Volume 14, Issue 12

Graphene hosts massless Dirac fermions owing to its linear electronic band structure. This distinctive feature underpins its extraordinary electronic properties, correlating to strong light–matter interactions on an extreme subwavelength scale. Over the past decade, intensive investigations have transitioned from fundamental graphene’s optical properties to practical application with the integration of graphene into metasurfaces, opening a new era of active flat optics. In this review, we provide a comprehensive overview of graphene-based metasurfaces, beginning with the intrinsic link between graphene’s optical response and its electronic properties. We highlight the development of actively tunable platforms and devices, including efficient modulators, high-sensitivity detectors, and advanced biosensing systems. We also discuss emerging approaches that enable ultrafast all-optical modulation and ultracompact device footprints, pushing the boundaries of performance. Finally, we explore the transformative prospects of non-Hermitian physics and inverse design strategies as novel frameworks for optimizing metasurface configurations. By synergizing graphene’s intrinsic tunability with innovative design methodologies, graphene-based metasurfaces hold immense potential to bridge the gap between fundamental science and real-world applications, defining a new frontier in next-generation photonic technologies.

A precise method for phase and amplitude detection in both the time and frequency domains of terahertz spectroscopy based on the weak-value amplification technique is proposed and demonstrated. Within the weak-value amplification scheme, the imaginary weak value enhances variations in the terahertz phase signals, whereas the real weak value amplifies changes in the terahertz amplitude signals. By employing various postselections in the terahertz weak measurement procedure in detecting minute changes of the phase and amplitude of the terahertz wave, we achieved a phase change range from −0.0187 rad to 0.0183 rad with an interval of 0.004 rad and an amplitude change range from −0.0238 rad to 0.0228 rad with an interval of 0.0056 rad. This results in a phase and amplitude measurement resolution of 10 −4  rad in the time domain. In the frequency domain, E $\left\vert E\right\vert $ spectra are calculated to assess phase and amplitude variations with respect to frequency or wavelength. We apply these methods to chiral detection, particularly in measuring optical activity such as circular dichroism (CD) and optical rotatory dispersion (ORD). Despite challenges such as strong terahertz wave absorption in aqueous solutions and weak optical responses from natural chiral materials in the terahertz band, we successfully conducted chiroptical spectroscopy on a relatively large volume (2.3 mL) of liquid (R)- and (S)-limonene, as well as lactose tablets with varying mass fractions. Furthermore, the carrier-envelope phase (CEP) shift, defined for one- or few-cycle time-domain terahertz pulses, was effectively achieved through the manipulation of a pair of terahertz polarizers in the terahertz beam path. Notably, when ϕ CEP = 0, a 150 % increase in the absorption coefficient of lactose was observed when weak measurement techniques were employed, compared to conditions without such measurements. This effort yielded THz-ORD and THz-CD spectra, demonstrating the potential of our methods to overcome traditional limitations and provide new insights into the optical response, dynamic properties, and low-frequency vibrational modes of biomolecules and materials in low-energy states, ultimately facilitating the identification of chiral stereoisomers.

Dry synthesis is a highly versatile method for the fabrication of nanoporous metal films, since it enables easy and reproducible deposition of single or multi-layers of nanostructured materials that can find intriguing applications in plasmonics, photochemistry and photocatalysis, to name a few. Here, we extend the use of this methodology to the preparation of copper nano-islands that represent an affordable and versatile example of disordered plasmonic substrates. Although the island morphology is disordered, the high density of these nanostructures with large surface area results in a good homogeneity on a macroscale, which is beneficial for plasmonic applications such as bio-sensing and photo-catalysis. With cathodoluminescence and electron-energy-loss spectroscopies we confirm the nano-islands as sources of the local field enhancement and identify the plasmonic resonance bands in the visible and near-infrared spectral range. The decay dynamics of the plasmonic signal are slower in the nano-island as compared to bulk copper films, which can be rationalized by a reduced energy dissipation in the nano-island films. Our study demonstrates a robust and lithography-free fabrication pathway to obtain nanostructured plasmonic copper substrates that represent a highly versatile low-cost alternative for future applications ranging from sensing to photochemistry and photocatalysis.

We employ polarization tomography to characterize the modal properties of a dielectric nanocavity with sub-wavelength mode confinement. Our analysis of reflection spectra shows that the Fano-lineshape depends strongly on the polarization in a confocal configuration, and that the lineshape can be transformed into a Lorentzian-like peak for a certain polarization. For this polarization setting, the background is almost fully suppressed in a finite range of frequencies. This enables us to identify another resonance that has not yet been experimentally reported for these nanocavities. Lastly, we use symmetry-forbidden polarizations and show that, surprisingly, the modal resonance features of the system remain visible.

Nature can significantly inspire humans. Chameleons, jellyfish, and many other creatures use unique camouflage methods. Multispectral camouflage materials are highly desirable to against progressive multispectral detection. The proposed structure should be simple and highly transparent to ensure a wide application range. In this study, a bio-inspired multispectral camouflage material with visible transparency, microwave diffusion, and infrared (IR) camouflage was designed, fabricated, and tested. Multispectral camouflage performance was achieved on a single metasurface by the following steps: First, a nanoscale multilayered film consisting of an oxide and metal was unitized to achieve a low IR emissivity and high visible transmittance. Then, two units were designed to obtain a phase difference, thus realizing the microwave diffusion performance. Based on the relationship between the area filling fraction and IR emissivity, the units can perform puzzled imaging under an IR thermal camera. The structural parameters were calculated and optimized through an equivalent circuit model-based artificial intelligence algorithm. Then, a 10 dB reduction in radar cross section from 7 GHz to 16 GHz, a puzzled IR thermal image, and a high optical transmittance (>0.7) were achieved. The work provides significant guidance for the design and fabrication of multispectral camouflage materials.

The metasurfaces are able to flexibly control electromagnetic waves. However, their design necessitates strict phase matching, which requires high computational load, and its control method has low flexibility. This paper proposes a novel method for the customization of the electric field of metalens with high degrees of freedom based on an improved Pixel Generative Adversarial Network (I-pixGAN). This I-pixGAN can design the phase distribution of the metalens according to the customized target electric field, and the constructed metalens based on this phase can modulate the target electric field. The results show that the proposed I-pixGAN can arbitrarily control the position of the focus in three-dimensional space, where the focus position is offset within one wavelength. Therefore, it is not need to strictly follow the electric field distribution characteristics, and it can flexibly construct unknown electric field, realizing a highly free electric field customization method in three-dimensional space, providing a new way for electric field operation. This paper promotes the development of electric focusing systems, not only improving focusing accuracy but also enhancing system flexibility and promoting the automation and intelligence of optical and terahertz systems.

The interaction between free electrons and laser-induced near-fields provides a platform to study ultrafast processes and quantum phenomena while enabling precise manipulation of electron wavefunctions through linear and orbital momentum transfer. Here, by introducing phase offset between two orthogonally polarized laser pulses exciting a gold nanorod, we generate a rotating plasmonic near-field dipole with clockwise and counterclockwise circulating orientations and investigate its interaction with a slow electron beam. Our findings reveal that the circulation direction of plasmonic fields plays a crucial role in modulating electron dynamics, enhancing coupling strength, and controlling recoil. Furthermore, synchronizing the interaction time of the electron beam with rotational dipolar plasmonic resonances results in significant transfer of angular momentum to the electron beams and deflects the electron wavepackets from their original trajectory. These findings highlight the potential of plasmon rotors for shaping electron wavepackets, offering promising applications in ultrafast microscopy, spectroscopy, and quantum information processing.

How topologies play a role in light–matter interaction is of great interest in control and transfer of topologically-protected structures. These topological structures such as skyrmions and merons have not yet been found in canonical momentum fields, which are fundamental in mechanical transfer between optical and matter fields. Here, we reveal the universality of generating skyrmionic structures in the canonical momentum of light in multipole Mie scattering fields. We demonstrate the distinct topological stability of canonical momentum skyrmions and merons, and compare with well-studied Poynting vector and optical spin fields. The study of these fields allow for a clean and direct approach to measuring and quantifying energetic structures in optical fields, through observable radiation pressure. Our work lays the foundation for exploring new topologically nontrivial phenomena in optical forces, metamaterial design, and light–matter interaction.

Terahertz vortex beams, carrying orbital angular momentum (OAM), are quite desirable for enhancing data transmission capability in telecommunication. However, it faces fundamental and technical challenges in a single metasurface to simultaneously generate orthogonal basis vortices with linear polarization ( x - and y -polarity) and circular polarization (left- and right-handed polarity) under the orthogonal polarized light incident. Here, we proposed a chip-integrated all-dielectric metasurface in the terahertz regime, to demonstrate the simultaneous generation of four-channel orthogonal polarized vortex beams at various topological charges under the x - and y -polarized light incident. The polarization multiplexed metasurface was designed only with a propagation phase strategy, consisting of polarization-maintaining and polarization-conversion meta-atoms. Simultaneous control of polarization and topological charges in vortex beams was realized by properly arranging birefringent meta-atom arrays to induce additional phases of x - and y -polarization as customized, showing more degrees of freedom for carrying information. The experimental results are in good agreement with the simulations. Such a metasurface approach provides complete polarization bases for further synthesis of diverse polarization vortices required for huge-capacity communication.

Janus metasurfaces have emerged as a promising platform to enable independent wave manipulation by fully exploiting the inherent propagation direction of electromagnetic waves. These structures allow achieving distinct wavefront functionalities based on the direction of wave propagation. Concurrently, metagratings have gathered significant attention as an innovative design scheme for wavefront manipulation, particularly in addressing the low efficiency issue commonly associated with conventional metasurfaces. This study introduces Janus metagratings as a means for tailoring efficient, direction-dependent absorption and reflection. Utilizing established analytical models, a precise analysis of diffraction modes is conducted in transmissive metagratings, facilitating asymmetric wavefront manipulation under the two incidence directions. By arranging distinct meta-atoms with specific load impedances on the upper and lower layers of the metagrating, efficient asymmetric wave responses are achieved. The design methodology is validated through full-wave simulations, which demonstrate strong consistency with theoretical predictions. Additionally, a Janus metagrating prototype is fabricated and tested in the microwave frequency regime, validating the direction-dependent wavefronts tailoring characteristics. The proposed design methodology offers a versatile platform for asymmetric propagation and advanced systems in future wireless and optical communication applications.