Shumei Chen

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.

Helicity multiplexed broadband metasurface holograms

Metasurfaces are engineered interfaces that contain a thin layer of plasmonic or dielectric nanostructures capable of manipulating light in a desirable manner. Advances in metasurfaces have led to various practical applications ranging from lensing to holography. Metasurface holograms that can be switched by the polarization state of incident light have been demonstrated for achieving polarization multiplexed functionalities. However, practical application of these devices has been limited by their capability for achieving high efficiency and high image quality. Here we experimentally demonstrate a helicity multiplexed metasurface hologram with high efficiency and good image fidelity over a broad range of frequencies. The metasurface hologram features the combination of two sets of hologram patterns operating with opposite incident helicities. Two symmetrically distributed off-axis images are interchangeable by controlling the helicity of the input light. The demonstrated helicity multiplexed metasurface hologram with its high performance opens avenues for future applications with functionality switchable optical devices.

Continuous control of the nonlinearity phase for harmonic generations

The capability of locally engineering the nonlinear optical properties of media is crucial in nonlinear optics. Although poling is the most widely employed technique for achieving locally controlled nonlinearity, it leads only to a binary nonlinear state, which is equivalent to a discrete phase change of π in the nonlinear polarizability. Here, inspired by the concept of spin-rotation coupling, we experimentally demonstrate nonlinear metasurfaces with homogeneous linear optical properties but spatially varying effective nonlinear polarizability with continuously controllable phase. The continuous phase control over the local nonlinearity is demonstrated for second and third harmonic generation by using nonlinear metasurfaces consisting of nanoantennas of C3 and C4 rotational symmetries, respectively. The continuous phase engineering of the effective nonlinear polarizability enables complete control over the propagation of harmonic generation signals. Therefore, this method seamlessly combines the generation and manipulation of harmonic waves, paving the way for highly compact nonlinear nanophotonic devices.

Symmetry-selective third-harmonic generation from plasmonic metacrystals.

Nonlinear processes are often governed by selection rules imposed by the symmetries of the molecular configurations. The most well-known examples include the role of centrosymmetry breaking for the generation of even harmonics, and the selection rule related to the rotational symmetry in harmonic generation for fundamental beams with circular polarizations. While the role of centrosymmetry breaking in second harmonic generation has been extensively studied in plasmonic systems, the investigation of selection rules pertaining to circular polarization states of harmonic generation is limited to crystals, i.e., symmetries at the atomic level. In this Letter we demonstrate the rotational symmetry dependent third harmonic generation from nonlinear plasmonic metacrystals. We show that the selection rule can be imposed by the rotational symmetry of metacrystals embedded into an isotropic organic nonlinear thin film. The results presented here may open new avenues for designing symmetry-dependent nonlinear optical responses with tailored plasmonic nanostructures.

Geometric Metasurface Fork Gratings for Vortex-beam Generation and Manipulation

In recent years, optical vortex beams possessing orbital angular momentum have received much attention due to their potential for high-capacity optical communications. This capability arises from the unbounded topological charges of orbital angular momentum (OAM) that provide infinite freedoms for encoding information. The two most common approaches for generating vortex beams are through fork diffraction gratings and spiral phase plates. While realization of conventional spiral phase plate requires complicated 3D fabrication, the emerging field of metasurfaces has provided a planar and facile solution for generating vortex beams of arbitrary orbit angular momentum. Among various types of metasurfaces, the geometric phase metasurface has shown great potential for robust control of light- and spin-controlled wave propagation. Here, we realize a novel type of geometric metasurface fork grating that seamlessly combine the functionality of a metasurface phase plate for vortex-beam generation, and that of a linear phase gradient metasurface for controlling the wave-propagation direction. The metasurface fork grating is therefore capable of simultaneously controlling both the spin and the orbital angular momentum of light.

Giant Nonlinear Optical Activity of Achiral Origin in Planar Metasurfaces with Quadratic and Cubic Nonlinearities.

3D chirality is shown to be unnecessary for introducing strong circular dichroism for harmonic generations. Specifically, near-unity circular dichroism for both second-harmonic generation and third-harmonic generations is demonstrated on suitably designed ultrathin plasmonic metasurfaces with only 2D planar chirality. The study opens up new routes for designing chip-type biosensing platform, which may allow for highly sensitive detection of bio- and chemical molecules with weak chirality.

Efficient energy exchange between plasmon and cavity modes via Rabi-analogue splitting in a hybrid plasmonic nanocavity

Plasmonic analogues of Rabi-splitting have been extensively studied in various metallic nanosystems hybridized with semiconductor quantum dots, nanocrystals and organic molecules, with a focus on the splitting energy gap where surface plasmon polaritons (SPPs) strongly couple with excitons. Similar strong coupling also occurs for individual metallic nanoparticles locating inside a photonic microcavity or nearby a waveguide due to the strong interaction between localized surface plasmons and photonic modes in the near-infrared wavelength range. In this work we study experimentally and theoretically the strong coupling between propagating SPPs and the Fabry-Perot (F-P) cavity mode in a metallic nanoparticle array-nanocavity hybrid system in the visible spectral range. The strong modal hybridization created giant modal anti-crossing which can be considered as the classical phenomenon of Rabi splitting i.e. a Rabi-analogue. In addition to the observation of a giant Rabi-analogue splitting energy of 148 meV at the strong coupling regime, we also reveal highly-efficient energy exchange between SPP and F-P modes at the low frequency dispersion branch through detailed numerical near-field studies and experimental phase delay analysis. The observed efficient mode conversion in the investigated plasmonic nanocavity is useful for designing novel nanophotonic devices, in which conventional photonic components need to be integrated with miniaturized plasmonic devices or vice versa.

Nonlinear Metasurface for Simultaneous Control of Spin and Orbital Angular Momentum in Second Harmonic Generation

The spin and orbital angular momentum (SAM and OAM) of light is providing a new gateway toward high capacity and robust optical communications. While the generation of light with angular momentum is well studied in linear optics, its further integration into nonlinear optical devices will open new avenues for increasing the capacity of optical communications through additional information channels at new frequencies. However, it has been challenging to manipulate the both SAM and OAM of nonlinear signals in harmonic generation processes with conventional nonlinear materials. Here, we report the generation of spin-controlled OAM of light in harmonic generations by using ultrathin photonic metasurfaces. The spin manipulation of OAM mode of harmonic waves is experimentally verified by using second harmonic generation (SHG) from gold meta-atom with 3-fold rotational symmetry. By introducing nonlinear phase singularity into the metasurface devices, we successfully generate and measure the topological charges of spin-controlled OAM mode of SHG through an on-chip metasurface interferometer. The nonlinear photonic metasurface proposed in this work not only opens new avenues for manipulating the OAM of nonlinear optical signals but also benefits the understanding of the nonlinear spin-orbit interaction of light in nanoscale devices.

Narrowband plasmonic excitation on gold hole-array nanostructures observed using spectroscopic ellipsometer

Surface plasmon polaritons (SPPs) modes on gold hole-array nanostructures were studied using spectroscopy ellipsometer in reflection mode. Using background free techniques in the optical ellipsometer, clear SPP bands on gold nanostructure/air interface were measured in UV-Visible-Near infrared (NIR) regime (300 nm-1800 nm). Plasmonic excitation with bandwidth of 13 nm (FWHM) was observed in reflection measurement, and it is much narrower than that observed in transmission measurement mode. In addition, the plasmonic excitation bands were characterized in both amplitude and phase domain. Theoretical analysis using reciprocal lattice vector method and rigorous coupled wave analysis (RCWA) agreed well with the experimental results. By measuring the TM and TE waves simultaneously, the spectroscopic ellipsometer provides an important method to analyze both amplitude and phase information in plasmonic nanostructures and metamaterials.

Sharp plasmonic resonance on gold gratings in amplitude and phase domains

In this work, we experimentally studied sharp plasmonic resonance on gold gratings in both amplitude and phase domains using a spectroscopic ellipsometry technique. We used numerical and analytical models to analyze the phase delay of TM and TE waves under a surface plasmon excitation condition, and the calculated result fits well with the experimental observations. In addition, the ellipsometry method used here provides an important tool to characterize the phase information in plasmonic and metamaterial devices.