In this paper, an improved algorithm for extracting the effective constitutive parameters of a metamaterial is derived. The procedure invokes the Kramers-Kronig relations to ensure the uniqueness of the solution. The accuracy of the method is demonstrated by retrieving the effective material parameters of a homogeneous slab. This study reveals under which conditions the calculation of the refractive index involves more than one branch of the complex logarithmic function. A metamaterial built up from wires and split-ring resonators is then investigated. The applicability and limits of the presented algorithm are explored by observing how the effective parameters of a metamaterial slab converge as its thickness is increased.

We present the next generation covert plasmonic security labels based on Ag nanowire structures and their polarization dependent surface-enhanced Raman scattering (SERS) imaging. The security labels consist of Ag nanowires fabricated by two-photon lithography and thermal evaporation, where molecular probes of choice are deposited. Simulation and experimental results show that the SERS signals from the embedded molecules depend significantly on the polarization of the incident field. The covert molecular information cannot be revealed directly from the physical features, but can only be read-out selectively by polarization-dependent SERS imaging. Our plasmonic security labels exhibit very narrow spectral fingerprint vibration, which is more specific than broadband colorimetry-based systems. The polarization-dependent SERS intensity, molecular fingerprint of SERS spectra, and versatile geometrical design by two-photon lithography have made our plasmonic Ag nanowire structures an ideal candidate as advanced security solutions for anti-counterfeiting application.

We review recent progress in the application of Deep Learning (DL) techniques for photonic nanostructure design and provide a perspective on current limitations and fruitful directions for further development.

The single mode hybrid dielectric-loaded plasmonic waveguide is presented at the wavelength of $1.55\mu \text{m}$ . We show that this waveguiding structure, consisting of a low-index ${\text{SiO}}_2$ -stripe sandwiched between a high-index Si-nanowire and a silver film, achieves both long propagation length and strong field confinement with high power intensity. Components such as 90°-circular and S-shaped bends, based on the proposed waveguide with an intensity confinement area of $50\times 200{\text{nm}}^2$ , can obtain a total transmission efficiency exceeding 85% for various bend radii. Finally, we demonstrate that the efficient directional couplers can be developed using two coupled waveguides. In particular, we determine the typical coupling lengths and maximum transfer power for different structural parameters of the coupler. These investigations provide the foundations for the design of chip-scale integrated plasmonic circuitry.