Wujiong Sun

Ultra-broadband terahertz metamaterial absorber

We demonstrated an ultra-broadband, polarization-insensitive, and wide-angle metamaterial absorber for terahertz (THz) frequencies using arrays of truncated pyramid unit structure made of metal-dielectric multilayer composite. In our design, each sub-layer behaving as an effective waveguide is gradually modified in their lateral width to realize a wideband response by effectively stitching together the resonance bands of different waveguide modes. Experimentally, our five layer sample with a total thickness 21 μm is capable of producing a large absorptivity above 80% from 0.7 to 2.3 THz up to the maximum measurement angle 40°. The full absorption width at half maximum of our device is around 127%, greater than those previously reported for THz frequencies. Our absorber design has high practical feasibility and can be easily integrated with the semiconductor technology to make high efficient THz-oriented devices.

A transparent metamaterial to manipulate electromagnetic wave polarizations

We design an anisotropic ultrathin metamaterial to allow perfect transmissions of electromagnetic (EM) waves for two incident polarizations within a common frequency interval. The transparencies are governed by different mechanisms, resulting in significant differences in transmission phase changes for two polarizations. The system can thus manipulate EM wave polarizations efficiently in transmission geometry, including polarization conversion and rotation. Microwave experiments performed on realistic samples are in excellent agreement with numerical simulations.

High-efficiency surface plasmon meta-couplers: concept and microwave-regime realizations

Surface plasmon polaritons (SPPs) and their low-frequency counterparts (i.e., spoof SPPs on artificial surfaces) have recently found numerous applications in photonics, but traditional devices to excite them (such as gratings and prism couplers) all suffer from problems of inherent low efficiency because the generated SPPs can decouple, returning to free space, and reflections at the device surface can never be avoided. Here, we propose a new SPP excitation scheme based on a transparent gradient metasurface and numerically demonstrate that it exhibits inherently high efficiency (~94%) because the designed meta-coupler suppresses both decoupling and surface reflections. As a practical realization of this concept, we fabricated a meta-coupler for operation in the microwave regime and performed near-field and far-field experiments to demonstrate that the achieved excitation efficiency for spoof SPPs reaches ~73%, which is several times higher than that achieved by other available devices in this frequency domain. Our findings can motivate the design and fabrication of high-performance plasmonic devices to harvest light–matter interactions, particularly those related to spoof SPPs in the low-frequency domain.

Analytic derivation of electrostrictive tensors and their application to optical force density calculations

Using multiple scattering theory, we derived for the first time analytical formulas for electrostrictive tensors for two-dimensional metamaterial systems. The electrostrictive tensor terms are found to depend explicitly on the symmetry of the underlying lattice of the metamaterial, and they also depend explicitly on the direction of a local effective wave vector. These analytical results enable us to calculate light induced body forces inside a composite system (metamaterial) using the Helmholtz stress tensor within the effective medium formalism in the sense that the fields used in the stress tensor are those obtained by solving the macroscopic Maxwell equation with the microstructure of the metamaterial replaced by an effective medium. Our results point to some fundamental questions of using an effective medium theory to determine optical force density. In particular, the fact that Helmholtz tensor carries electrostrictive terms that are explicitly symmetry dependent means that the standard effective medium parameters cannot give sufficient information to determine body force density, even though they can give the correct total force. A more challenging issue is that the electrostrictive terms are related to a local effective wave vector, and it is not always obtainable in systems with boundary reflections within the context of a standard effective medium approach.

Manipulating electromagnetic waves with metamaterials: Concept and microwave realizations

Our recent efforts in manipulating electromagnetic (EM) waves using metamaterials (MTMs) are reviewed with emphasis on 1) manipulating wave polarization and transporting properties using homogeneous MTMs, 2) manipulating surface-wave properties using plasmonic MTMs, and 3) bridging propagating and surface waves using inhomogeneous meta-surfaces. For all these topics, we first illustrate the physical concepts and then present several typical practical realizations and applications in the microwave regime.

A chirality switching device designed with transformation optics

Based on transformation optics theory, we designed a chirality switching device, such that an object hidden inside would exhibit a reversed chirality (i.e., from left-handedness to right-handedness) for an observer at the far field. Distinct from a perfect mirror which also creates a chirality-reversed image, our device makes the original object completely invisible to the far field observer. Numerical simulations are employed to demonstrate the functionalities of the designed devices in both two- and three-dimensional spaces.

Recent advances on metasurfaces

We briefly summarize our recent efforts in employing meta-surfaces to control electromagnetic waves, including realizing high-efficiency photonic spin-hall effect and surface-plasmon couplers, and controlling phases with graphene-based meta-surfaces.

Metamaterials to bridge propagating waves with surface waves and control electromagnetic waves

In this work, we demonstrate theoretically and experimentally that a specific gradient-index meta-surface can convert a PW to a SW with nearly 100% efficiency. Distinct from conventional devices such as prism or grating couplers, the momentum mismatch between PW and SW is compensated by the reflection-phase gradient of the meta-surface, and a nearly perfect PW-SW conversion can happen for any incidence angle larger than a critical value. Experiments in the microwave region, including both far-field and near-field characterizations, are in excellent agreement with full-wave simulations. Based on gradient metasurfaces idea, we also realized a gradient meta-surface supporting high-efficiency anomalous reflection for near-infrared light and a flat focusing lens for microwave.

Reflectionless ultrathin microwave waveplate based on metamaterials

We design an anisotropic ultrathin metamaterial to allow perfect transmissions of electromagnetic (EM) waves for two incident polarizations within a common frequency interval. The transparencies are governed by different mechanisms, resulting in significant differences in transmission phase changes for two polarizations. The system can thus manipulate EM wave polarizations efficiently in transmission geometry, including polarization conversion and rotation. Microwave experiments performed on realistic samples are in excellent agreement with numerical simulations.