Polarization is one of the basic properties of electromagnetic waves conveying valuable information in signal transmission and sensitive measurements. Conventional methods for advanced polarization control impose demanding requirements on material properties and attain only limited performance. We demonstrated ultrathin, broadband, and highly efficient metamaterial-based terahertz polarization converters that are capable of rotating a linear polarization state into its orthogonal one. On the basis of these results, we created metamaterial structures capable of realizing near-perfect anomalous refraction. Our work opens new opportunities for creating high-performance photonic devices and enables emergent metamaterial functionalities for applications in the technologically difficult terahertz-frequency regime.

Terahertz Metamaterials for Linear Polarization Conversion and Anomalous Refraction

Metamaterials are composed of periodic subwavelength metal/dielectric structures that resonantly couple to the electric and/or magnetic components of the incident electromagnetic fields, exhibiting properties that are not found in nature. This class of micro- and nano-structured artificial media have attracted great interest during the past 15 years and yielded ground-breaking electromagnetic and photonic phenomena. However, the high losses and strong dispersion associated with the resonant responses and the use of metallic structures, as well as the difficulty in fabricating the micro- and nanoscale 3D structures, have hindered practical applications of metamaterials. Planar metamaterials with subwavelength thickness, or metasurfaces, consisting of single-layer or few-layer stacks of planar structures, can be readily fabricated using lithography and nanoprinting methods, and the ultrathin thickness in the wave propagation direction can greatly suppress the undesirable losses. Metasurfaces enable a spatially varying optical response (e.g. scattering amplitude, phase, and polarization), mold optical wavefronts into shapes that can be designed at will, and facilitate the integration of functional materials to accomplish active control and greatly enhanced nonlinear response. This paper reviews recent progress in the physics of metasurfaces operating at wavelengths ranging from microwave to visible. We provide an overview of key metasurface concepts such as anomalous reflection and refraction, and introduce metasurfaces based on the Pancharatnam-Berry phase and Huygens' metasurfaces, as well as their use in wavefront shaping and beam forming applications, followed by a discussion of polarization conversion in few-layer metasurfaces and their related properties. An overview of dielectric metasurfaces reveals their ability to realize unique functionalities coupled with Mie resonances and their low ohmic losses. We also describe metasurfaces for wave guidance and radiation control, as well as active and nonlinear metasurfaces. Finally, we conclude by providing our opinions of opportunities and challenges in this rapidly developing research field.

A review of metasurfaces: physics and applications.

Metamaterials are composed of periodic subwavelength metal/dielectric structures that resonantly couple to the electric and/or magnetic components of the incident electromagnetic fields, exhibiting properties that are not found in nature. Planar metamaterials with subwavelength thickness, or metasurfaces, consisting of single-layer or few-layer stacks of planar structures, can be readily fabricated using lithography and nanoprinting methods, and the ultrathin thickness in the wave propagation direction can greatly suppress the undesirable losses. Metasurfaces enable a spatially varying optical response, mold optical wavefronts into shapes that can be designed at will, and facilitate the integration of functional materials to accomplish active control and greatly enhanced nonlinear response. This paper reviews recent progress in the physics of metasurfaces operating at wavelengths ranging from microwave to visible. We provide an overview of key metasurface concepts such as anomalous reflection and refraction, and introduce metasurfaces based on the Pancharatnam-Berry phase and Huygens' metasurfaces, as well as their use in wavefront shaping and beam forming applications, followed by a discussion of polarization conversion in few-layer metasurfaces and their related properties. An overview of dielectric metasurfaces reveals their ability to realize unique functionalities coupled with Mie resonances and their low ohmic losses. We also describe metasurfaces for wave guidance and radiation control, as well as active and nonlinear metasurfaces. Finally, we conclude by providing our opinions of opportunities and challenges in this rapidly developing research field.

A review of metasurfaces: physics and applications

In this paper, a double V-shaped metasurface that can efficiently convert linear polarizations of electromagnetic (EM) waves in wideband is proposed. Based on the electric and magnetic resonant features of a single V-shaped particle, four EM resonances are generated in a V-shaped pair, leading to significant bandwidth expansion of cross-polarized reflections. The simulation results show that the proposed metasurface is able to convert linearly polarized waves into cross-polarized waves in ultrawideband from 12.4 to 27.96 GHz, with an average polarization conversion ratio (PCR) of 90%. The experimental results are in good agreement with the numerical simulations. Compared to published designs, the proposed polarization converter has a simple geometry but an ultrawideband and hence can be used in many applications, such as reflector antennas, imaging systems, remote sensors, and radiometers. The method can also be extended to the terahertz band.

Ultrawideband and High-Efficiency Linear Polarization Converter Based on Double V-Shaped Metasurface

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.

Gradient metasurfaces: a review of fundamentals and applications.

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.

http://www.physics.fudan.edu.cn/tps/people/lzhou/papers/111-APL%20105,%20021102%20(2014).pdf

Ultra-broadband terahertz metamaterial absorber

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.

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

DOI: 10.1002/adom.201901548 energy with 100% efficiency can be very useful in energy harvesting and invisibility-related applications.[2–4] Moreover, it is even more fascinating if one can combine those two functionalities into one single device, which can behave either as an ideal filter (as shown in Figure 1a) or a perfect absorber (as shown in Figure 1b), controlled dynamically by certain external knobs. Such tunable devices can be potentially useful in many application scenarios (e.g., smart radar radomes and solar cells), but are extremely challenging to realize since the two EM-wave properties (i.e., scattering and absorption) of an ultrathin slab are strongly coupled with each other. In fact, early theoretical studies have demonstrated that the largest absorption enabled by an ultrathin screen is 50% (as shown in Figure 1c,d),[5] not mentioning further difficulties in making the device actively tunable. Recent developments on metasurface (particularly on tunable metasurfaces) provide possible solutions to make such optical devices. Metasurfaces,[6–12] ultrathin metamaterials composed by planar meta-atoms with tailored EM properties, have exhibited strong abilities to control EM waves,[6,7,13–16] with perfect transmission[17] and absorption[18] of EM waves both realized. With active elements integrated, one can further construct tunable metasurfaces with dynamically controlled functionalities[19–29] including tunable absorbers and filters. However, most tunable metadevices realized so far are based on reflection geometry[20–23,26–29] which are relatively easy to design, since only two channels exist in such systems for EM waves to radiate or dissipate (i.e., the reflection port and absorption).[30] In contrast, high-performance tunable metadevices in transmission modes are rarely seen, not mentioning those exhibiting dynamically switched dual functionalities. The intrinsic physics is that a transmissive metasurface has three channels (i.e., transmission and reflection ports, and absorption) to transport/dissipate energy, which must be carefully balanced to yield the desired functionalities (i.e., perfect transmission or perfect absorption). However, the competitions between these three channels are very complicated, which make a transmission-absorption switchable metadevice very difficult to realize, especially on an ultrathin platform. In this paper, we design and experimentally realize such a metadevice with ultrathin thickness. In Section 2, we start from examining the EM properties a trilayer metasurface with PIN diodes loaded and controlled by the same voltages, and show that such a device cannot exhibit freely controlled transmission and absorption properties. We next employ the coupled-mode A thin screen exhibiting dynamically switchable transmission/absorption functionalities is highly desired in practice. However, a trilayer transmissive metasurface with active elements controlled in a uniform manner cannot exhibit independently controlled transmission and absorption properties due to intriguing interplays between the scattering and absorbing properties in systems exhibiting inversion symmetries. This motivates to employ the coupled-mode theory to establish a generic phase diagram for such transmissive metasurfaces with active elements loaded in different layers tuned independently and to guide researchers design tunable metadevices with completely decoupled transmission/absorption responses. Based on such a phase diagram, a microwave metasurface is designed/fabricated with PIN diodes incorporated, and it is experimentally demonstrated that its functionality can switch from perfect transparency to perfect absorption, controlled by external voltages applied across the diodes. In addition to finding immediate applications in practice (e.g., smart radomes), the results of this study also provide a new type of tunable meta-atom for building metasurfaces with flexible wave-front control abilities.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adom.201901548

A Tunable Metasurface with Switchable Functionalities: From Perfect Transparency to Perfect Absorption

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.

http://www.physics.fudan.edu.cn/tps/people/lzhou/papers/73-OL%2036,927.pdf

A transparent metamaterial to manipulate electromagnetic wave polarizations.

A theoretical formalism is developed to calculate the optical force induced within a 2D metamaterial, based on the electrostrictive tensor. It turns out that it is not always straightforward to use the usual effective-medium parameters $ϵ$ and $\ensuremath{\mu}$ of the metamaterial for obtaining the body force density. The findings are tested via a 2D array of parallel infinitely long dielectric cylinders and the analytical theory is in excellent agreement with numerics. The correct stress tensor to describe the force density in an effective medium is the Helmholtz one rather than the Maxwell. The theory, though, is not exact and points to a fundamental limitation of effective medium theory in describing light-induced body forces.

/pdf/analytic-derivation-of-electrostrictive-tensors-and-their-32k5mvpsxa.pdf

Wujiong Sun

Papers

Ultra-broadband terahertz metamaterial absorber

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

A Tunable Metasurface with Switchable Functionalities: From Perfect Transparency to Perfect Absorption

A transparent metamaterial to manipulate electromagnetic wave polarizations.

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