Holger Mühlenbernd

Metasurface holograms reaching 80% efficiency

Surfaces covered by ultrathin plasmonic structures--so-called metasurfaces--have recently been shown to be capable of completely controlling the phase of light, representing a new paradigm for the design of innovative optical elements such as ultrathin flat lenses, directional couplers for surface plasmon polaritons and wave plate vortex beam generation. Among the various types of metasurfaces, geometric metasurfaces, which consist of an array of plasmonic nanorods with spatially varying orientations, have shown superior phase control due to the geometric nature of their phase profile. Metasurfaces have recently been used to make computer-generated holograms, but the hologram efficiency remained too low at visible wavelengths for practical purposes. Here, we report the design and realization of a geometric metasurface hologram reaching diffraction efficiencies of 80% at 825 nm and a broad bandwidth between 630 nm and 1,050 nm. The 16-level-phase computer-generated hologram demonstrated here combines the advantages of a geometric metasurface for the superior control of the phase profile and of reflectarrays for achieving high polarization conversion efficiency. Specifically, the design of the hologram integrates a ground metal plane with a geometric metasurface that enhances the conversion efficiency between the two circular polarization states, leading to high diffraction efficiency without complicating the fabrication process. Because of these advantages, our strategy could be viable for various practical holographic applications.

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.

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.

Effect of alignment on a liquid crystal/split-ring resonator metasurface.

A metasurface comprising a two-dimensional array of split-ring resonators with resonance frequencies in the near-infrared region is fabricated and embedded in a uniformly aligned liquid crystal. The change of the dielectric permittivity in proximity to the plasmonic structure by the replacement of air with the liquid crystal results in a decrease in resonance frequencies. The resonance shift can be attributed to the interaction of the evanescent field of the excited resonant plasmon modes with the liquid crystal. This shift in resonance frequency is found to depend on the liquid-crystal alignment and to vary for different modes. Also, the resulting effects of changes in temperature or applied external electric field on the metasurface depend on the liquid-crystal alignment and may differ from mode to mode. These observations indicate that the characteristic frequencies of the resonant split-ring resonator modes may depend on different evanescent field components interacting with the liquid crystal. Consequently, certain design rules should be taken into account for the development of tunable metasurfaces based on liquid crystals.

Electro-optic tuning of split ring resonators embedded in a liquid crystal

Two-dimensional arrays of split ring resonators for near-infrared frequencies are embedded in a liquid crystal (LC) and the influences of LC alignment, temperature, and electric fields on the resonance frequencies are studied. The results show that tunability can not only be achieved by influencing the state of polarization of the incident radiation, but also by direct interaction of the evanescent field of the resonant modes with the LC. Depending on the LC alignment, the field-induced shift of the resonance frequency is found to vary for different excited modes. Some guidelines for the design of tunable frequency selective metasurfaces can be deduced from these experimental results.

Single-pixel computational ghost imaging with helicity-dependent metasurface hologram

A helicity-dependent computational ghost image generated by a metasurface hologram offers a promising optical encryption scheme. Different optical imaging techniques are based on different characteristics of light. By controlling the abrupt phase discontinuities with different polarized incident light, a metasurface can host a phase-only and helicity-dependent hologram. In contrast, ghost imaging (GI) is an indirect imaging modality to retrieve the object information from the correlation of the light intensity fluctuations. We report single-pixel computational GI with a high-efficiency reflective metasurface in both simulations and experiments. Playing a fascinating role in switching the GI target with different polarized light, the metasurface hologram generates helicity-dependent reconstructed ghost images and successfully introduces an additional security lock in a proposed optical encryption scheme based on the GI. The robustness of our encryption scheme is further verified with the vulnerability test. Building the first bridge between the metasurface hologram and the GI, our work paves the way to integrate their applications in the fields of optical communications, imaging technology, and security.

Liquid crystals and precious metal: from nanoparticle dispersions to functional plasmonic nanostructures

ABSTRACT Precious metals and liquid crystals (LC) are quite different yet share some common features, for example, their beauty, distinguished optical properties and a competition of high prices per gramme. Triggered by the vision of artificial materials with extremely unusual properties (metamaterials), facilitated by the methods of modern nanotechnology and motivated by John Goodby’s and other colleagues’ synthetic activities, combinations of LCs and gold nanoparticles or nanostructures have attracted much attention during the last decade. This noncomprehensive article describes some examples and insights in this field. Perspectives and opportunities of further research are discussed. Graphical Abstract

Metasurface holography with multiple channels

Holographic multiplexing is capable of recording multiple images in the same area to increase the information capacity and make the optimum use of the space-bandwidth product. Unlike volume holographic recording systems with Bragg-based selectivity, holographic multiplexing based on metasurfaces is a quite distinct field. Here, by combining the synthetic spectrum method with a geometric phase metasurface that can generate a helicity dependent phase profile, we achieve two circularly polarized multiplexing channels for holography. We experimentally demonstrate polarization and position holographic multiplexing in the visible and near IR range with pairs of superimposed images. Furthermore, the broadband wavelengths reconstruction abilities of such metasurfaces holograms are experimentally characterized. In addition numerical analysis of the factors that affect the multiplexing capacity are presented. The metasurface holography multiplexing demonstrated may serve as a platform for large capacity optical data storage, pattern recognition, spatial-temporal filter and information processing owing to its unique advantages of parallel recording and plenty of multiplexing methods.