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Artificially designed modulators that enable a wealth of freedom in manipulating the terahertz (THz) waves at will are an essential component in THz sources and their widespread applications. Dynamically controlled metasurfaces, being multifunctional, ultrafast, integrable, broadband, high contrasting, and scalable on the operating wavelength, are critical in developing state-of-the-art THz modulators. Recently, external stimuli-triggered THz metasurfaces integrated with functional media have been extensively explored. The vanadium dioxide (VO2)-based hybrid metasurfaces, as a unique path toward active meta-devices, feature an insulator–metal phase transition under the excitation of heat, electricity, and light, etc. During the phase transition, the optical and electrical properties of the VO2 film undergo a massive modification with either a boosted or dropped conductivity by more than four orders of magnitude. Being benefited from the phase transition effect, the electromagnetic response of the VO2-based metasufaces can be actively controlled by applying external excitation. In this review, we present recent advances in dynamically controlled THz metasurfaces exploiting the VO2 phase transition categorized according to the external stimuli. THz time-domain spectroscopy is introduced as an indispensable platform in the studies of functional VO2 films. In each type of external excitation, four design strategies are employed to realize external stimuli-triggered VO2-based THz metasurfaces, including switching the transreflective operation mode, controlling the dielectric environment of metallic microstructures, tailoring the equivalent resonant microstructures, and modifying the electromagnetic properties of the VO2 unit cells. The microstructures' design and electromagnetic responses of the resulting active metasurfaces have been systematically demonstrated, with a particular focus on the critical role of the VO2 films in the dynamic modulation processes. [ABSTRACT FROM AUTHOR]

Geometric phase metasurfaces, as one of the main branches of meta‐optics, have attracted enormous interest in the last two decades. Recently, through rotating a set of subwavelength dipole sources, geometric phase concept has been extended to near‐field regime for the control of surface plasmons (SPs). Despite this progress, puzzles and shortcomings still exist: it is curious that geometric phases equal to once and twice the rotation angle of dipole source are both reported for SP controls, and the control strategies examined thus far only work for a single wavelength. Hereby, a rigorous derivation of the SP excitation of dipole sources upon circularly polarized illumination is performed, and the rotation dependence and in‐plane coordinate correlation of geometric phase control of SPs is clarified. Moreover, a holographic approach is proposed to implement multiplexed geometric phase control, experimentally demonstrating several metalenses that can couple and steer the incident circular polarizations of four wavelengths and two spin directions to different SP focusing beams. This work will pave an avenue toward the development of integrated and multiplexed SP devices. [ABSTRACT FROM AUTHOR]

Light is a complex vectorial field characterized by its amplitude, phase, and polarization properties, which can be further represented by four basic parameters, that is, amplitudes and phases of two orthogonally polarized components. Controlling these parameters simultaneously and independently at will using metasurfaces are essential in arbitrarily manipulating the light propagation. However, most of the studies so far commonly require a great number of different meta‐atoms or rely on diffraction under oblique incidence, which lack convenience and flexibility in design and implementation. Here, a new metasurface paradigm is proposed that can completely manipulate the amplitudes and phases of two spin components based on the interference effect, where only two different meta‐atoms are applied. For proof‐of‐concept demonstration, two five‐channel meta‐holograms for imaging and information encryption are designed and experimentally characterized. The interference method provides a simple route toward compact complex and multifunctional meta‐devices. [ABSTRACT FROM AUTHOR]

Coupling free‐space light into surface plasmons (SPs) has traditionally faced challenges in arbitrarily controlling the directionality of the SPs with respect to the incident polarization. Recently reported polarization‐dependent directional coupling of SPs using metasurfaces has attracted enormous attention for its promise in developing innovative and on‐demand plasmonic devices. However, further progress has been hampered by the limitation in designating the directional coupling with tailorable polarization‐dependent feature and compatible ability to incidence of complex polarization distribution. Here, a directional coupling strategy is introduced to overcome these limits. It is demonstrated that, by utilizing the interference between two pairs of slit resonators with different resonance responses, the directional SP coupling can be expanded to arbitrary group of polarized incidence by simply varying their relative distance, and the SP coupling direction will reverse under the corresponding orthogonally polarized incidence without affecting the efficiency. To exhibit the versatile design capability of the strategy, it is further utilized to construct several polarization‐dependent plasmonic devices for controlling SP propagations, including two SP metalenses and a plasmonic Airy beam launcher, in which it is shown that the device can even be designed to work for non‐uniformly polarized incidence. The strategy paves a reliable route towards many practical on‐chip applications. [ABSTRACT FROM AUTHOR]

Extreme terahertz (THz) science and technologies, the next disruptive frontier in nonlinear optics, provide multifaceted capabilities for exploring strong light‐matter interactions in a variety of physical systems. However, current techniques involve the need for an extremely high‐field free space THz source that is difficult to generate and has limited investigations to a rather weak and linear regime of light‐matter interactions. Therefore, new approaches are being sought for the tight confinement of THz waves that can induce nonlinear effects. Here, a nonlinear "tera‐nano" metasurface is demonstrated exhibiting extremely large THz nonlinearity and sensitive self‐modulation of resonances at moderate incident THz field strengths. A record deep‐subwavelength (≈λ/33 000) confinement of strongly enhanced (≈3200) THz field in a nano‐gap (15 nm) exhibits remarkable THz field‐tailored nonlinearity. Further, ultrafast injection of photocarriers reveals a competition between nonlinear THz field‐induced intervalley scattering and optically driven interband excitations. The results on "tera‐nano" metasurfaces enable a novel platform to realize enhanced nonlinear nano/micro composites for field‐sensitive extreme THz nonlinear applications without the need for intense THz light sources. [ABSTRACT FROM AUTHOR]

Conventional metasurfaces often manipulate light propagation in a passive way, which cannot satisfy the dynamic operation requirements of many practical applications. Integrating functional materials or components into the structure design is one effective method, which, however, often introduces additional loss to the resonances, thus greatly affecting the efficiency of modulation. Here, we numerically propose a reflection-type dielectric metasurface design that could allow switchable and efficient wavefront control in the terahertz regime. The metasurfaces are composed of dielectric structures suspended on a ground metallic film, which can exhibit different phase responses at different gap distances between them owing to the variation of the Fabre-Pérot (FP) resonance. Two metasurfaces are designed and numerically demonstrated, which can achieve mechanically switchable beam steering and wave focusing responses, respectively, by only changing the gap distance by a small range of 1/3 working wavelength. The maximum efficiency can reach around 89%. The proposed method provides a new avenue towards efficient and active terahertz wavefront control. [ABSTRACT FROM AUTHOR]

Spoof surface plasmons (SSPs) play crucial roles in terahertz (THz) near‐field photonics. However, both high‐efficiency excitation and wavefront engineering of SSPs remain great challenges, which hinder their wide applications in practice. Here, a scheme is proposed to simultaneously achieve these two goals efficiently using a single ultracompact device. First, it is shown that a gradient meta‐coupler constructed by high‐efficiency Pancharatnam–Berry (PB) meta‐atoms can convert circularly polarized (CP) THz beams into SSPs with absolute efficiency up to 60%. Encoding a parabolic phase profile into the meta‐coupler based on the PB mechanism, it is demonstrated that the device can covert CP beams into SSPs with focusing or defocusing wavefronts, dictated by the chirality of the incident wave. Finally, two distinct chirality‐dependent phase distributions are encoded into the meta‐coupler design by combining the PB and resonance phase mechanisms, and it is demonstrated that the resulting meta‐device can achieve SSP excitations with chirality‐delinked bifunctional wavefront engineering. THz near‐field experiments are performed to characterize all three devices, in excellent agreement with full‐wave simulations. The results pave the road to realize ultracompact devices integrating different functionalities on near‐field manipulations, which can find many applications (e.g., optical sensing, imaging, on‐chip photonics, etc.) in different frequency domains. [ABSTRACT FROM AUTHOR]

Seeking the unprecedented ability of metasurfaces to manipulate terahertz waves in a desired manner has attracted increasing interest. We design and investigate a kind of multi-channel, polarization-dependent transmission-type dielectric metasurface-based terahertz beam splitter. It is numerically demonstrated that such metasurfaces can manipulate terahertz beam, and split it into four channels with alterable angles and orthogonal polarizations, under the linearly polarized incidence with both the x- and y–polarization components. The suggested regime may alleviate the shortage of advanced multifunctional devices and provide extra freedom for terahertz functional device design. [ABSTRACT FROM AUTHOR]

Abstract: Surface plasmons (SPs) promise versatile potential applications in many aspects and thus have been a subject of enormous interest. As the most essential functionality, efficient coupling of free‐space light into complex SP field is of particular interest in current research. Most existing methods only focus on either efficient coupling or complex SP field generation, which is insufficient to support the ever‐increasing demand of practical applications. Herein, it is demonstrated both theoretically and experimentally that a metasurface composed of properly designed C‐shaped slit resonators can function as an efficient SP coupler. More importantly, it is shown that the phase and amplitude of the coupled SPs can be simultaneously manipulated, which would offer considerable design flexibilities for complex SP launching. This methodology can provide a wide platform for the design and fabrication of high‐performance plasmonic devices in the future, especially large‐area ones in practical applications. [ABSTRACT FROM AUTHOR]

Metasurfaces, 2D artificial electromagnetic media, open up a new frontier of functional device design ranging from radio waves to the visible region. Particularly, metasurface-based lenses are indispensable in various practical terahertz applications. The authors aim at achieving flexible and robust metalenses for efficient terahertz wave control. In general, resolution and efficiency are two inevitable parameters in determining the focusing and imaging abilities of lenses, which however are rarely experimentally demonstrated in the terahertz band. In this Communication, three broadband and robust metalenses with nonlinear phase profiles are proposed, all of which are experimentally investigated by using near-field scanning terahertz microscopy (NSTM) with a spatial resolution of 50 µm. The measurement shows that the metalens can focus a 0.95 THz wave to a spot size of 580 µm and achieve a transmittance efficiency of 45%. In addition, the NSTM system facilitates an experimental investigation of the incidence angle dependence of the terahertz metalens, which proves the robust focusing feature of the proposed device. This demonstration delivers a promising metasurface design for potential applications in imaging and information processing that may be of interest for the entire electromagnetic spectrum. [ABSTRACT FROM AUTHOR]

Orbital angular momentum (OAM) has been recently introduced to plasmonics for generating plasmonic vortices with a helical wavefront, opening avenues for exotic on‐chip applications such as quantum information processing and communications. In previous demonstrations, carefully designed optical elements are used to convert left‐ and right‐circular polarizations into plasmonic vortices with different topological charges, resulting in conversion from optical spin angular momentum (SAM) to plasmonic OAM. Here, it is demonstrated theoretically and experimentally that by utilizing the near‐field coupling between paired resonators in a metasurface, selective conversion from optical SAM to plasmonic OAM is realized, where generation of plasmonic vortices can be achieved for incident light of one circular polarization while significantly suppressed for the other circular polarization. The proposed design scheme may motivate the design and fabrication of future practical plasmonic devices. [ABSTRACT FROM AUTHOR]

Metasurfaces become an effective route to the control of not only free‐space waves but also surface plasmon polaritons (SPPs). SPPs, propagating along the interface between a metal and a dielectric, show properties of strong enhancement of the local field. Thanks to these essential functionalities, manipulating SPPs in metasurfaces is of particular interest in current research. Here, observation of anomalous SPP propagation is presented when SPPs traverse an interface between two metasurfaces whose topologies of eigenmode equal‐frequency contours are different. By combining various complementary meta‐resonators, SPP propagation was ready to be redirected at the topological transition interface at will. The proposed integrated metasurfaces open unprecedented avenues for anomalous wave propagation and guidance, showing exceptional capabilities for designing photonic devices. [ABSTRACT FROM AUTHOR]