Refractive micro-optical elements for surface plasmons: from classical to gradient index optics

Excite Spoof Surface Plasmons with Tailored Wavefronts Using High-Efficiency Terahertz Metasurfaces

Spoof surface plasmons (SSPs) play crucial roles in terahertz (THz) near-field photonics. However, both high-efficiency excitation and wavefront engineering of SSPs remain great challenges, which hinder their wide applications in practice. Here, a scheme is proposed to simultaneously achieve these two goals efficiently using a single ultracompact device. First, it is shown that a gradient meta-coupler constructed by high-efficiency Pancharatnam-Berry (PB) meta-atoms can convert circularly polarized (CP) THz beams into SSPs with absolute efficiency up to 60%. Encoding a parabolic phase profile into the meta-coupler based on the PB mechanism, it is demonstrated that the device can covert CP beams into SSPs with focusing or defocusing wavefronts, dictated by the chirality of the incident wave. Finally, two distinct chirality-dependent phase distributions are encoded into the meta-coupler design by combining the PB and resonance phase mechanisms, and it is demonstrated that the resulting meta-device can achieve SSP excitations with chirality-delinked bifunctional wavefront engineering. THz near-field experiments are performed to characterize all three devices, in excellent agreement with full-wave simulations. The results pave the road to realize ultracompact devices integrating different functionalities on near-field manipulations, which can find many applications (e.g., optical sensing, imaging, on-chip photonics, etc.) in different frequency domains.

Two-dimensional polymer grating and prism on Bloch surface waves platform

A one-dimensional photonic crystal sustaining Bloch Surface Waves (BSWs) is used as a platform for two-dimensional integrated optics. The dielectric platform shows low loss, long propagation distance and high surface field enhancement. In order to study the potential of the platform for future photonic chips, polymer ultra-thin prisms and gratings (~100 nm) are engineered on the top of the platform. This polymer layer modifies the BSWs effective index enabling a direct manipulation of light. The BSW deflection effects caused by surface prisms are observed in the near-field and Snells law is verified; then the BSW diffractions through surface gratings are experimentally and theoretically characterized. The results show a robust platform that can be used for integrated optics that includes different optical components. One of the main advantages is that these 2D photonic devices can have arbitrary shapes, which is difficult to obtain in 3D.

Manipulation of Airy surface plasmon beams

We demonstrate experimentally the manipulation of Airy surface plasmon beams in a linear potential. For this purpose, we fabricate dielectric-loaded plasmonic structures with a graded refractive index by negative-tone gray-scale electron beam lithography. Using such carefully engineered potentials, we show that the bending of an Airy surface plasmon beam can be fully reversed by the potential.

Self-collimation of surface plasmon beams

We generate self-collimating surface plasmon beams in a doubly periodic plasmonic grating. The self-collimation effect is understood from the local anisotropy of the isofrequency surface of the grating in the vicinity of the bandgap. The properties of the beams are analyzed by leakage radiation microscopy and show to an unprecedented level significantly reduced diffraction as compared with plasmon beams propagating on a flat metal film.

Versatile gradients of chemistry, bound ligands and nanoparticles on alumina nanopore arrays

Nanoporous alumina (PA) arrays produced by self-ordering growth, using electrochemical anodization, have been extensively explored for potential applications based upon the unique thermal, mechanical and structural properties, and high surface-to-volume ratio of these materials. However, the potential applications and functionality of these materials may be further extended by molecular-level engineering of the surface of the pore rims. In this paper we present a method for the generation of chemical gradients on the surface of PA arrays based upon plasma co-polymerization of two monomers. We further extend these chemical gradients, which are also gradients of surface charge, to those of bound ligands and number density gradients of nanoparticles. The latter represent a highly exotic new class of materials, comprising aligned PA, capped by gold nanoparticles around the rim of the pores. Gradients of chemistry, ligands and nanoparticles generated by our method retain the porous structure of the substrate, which is important in applications that take advantage of the inherent properties of these materials. This method can be readily extended to other porous materials.

Plasmonic Airy beams with dynamically controlled trajectories

We report the experimental generation and dynamic trajectory control of plasmonic Airy beams (PABs). The PABs are created by directly coupling free-space Airy beams to surface plasmon polaritons through a grating coupler on a metal surface. We show that the ballistic motion of the PABs can be reconfigured in real time by either a computer addressed spatial light modulator or mechanical means.

Plasmonic Luneburg and Eaton lenses

Plasmonics takes advantage of the properties of surface plasmon polaritons, which are localized or propagating quasiparticles in which photons are coupled to the quasi-free electrons in metals. In particular, plasmonic devices can confine light in regions with dimensions that are smaller than the wavelength of the photons in free space, and this makes it possible to match the different length scales associated with photonics and electronics in a single nanoscale device. Broad applications of plasmonics that have been demonstrated to date include biological sensing, sub-diffraction-limit imaging, focusing and lithography and nano-optical circuitry. Plasmonics-based optical elements such as waveguides, lenses, beamsplitters and reflectors have been implemented by structuring metal surfaces or placing dielectric structures on metals to manipulate the two-dimensional surface plasmon waves. However, the abrupt discontinuities in the material properties or geometries of these elements lead to increased scattering of surface plasmon polaritons, which significantly reduces the efficiency of these components. Transformation optics provides an alternative approach to controlling the propagation of light by spatially varying the optical properties of a material. Here, motivated by this approach, we use grey-scale lithography to adiabatically tailor the topology of a dielectric layer adjacent to a metal surface to demonstrate a plasmonic Luneburg lens that can focus surface plasmon polaritons. We also make a plasmonic Eaton lens that can bend surface plasmon polaritons. Because the optical properties are changed gradually rather than abruptly in these lenses, losses due to scattering can be significantly reduced in comparison with previously reported plasmonic elements.

Surface plasmon mode steering and negative refraction

The propagation of surface plasmon beams through singly and doubly periodic metallic gratings is analyzed both in real and Fourier spaces. Large beam steering effects are experimentally revealed by probing the isofrequency surfaces (IFS) related to propagating plasmonic Bloch waves inside the gratings. In particular, negative refraction is demonstrated close to the Bragg condition. We analyze how the local structure of the IFS can amplify the sensitivity of surface plasmon-based sensors.