Parallel temporal signal processing enabled by polarization-multiplexed programmable THz metasurfaces.

Under the trends of multifunctionality, tunability, and compactness in modern wave-based signal processors, in this paper, we propose a polarization-multiplexed graphene-based metasurface to realize distinct mathematical operators on the parallel time-domain channels enabled by vertical and horizontal polarizations. The designed metasurface is composed of two perpendicularly-oriented graphene strips for each of which the chemical potential can be dynamically tuned through a DC biasing circuit. The programmable metasurface exhibits two orthogonal channels through which the time-domain input signals are elaborately processed by separate mathematical functions. Several illustrative examples are presented demonstrating that the proposed device can operate on different time-domain analog computing modes such as fractional-order differentiator and phaser at the same time. The strategy introduced in this paper will enable real-time parallel temporal analog computing and has potentially essential applications in terahertz spectroscopy architectures, communication systems, and computing technologies.

Parallel wave-based analog computing using metagratings.

Wave-based signal processing has witnessed a significant expansion of interest in a variety of science and engineering disciplines, as it provides new opportunities for achieving high-speed and low-power operations. Although flat optics desires integrable components to perform multiple missions, yet, the current wave-based computational metasurfaces can engineer only the spatial content of the input signal where the processed signal obeys the traditional version of Snell's law. In this paper, we propose a multi-functional metagrating to modulate both spatial and angular properties of the input signal whereby both symmetric and asymmetric optical transfer functions are realized using high-order space harmonics. The performance of the designed compound metallic grating is validated through several investigations where closed-form expressions are suggested to extract the phase and amplitude information of the diffractive modes. Several illustrative examples are demonstrated to show that the proposed metagrating allows for simultaneous parallel analog computing tasks such as first- and second-order spatial differentiation through a single multichannel structured surface. It is anticipated that the designed platform brings a new twist to the field of optical signal processing and opens up large perspectives for simple integrated image processing systems.

A theoretical investigation on reciprocity-inspired wide-angle spectrally-selective THz absorbers augmented by anisotropic metamaterials.

In this paper, a theoretical framework relying on the reciprocity theorem is proposed to accurately design a spectrally-selective THz superstrate-loaded metamaterial absorber (SLMA) exhibiting wide-angle feature. By leveraging high-order Floquet harmonics in a generalized transmission line model characterizing the conventional metamaterial absorbers (MAs), it is demonstrated that MAs suffer from impedance mismatch, especially at near grazing angles. From an impedance matching viewpoint, this major challenge is tackled in this paper via two different designs, exploiting a magneto-electric anisotropic Huygens' metamaterial and a multilayer dielectric structure at a certain distance over the MA plane. The numerical results corroborate well the theoretical predictions, elucidating that the proposed SLMA significantly broadens the angular performance of the MA up to near grazing angles (about 80°), where high absorptivity is still achieved in both principal planes. The deteriorating effect of diffraction modes has been comprehensively analyzed. In comparison to the previous wide-angle MA reports based on intricate particle geometries and brute-force optimizations, the proposed design features a straightforward semi-analytical algorithm, which can also be re-developed for microwave, mid-infrared, and optical frequency bands and for any type of MA element. The proposed SLMA would be very promising for various wavelength-selective applications such as sensors and imaging.

Self-biased tri-state power-multiplexed digital metasurface operating at microwave frequencies.

Exploiting of nonlinearity has opened doors into undiscovered areas to achieve multiplexed performances in recent years. Although efforts have been made to obtain diverse nonlinear architectures at visible frequencies, the room is still free for incorporating non-linearity into the design of microwave metasurfaces. In this paper, a passive dual-band power intensity-dependent metasurface is presented, which is composed of two different linear and nonlinear meta-atoms accommodating a capacitor and a PIN-diode, respectively. The proposed digital metasurface has three operational states: 1) it acts as a normal reflector at low power intensities while providing a dual-band nonlinear response upon illuminating by high-power incidences where 2) it perfectly absorbs the radiations at f1=6.7 GHz and 3) re-distributes the scattered beams by arranging the meta-atoms with a certain coding pattern at f2=9.4 GHz. The performance of the designed coding elements has been characterized by using the scattering parameters captured in the full-wave simulations and the nonlinear analysis performed in ADS software where the accurate model of diodes is involved. The emergence of microwave self-biased metasurfaces with smart re-actions against incident waves with different power levels reveals great opportunities for designing smart windows, smart camouflage coating surfaces, and so on.

Real-time terahertz wave channeling via multifunctional metagratings: a sparse array of all-graphene scatterers.

Acquiring full control over a large number of diffraction orders can be strongly attractive in the case of realizing multifunctional devices such as multichannel reflectors. Recently, the concept of metagrating has been introduced, which enables obtaining the desired diffraction pattern through a sparse periodic array of engineered scatterers. In this Letter, for the first time, to the best of our knowledge, a tunable all-graphene multichannel meta-reflector is proposed for operating at terahertz (THz) frequencies. In the supercell level, the designed metagrating is composed of three graphene ribbons of different controllable chemical potentials which can be regarded as a five-channel THz meta-reflector. By choosing proper distribution of DC voltages feeding the ribbons, our design can realize different intriguing functionalities such as anomalous reflection, retroreflection, and three-channel power splitting within a single shared aperture and with high efficiency. This Letter paves the way toward designing highly efficient and tunable THz multichannel meta-reflectors with many potential applications in photonics and optoelectronics.

Asymmetric Spatial Power Dividers Using Phase-Amplitude Metasurfaces Driven by Huygens Principle.

Recent years have witnessed an extraordinary spurt in attention toward the wave-manipulating strategies revealed by phase-amplitude metasurfaces. Recently, it has been shown that, when two different phase-encoded metasurfaces responsible for doing separate missions are added together based on the superposition theorem, the mixed digital phase distribution will realize both missions at the same time. In this paper, via a semi-analytical procedure, we demonstrate that such a theorem is not necessarily valid when using phase-only metasurfaces or ignoring the element pattern functions. We introduce the concept of asymmetric spatial power divider (ASPD) with arbitrary power ratio levels in which modulating both amplitude and phase of the meta-atoms is inevitable to fully control the power intensity pattern of a reflective metasurface. Numerical simulations illustrate that the proposed ASPD designed by proper phase and amplitude distribution over the surface can directly generate a desired number of beams with predetermined orientations and power budgets. The C-shaped Pancharatnam-Berry meta-atoms locally realize the optimal phase and amplitude distribution in each case, and the good conformity between simulations and theoretical predictions verifies the presented formalism. A prototype of our ASPD designs is also fabricated and measured, and the experimental results corroborate well our numerical and semi-analytical predictions. Our findings not only offer possibilities to realize arbitrary spatial power dividers over subwavelength scale but also reveal an economical and simple alternative for a beamforming array antenna.

Polarization-insensitive, ultra-broadband, and compact metamaterial-inspired optical absorber via wide-angle and highly efficient performances.

Because of exhibiting extraordinary features, metamaterial absorbers have captured considerable attention in recent years, especially at visible frequencies. In this paper, a new design of a metamaterial-inspired perfect visible absorber (MIPVA) is investigated, which exhibits ultra-broadband, polarization-independent, and wide-angle performances. The proposed MIPVA provides a flat and near unity absorbance (>99%) in an ultra-broad range of radiation wavelengths from λ=500 to 625 nm, while retaining its convincing absorptivity over the entire visible wavelengths. A comprehensive parametric study is accomplished to demonstrate the effects of structural parameters on the absorptivity of the designed MIPVA. To clarify the physical mechanism of absorption, the electric field and surface current distributions of MIPVA are also monitored and elaborately discussed throughout the paper. The results show that the proposed MIPVA exhibits a polarization-insensitive absorption behavior in a wide range of incident wave angles. The interference theory is also utilized to verify the results. In addition, our MIPVA has a compact and low-profile design, while its ability to absorb solar radiation is significantly improved with respect to preceding studies in terms of both the frequency bandwidth and absorptivity; thereby, it is a worthy candidate to play an essential role in different visible-range applications.

An Information Theory-Inspired Strategy for Design of Re-programmable Encrypted Graphene-based Coding Metasurfaces at Terahertz Frequencies.

Inspired by the information theory, a new concept of re-programmable encrypted graphene-based coding metasurfaces was investigated at terahertz frequencies. A channel-coding function was proposed to convolutionally record an arbitrary information message onto unrecognizable but recoverable parity beams generated by a phase-encrypted coding metasurface. A single graphene-based reflective cell with dual-mode biasing voltages was designed to act as "0" and "1" meta-atoms, providing broadband opposite reflection phases. By exploiting graphene tunability, the proposed scheme enabled an unprecedented degree of freedom in the real-time mapping of information messages onto multiple parity beams which could not be damaged, altered, and reverse-engineered. Various encryption types such as mirroring, anomalous reflection, multi-beam generation, and scattering diffusion can be dynamically attained via our multifunctional metasurface. Besides, contrary to conventional time-consuming and optimization-based methods, this paper convincingly offers a fast, straightforward, and efficient design of diffusion metasurfaces of arbitrarily large size. Rigorous full-wave simulations corroborated the results where the phase-encrypted metasurfaces exhibited a polarization-insensitive reflectivity less than -10 dB over a broadband frequency range from 1 THz to 1.7 THz. This work reveals new opportunities for the extension of re-programmable THz-coding metasurfaces and may be of interest for reflection-type security systems, computational imaging, and camouflage technology.

Multilayer graphene-based metasurfaces: robust design method for extremely broadband, wide-angle, and polarization-insensitive terahertz absorbers.

In this study, by using an equivalent circuit method, a polarization-insensitive terahertz (THz) absorber based on multilayer graphene-based metasurfaces (MGBMs) is systematically designed, providing an extremely broad absorption bandwidth (BW). The proposed absorber is a compact, three-layer structure, comprising square-, cross-, and circular-shaped graphene metasurfaces embedded between three separator dielectrics. The equivalent-conductivity method serves as a parameter retrieval technique to characterize the graphene metasurfaces as the components of the proposed circuit model. Good agreement is observed between the full-wave simulations and the equivalent-circuit predictions. The optimum MGBM absorber exhibits >90% absorbance in an extremely broad frequency band of 0.55-3.12 THz (BW=140%). The results indicate a significant BW enhancement compared with both the previous metal- and graphene-based THz absorbers, highlighting the capability of the designed MGBM absorber. To clarify the physical mechanism of absorption, the surface current and the electric-field distributions, as well as the power loss density of each graphene metasurface, are monitored and discussed. The MGBM functionality is evaluated under a wide range of incident wave angles to prove that the proposed absorber is omnidirectional and polarization-insensitive. These superior performances guarantee the applicability of the MGBM structure as an ultra-broadband absorber for various THz applications.