparticle arrays

Modeling a nanoparticle array (NPA) inside a thin glass slab as a lumped optical resonator, we propose a systematic approach to design for an efficient optical filter with bandwidth tunability. The quality factor and bandwidth of the resonator are related to the physical, geometrical, and electrical parameters of an NPA and its surrounding medium, whose permittivity is varied to change the bandwidth. We propose a structure amenable to our design approach consisting of an NPA slab surrounded by liquid crystal whose permittivity can be altered. We validate the design procedure with examples of tunable-bandwidth filters at different frequency regimes from NIR to blue.

A figure of merit for focusing metasurfaces

We propose and define a figure of merit (FoM) for designing focusing metasurfaces by establishing a reference case. We emphasize the important role of the focal point concept for obtaining the desired functionality. We suggest several design considerations along with the FoM for a careful design and performance evaluation of the prototype, without which the focusing property of a designed metasurface may be only slightly better than that of a planar scattering surface.

Spatial filter for optical frequencies using plasmonic metasurfaces

We present spatial filters using two-dimensional (2-D) nanoparticle arrays (NPAs). Using interaction polarizability, which has contributions from the isolated-particle response as well as the interparticle interaction, the resonance behavior of the NPA as a shunt resonator is extracted. Then, by cascading two or more NPAs, a spatial filter with desired bandwidth can be realized. The method is verified by designing a sample filter subject to a specified bandwidth in the visible.

Nanoparticle array for visual dielectric characterization of powder

We present a nanoparticle array (NPA) to visually predict the optical dielectric constant of a thin layer of powder. We validate the proposed concept with the full-wave simulation of a design example.