Band 14, Heft 17

We present a systematic investigation of the optical response to circularly polarized illumination in twisted stacked plasmonic nanostructures. The system consists in two identical, parallel gold triskelia, centrally aligned and rotated at a certain angle relative to each other. Sample fabrication was accomplished through a novel multilevel high-resolution electron beam lithography. This stack holds two plasmonic modes of multipolar character in the near-infrared range, showing a strong dependence of their excitation intensities on the handedness of the circularly polarized incident light. This translates into a large circular dichroism which can be modulated by adjusting the twist angle of the stack. Fourier-transform infrared (FTIR) spectroscopy and numerical simulations were employed to characterize the spectral features of the modes. Remarkably, in contrast to previous results in other stacked nanostructures, the system’s response exhibits a behavior analogous to that of two interacting dipoles only at small angles. As the angle approaches 15°, where maximum dichroism is observed, more complex modes of the stack emerge. These modes evolve towards two in-phase multipolar excitations of the two triskelia as the angle increases up to 60°. Finally, simulations for a triangular array of such stacked elements show a sharp mode arising from the hybridization of a surface lattice resonance with the low-energy mode of the stack. This hybridized mode demonstrates the capability to be selectively switched on and off through the light polarization handedness.

The high extinction ratio mode (de)multiplexer is a pivotal component in high capacity mode-division multiplexing data communication and nascent on-chip intermodal acousto-optic modulators. Up to now, high performance on-chip mode (de)multiplexers are still lacking for integrated AOMs on the lithium niobate-on-insulator platform. In this paper, we propose and demonstrate an innovative scheme to achieve high extinction ratio signal routing for acousto-optic modulation, by leveraging a two-mode (de)multiplexer in conjunction with a high- Q racetrack microring resonator. The integrated devices are fabricated with one-step electron beam lithography and dry etching processes. The demonstrated two-mode (de)multiplexer boasts the excellent intermodal crosstalk below −20 dB and the on-chip insertion loss of less than 1.92 dB within the wavelength range of 1,514–1,580 nm. With the reinforcement of the microring resonator filter, the carrier signal can be suppressed thoroughly and the measured extinction ratio attains over 30 dB. Our proof-of-principle investigations have provided a feasible and compact solution to implement practical intermodal AOMs in LNOI for photonic and quantum information process, microwave photonics, and LiDAR.

Optically variable features are widely used in product design and anti-counterfeiting. However, current industrial methods rely heavily on chemical inks, which pose environmental concerns and suffer from poor wear and corrosion resistance. We experimentally demonstrate the generation of non-iridescent yet angle-dependent structural colors on titanium surfaces using a nanosecond laser-induced oxidation. Unlike conventional optical color-change methods that rely on multilayer interference, grating diffraction, or surface plasmons, this technique leverages a periodically arranged stepped structure to achieve abrupt color changes under small angle variations. The color shift originates from morphological differences among structures at different heights, which reflect light at distinct angles and produce varying colors through interference effects. The formation mechanism is elucidated through numerical simulations of the processing temperature, revealing that controlled laser ablation, oxidation, and thermal radiation on the sample surface create the unique structure. By tuning the point distance and dwell time, the affected area and intensity of these processes can be regulated. This advancement not only provides new ideas for anti-counterfeiting applications but also broadens the capabilities of laser coloring technology.

Vortex beams, characterized by orbital angular momentum (OAM), hold significant potential in optical communications, quantum information processing, and optical manipulation. However, existing metasurface designs are largely confined to single-degree-of-freedom control, such as static OAM generation or fixed focal points, which limiting their ability to integrate polarization multiplexing with dynamic focal tuning. To address this challenge, we propose a tunable multifunctional cascaded metasurface that synergizes polarization-sensitive phase engineering with interlayer rotational coupling, overcoming conventional device limitations. The designed metasurface independently generates distinct OAM states in orthogonal circular polarization channels under right-handed circularly polarized (RCP) incidence, that is, a vortex beam with topological charge ℓ = −1 in the left-handed circularly polarized (LCP) channel and a superimposed vortex state (ℓ = +1, −1) in the RCP channel. Continuous focal tuning is achieved via interlayer rotation in the axis-direction, with experimental validation at target frequency. Experimental results demonstrate the focal length modulation range from 25.9λ to 9.5λ as the interlayer rotation angle varies between 90° and 240°. This multi-degree-of-freedom control strategy establishes a new method for high-capacity optical communications, dynamic holography, and quantum state manipulation, while advancing the development of intelligent metasurfaces for 6G networks and integrated photonic systems.

Optical neural networks (ONNs) have demonstrated unique advantages in overcoming the limitations of traditional electronic computing through their inherent physical properties, including high parallelism, ultra-wide bandwidth, and low power consumption. As a crucial implementation of ONNs, on-chip diffractive optical neural network (DONN) offers an effective solution for achieving highly integrated and energy-efficient machine learning tasks. Notably, wavelength, as a fundamental degree of freedom in optical field manipulation, exhibits multidimensional multiplexing capabilities that can significantly enhance computational parallelism. However, existing DONNs predominantly operate under single-wavelength mechanisms, limiting the computational throughput. Here, we propose a multi-wavelength visual classification architecture termed PhC-DONN, which integrates two-dimensional photonic crystal (PhC) components with diffractive computing units. The architecture comprises three functional modules: (1) a PhC convolutional layer that enables multi-wavelength feature extraction; (2) a three-stage diffraction layer performing parallel modulation of optical fields; and (3) a PhC nonlinear activation layer implementing wavelength nonlinear computation. The results demonstrate that the PhC-DONN achieves classification accuracies of 99.09 % on the MNIST dataset, 66.41 % on the CIFAR-10 dataset, and 92.25 % on KTH human action recognition. By introducing a wavelength-parallel classification mechanism, the architecture accomplishes multi-channel inference during a single light propagation pass, resulting in a 32-fold enhancement in computational throughput compared to conventional DONNs while improving classification accuracy. This work not only establishes a novel optical classification paradigm for multi-wavelength optical neural network, but also provides a viable pathway towards constructing large-scale photonic intelligence parallel processors.

Frequency modulated continuous wave (FMCW) light detection and ranging (LiDAR) has recently become a research hotspot in the fields of autonomous driving and intelligent perception due to its high-precision ranging and velocity measurement capabilities. However, the existing LiDAR systems are usually challenged in expanding the field-of-view (FOV), which often comes at the expense of beam quality and degrades the detection accuracy and signal-to-noise ratio. On the other hand, the complexity of data processing algorithms may introduce significant measurement inaccuracies, potentially leading to substantial deviations in the final results. These two constraints limit the performance of LiDAR in complex scenarios. To address these issues, this paper proposes a new architecture for FMCW LiDAR that employs a geometric metasurface as a polarization splitter for expanded FOV of beam steering. With the combination of mechanical scanning mirror and metasurface, the scanning FOV has been successfully enlarged from 64° × 20° to 64° × 40°. Simultaneously, millimeter-level precision was achieved in distance measurement, along with an average relative error of 9 mm/s in velocity measurement, which confirms stable and precise system performance. This approach not only broadens the scanning range but also preserves the measurement accuracy of FMCW technology. This paper innovatively combines polarization beam-splitting metasurface with FMCW technology to achieve high-precision measurement over a wide field of view, providing a promising new technical pathway for the technological evolution of future LiDAR systems.

The terahertz (THz) frequency band has abundant spectrum resources, which is suitable for constructing communication systems with ultra-high data rates and extremely low latency. Multiple input multiple output (MIMO) devices are crucial for realizing THz communication, and the synchronous transmission and noncorrelation of different channels are the keys to MIMO technology. This paper proposes a graphene-based polarization spatial diversity and multiplexing MIMO surface (PDM-MIMOS) with 2 × 2 metasurface arrays. Dual-polarized channels can be modulated synchronously by the same metasurface modulator and received by the receiver (RX) without crosstalk. Experimental results demonstrate that the modulation cut-off frequency can reach up to 30 kHz. By constructing a continuous THz wave communication system, it is demonstrated that PDM-MIMOS can achieve spatial diversity and multiplexing, thereby improving communication quality and data rate. Furthermore, we compare the signal quality of THz communication and visible light communication under villainous weather conditions. The experiment proves that the communication reliability of THz communication is 19.4 times that of visible light communication. This work offers potential for compact, dual-polarized modulators that can be applied in THz communication, detection, and imaging.

Metasurfaces, capable of manipulating light at subwavelength scales, hold great potential for advancing optoelectronic applications. Generative models, particularly Generative Adversarial Networks (GANs), offer a promising approach for metasurface inverse design by efficiently navigating complex design spaces and capturing underlying data patterns. However, existing generative models struggle to achieve high electromagnetic fidelity and structural diversity. These challenges arise from the lack of explicit electromagnetic constraints during training, which hinders accurate structure-to-electromagnetic mapping, and the absence of mechanisms to handle one-to-many mappings dilemma, resulting in insufficient structural diversity. To address these issues, we propose the Anchor-controlled Generative Adversarial Network (AcGAN), a novel framework that improves both electromagnetic fidelity and structural diversity. To achieve high electromagnetic fidelity, AcGAN proposes the Spectral Overlap Coefficient (SOC) for precise spectral fidelity assessment and develops AnchorNet, which provides real-time physics-guided feedback on electromagnetic performance to refine the structure-to-electromagnetic mapping. To enhance structural diversity, AcGAN incorporates a cluster-guided controller that refines input processing and ensures multilevel spectral integration, guiding the generation process to explore multiple configurations. Empirical analysis shows that AcGAN reduces the Mean Squared Error (MSE) by 73 % compared to current state-of-the-art and significantly expands the design space to generate diverse metasurface architectures that meet precise spectral demands.

Electromagnetic scattering control of optical windows has significant challenges in improving optical transmission and compatibility, especially for multispectral and large-angle incidences, due to material and structure mismatches. This paper presents trans-scale hierarchical metasurfaces (THM) to achieve wide-angle optical transmission enhancement and electromagnetic scattering-compatible regulation in dual-band lasers, and infrared and microwave ranges. THM comprises an ultrafine hollow metal array (UHMA) and a transmission-enhanced micro-nanocone array (TMCA). The UHMA regulates microwave radar cross-section (RCS) echo diffuse reflection, while the upper-layer TMCA enables wide-angle optical transmission enhancement. A THM sample of 200 × 200 mm 2 was fabricated using multistage nanolithography, demonstrating exceptional multifunctional compatibility and optical performance. Results show that the THM sample achieves 10 dB scattering reduction in the 9.5–17.5 GHz microwave band, with average optical transmittance exceeding 90 % at 0°–60° incidence angles within optical ranges of 1.42, 1.7, and 3–5 μm. Compared to a zinc sulfide (ZnS) window with a UHMA on its surface, the THM improved the average transmission by 34.3 % over wide angles while allowing microwave scattering control. Broadband polarization-independent, low-crosstalk imaging, and hydrophobic characteristics were demonstrated. This study provides a design approach for multifunctional devices with synergistic optical and microwave regulation, particularly for optical transparency in microwave devices.

One-dimensional photonic crystal (1D-PhC) pillar cavities allow transducing mechanical pillar vibrations to the optical domain, thereby relaxing the requirements typically associated with mechanical motion detection. In this study, we integrate these geometries into a silicon-on-insulator photonics platform and explore their optical and mechanical properties. The 1D-PhC structures consist of a linear array of high aspect ratio nanopillars with nanometer-sized diameters, designed to enhance the interaction between transverse-magnetic (TM) polarized optical fields and mechanical vibrations and to minimize optical leaking to the substrate. Integrated waveguides are engineered to support TM-like modes, which enable optimized coupling to the 1D-PhC optical cavity modes via evanescent wave interaction. Finite element method simulations and experimental analyses reveal that these cavities achieve relatively high optical quality factors ( Q ∼ 4 × 10 3 ). In addition, both simulated and experimentally measured mechanical vibrational frequencies show large optomechanical coupling rates exceeding 1 MHz for the fundamental cantilever-like modes. By tuning the separation between the 1D-PhC and the waveguide, we achieve optimal optical coupling conditions that enable the transduction of thermally activated mechanical modes across a broad frequency range – from tens to several hundreds of MHz. This enhanced accessibility and efficiency in mechanical motion transduction significantly strengthens the viability of established microelectromechanical (MEMS) and nanoelectromechanical systems (NEMS) technologies based on nanowires, nanorods, and related structures, particularly in applications such as force sensing and biosensing.

Under-display camera (UDC) systems enable full-screen displays in smartphones by embedding the camera beneath the display panel, eliminating the need for notches or punch holes. However, the periodic pixel structures of display panels introduce significant optical diffraction effects, leading to imaging artifacts and degraded visual quality. Conventional approaches to mitigate these distortions, such as deep learning-based image reconstruction, are often computationally expensive and unsuitable for real-time applications in consumer electronics. This work introduces an inverse-designed metasurface for wavefront restoration, addressing diffraction-induced distortions without relying on external software processing. The proposed metasurface effectively suppresses higher-order diffraction modes caused by the metallic pixel structures, restores the optical wavefront, and enhances imaging quality across multiple wavelengths. By eliminating the need for software-based post-processing, our approach establishes a scalable, real-time optical solution for diffraction management in UDC systems. This advancement paves the way to achieve software-free real-time image restoration frameworks for many industrial applications.