Benfeng Bai

Three-dimensional optical holography using a plasmonic metasurface

Benefitting from the flexibility in engineering their optical response, metamaterials have been used to achieve control over the propagation of light to an unprecedented level, leading to highly unconventional and versatile optical functionalities compared with their natural counterparts. Recently, the emerging field of metasurfaces, which consist of a monolayer of photonic artificial atoms, has offered attractive functionalities for shaping wave fronts of light by introducing an abrupt interfacial phase discontinuity. Here we realize three-dimensional holography by using metasurfaces made of subwavelength metallic nanorods with spatially varying orientations. The phase discontinuity takes place when the helicity of incident circularly polarized light is reversed. As the phase can be continuously controlled in each subwavelength unit cell by the rod orientation, metasurfaces represent a new route towards high-resolution on-axis three-dimensional holograms with a wide field of view. In addition, the undesired effect of multiple diffraction orders usually accompanying holography is eliminated.

Dual-polarity plasmonic metalens for visible light

Surface topography and refractive index profile dictate the deterministic functionality of a lens. The polarity of most lenses reported so far, that is, either positive (convex) or negative (concave), depends on the curvatures of the interfaces. Here we experimentally demonstrate a counter-intuitive dual-polarity flat lens based on helicity-dependent phase discontinuities for circularly polarized light. Specifically, by controlling the helicity of the input light, the positive and negative polarity are interchangeable in one identical flat lens. Helicity-controllable real and virtual focal planes, as well as magnified and demagnified imaging, are observed on the same plasmonic lens at visible and near-infrared wavelengths. The plasmonic metalens with dual polarity may empower advanced research and applications in helicity-dependent focusing and imaging devices, angular-momentum-based quantum information processing and integrated nano-optoelectronics.

Dispersionless phase discontinuities for controlling light propagation.

Ultrathin metasurfaces consisting of a monolayer of subwavelength plasmonic resonators are capable of generating local abrupt phase changes and can be used for controlling the wavefront of electromagnetic waves. The phase change occurs for transmitted or reflected wave components whose polarization is orthogonal to that of a linearly polarized (LP) incident wave. As the phase shift relies on the resonant features of the plasmonic structures, it is in general wavelength-dependent. Here, we investigate the interaction of circularly polarized (CP) light at an interface composed of a dipole antenna array to create spatially varying abrupt phase discontinuities. The phase discontinuity is dispersionless, that is, it solely depends on the orientation of dipole antennas, but not their spectral response and the wavelength of incident light. By arranging the antennas in an array with a constant phase gradient along the interface, the phenomenon of broadband anomalous refraction is observed ranging from visible to near-infrared wavelengths. We further design and experimentally demonstrate an ultrathin phase gradient interface to generate a broadband optical vortex beam based on the above principle.

Helicity dependent directional surface plasmon polariton excitation using a metasurface with interfacial phase discontinuity

Researchers have now demonstrated a reconfigurable, unidirectional coupling scheme for excitation of collective oscillations of the electrons at a metal surface, the so-called surface plasmon-polaritons. Lingling Huang and co-workers achieved an efficient and controllable coupling of photons in free space to these surface states on a metal film — a task important for the future development of nanoscale optoelectronic circuitry — by employing a nanostructured thin metal film on a glass substrate. The nanostructured metal film features an array of rectangular nano-apertures arranged in a carefully designed orientation and pattern. Such a ‘plasmonic metasurface’ couples photons to surface plasmon-polaritons while depending crucially on the circular polarization state of the incident light. As a result, when circularly polarized light strikes the surface, the handedness of the light dictates the propagation direction of the resulting surface plasmon-polaritons.

Integrated plasmonic semi-circular launcher for dielectric-loaded surface plasmon-polariton waveguide

A semi-circular plasmonic launcher integrated with dielectric-loaded surface plasmon-polaritons waveguide (DLSPPW) is proposed and analyzed theoretically, which can focus and efficiently couple the excited surface plasmon polaritons (SPPs) into the DLSPPW via the highly matched spatial field distribution with the waveguide mode in the focal plane. By tuning the incident angle or polarization of the illuminating beam, it is shown that the launcher may be conveniently used as a switch or a multiplexer that have potential applications in plasmonic circuitry. Furthermore, from an applicational point of view, it is analyzed how the coupling performance of the launcher can be further improved by employing multiple semi-circular slits.

Experimental demonstration of tunable directional excitation of surface plasmon polaritons with a subwavelength metallic double slit

We demonstrate experimentally the directional excitation of surface plasmon polaritons (SPPs) on a metal film by a subwavelength double slit under backside illumination, based on the interference of SPPs generated by the two slits. By varying the incident angle, the SPPs can be tunably directed into two opposite propagating directions with a predetermined splitting ratio. Under certain incident angle, unidirectional SPP excitation can be achieved. This compact directional SPP coupler is potentially useful for many on-chip applications. As an example, we show the integration of the double-slit couplers with SPP Bragg mirrors, which can effectively realize selective coupling of SPPs into different ports in an integrated plasmonic chip.

Single-layer one-dimensional nonpolarizing guided-mode resonance filters under normal incidence

We demonstrate that properly designed one-dimensional guided-mode resonance filters (GMRFs) with only one grating layer can exhibit a nonpolarizing resonant filtering effect under normal incidence. A sinusoidal profile nonpolarizing GMRF is realized by photoinduced surface-relief grating formation on thin films of polymer-azobenzene complexes and subsequent atomic layer deposition, showing the feasibility of fabrication of such compact GMRFs.

Effect of surface plasmon resonance on the optical activity of chiral metal nanogratings.

We examine the mechanism responsible for the optical activity of a two-dimensional array of gold nanostructures with no mirror symmetry on a dielectric substrate. Measurements with different incident angles, polarizations and sample orientations allow us to reveal that observed polarization effect is enhanced by surface plasmon resonance. By performing numerical simulation with rigorous diffraction theory we also show that the grating chirality can be described in terms of the non-coplanarity of the electric field vectors at the front (air-metal) and back (substrate-metal) sides of the grating layer.

Broadband Hybrid Holographic Multiplexing with Geometric Metasurfaces

An effective way for broadband holographic multiplexing based on geometric metasurfaces is demonstrated by the integration of several recording channels into a single device. Each image can be individually addressed with a unique set of parameters, such as circular polarization, position, and angle. Such a technique paves the way for a wide range of applications related to optical patterning, encryption, and information processing.

Observation of extraordinary optical activity in planar chiral photonic crystals.

Control of light polarization is a key technology in modern photonics including application to optical manipulation of quantum information. The requisite is to obtain large rotation in isotropic media with small loss. We report on extraordinary optical activity in a planar dielectric on-waveguide photonic crystal structure, which has no in-plane birefringence and shows polarization rotation of more than 25 degrees for transmitted light. We demonstrate that in the planar chiral photonic crystal, the coupling of the normally incident light wave with low-loss waveguide and Fabry-Pérot resonance modes results in a dramatic enhancement of the optical activity.