Two-dimensional beam steering with tunable metasurface in infrared regime

Electrically programmable pixelated coherent mid-infrared thermal emission

Active metasurfaces have recently emerged as compact, lightweight, and efficient platforms for dynamic control of electromagnetic fields and optical responses. However, the complexities associated with their post-fabrication tunability significantly hinder their widespread applications, especially for the mid-infrared range due to material scarcity and design intricacy. Here, we experimentally demonstrate highly dynamic, pixelated modulations of coherent mid-infrared emission based on an electrically programmable plasmonic metasurface integrated with graphene field-effect transistors (Gr-FETs). The ultrabroad infrared transparency of graphene allows for customized control over plasmonic meta-atoms, thus achieving coherent mid-infrared states across a broad range of wavelengths, directions, and polarizations. The spatial temperature modulation generated by Gr-FETs is effectively synergized with the emissivity control by the localized surface plasmon polaritons and quasi-bound states in the continuum from gold nanoantennas. This integrated temperature-emissivity modulation of metasurfaces is systematically extended to form a pixelated 2D array with low crosstalk, envisioning advanced approaches toward scalable 2D electrical wiring for densely packed, independently addressable pixels.

Optoelectronic metadevices

Metasurfaces have introduced new opportunities in photonic design by offering unprecedented, nanoscale control over optical wavefronts. These artificially structured layers have largely been used to passively manipulate the flow of light by controlling its phase, amplitude, and polarization. However, they can also dynamically modulate these quantities and manipulate fundamental light absorption and emission processes. These valuable traits can extend their application domain to chipscale optoelectronics and conceptually new optical sources, displays, spatial light modulators, photodetectors, solar cells, and imaging systems. New opportunities and challenges have also emerged in the materials and device integration with existing technologies. This Review aims to consolidate the current research landscape and provide perspectives on metasurface capabilities specific to optoelectronic devices, giving new direction to future research and development efforts in academia and industry.

Electrically switchable 2N-channel wave-front control for certain functionalities with N cascaded polarization-dependent metasurfaces

Metasurfaces with tunable functionalities are greatly desired for modern optical system and various applications. To increase the operating channels of polarization-multiplexed metasurfaces, we proposed a structure of N cascaded dual-channel metasurfaces to achieve 2N electrically switchable channels without intrinsic loss or cross-talk for certain functionalities, including beam steering, vortex beam generation, lens, etc. As proof of principles, we have implemented a 3-layer setup to achieve 8 channels. In success, we have demonstrated two typical functionalities of vortex beam generation with switchable topological charge of l = -3 ~ +4 or l = -1 ~ -8, and beam steering with the deflection direction switchable in an 8×1 line or a 4×2 grid. We believe that our proposal would provide a practical way to significantly increase the scalability and extend the functionality of polarization-multiplexed metasurfaces. Although this method is not universal, it is potential for the applications of LiDAR, glasses-free 3D display, OAM (de)multiplexing, and varifocal meta-lens.

Time-varying metasurface driven broadband radar jamming and deceptions

Conventional radar jamming and deception systems typically necessitate the custom design of complex circuits and algorithms to transmit an additional radio signal toward a detector. Consequently, they are often cumbersome, energy-intensive, and difficult to operate in broadband electromagnetic environment. With the ongoing trend of miniaturization of various devices and the improvement of radar system performance, traditional techniques no longer meet the requirements for broadband, seamless integration, and energy efficiency. Time-varying metasurfaces, capable of manipulating electromagnetic parameters in both temporal and spatial domains, have thus inspired many contemporary research studies to revisit established fields. In this paper, we introduce a time-varying metasurface driven radar jamming and deception system (TVM-RJD), which can perfectly overcome the aforementioned intrinsic challenges. Leveraging a programmable bias voltage, the TVM-RJD can alter the spectrum distribution of incident waves, thereby deceiving radar into making erroneous judgments about the target's location. Experimental outcomes affirm that the accuracy deviation of the TVM-RJD system is less than 0.368 meters, while achieving a remarkable frequency conversion efficiency of up to 96.67%. The TVM-RJD heralds the expansion into a wider application of electromagnetic spatiotemporal manipulation, paving the way for advancements in electromagnetic illusion, radar invisibility, etc.

Broadband achromatic and wide field of view metalens-doublet by inverse design

Metalenses, composed of patterned meta-atoms in various dimensions, offer tailored modulation of phase, amplitude, and polarization for diverse imaging applications across the visible and near-infrared spectra. However, simultaneously achieving achromatic and wide field of view (WFOV) imaging remains a significant challenge. In this paper, we propose a general inverse design framework for metalens-doublets that simultaneously enables broadband achromatic and WFOV imaging. The broadband achromatic and WFOV (BA&WFOV) metalens-doublet comprises a propagation phase metalens and a geometric phase metalens positioned on opposite sides of the substrate. This framework requires only once polarization conversion and mitigates aperture size constraints imposed by the limited group delay (GD) range of meta-atoms. We present a BA&WFOV metalens-doublet with an f-number of 3.9, a full field of view (FOV) of 68°, and a wavelength range from 640nm to 820nm. This metalens-doublet exhibits diffraction-limited focusing with an average absolute focusing efficiency of 16% and an average relative focusing efficiency of 60%. This innovative framework holds significant promise for applications in fields such as phone cameras, VR/AR, and endoscopes.

Multi-band reprogrammable phase-change metasurface spectral filters for on-chip spectrometers

Active optical metasurfaces provide a platform for dynamic and real-time manipulation of light at subwavelength scales. However, most active metasurfaces are unable to simultaneously possess a wide wavelength tuning range and narrow resonance peaks, thereby limiting further advancements in the field of high-precision sensing or detection. In the paper, we proposed a reprogrammable active metasurface that employs the non-volatile phase change material Ge2Sb2Te5 and demonstrated its excellent performance in on-chip spectrometer. The active metasurfaces support magnetic modes and feature Friedrich-Wintgen quasi bound states in the continuum, capable of achieving multi-resonant near-perfect absorption, a multilevel tuning range, and narrowband performance in the infrared band. Meanwhile, we numerically investigated the coupling phenomenon and the intrinsic relationship between different resonance modes under various structural parameters. Furthermore, using the active metasurfaces as tunable filters and combined with compressive sensing algorithms, we successfully reconstructed various types of spectral signals with an average fidelity rate exceeding 0.99, utilizing only 51 measurements with a single nanostructure. A spectral resolution of 0.5 nm at a center wavelength 2.538 µm is predicted when the crystallization fractions of GST change from 0 to 20%. This work has promising potential in on-site matter inspection and point-of-care (POC) testing.

Electrically tunable plasmonic metasurface as a matrix of nanoantennas

We report the fabrication and characterization of a plasmonic metasurface comprising electrically-contacted sub-wavelength gold dipole nanoantennas, conformally coated by a thin hafnia film, an indium tin oxide layer and a backside mirror, forming metal-oxide-semiconductor (MOS) capacitors, for use as an electrically-tunable reflectarray or metasurface. By voltage biasing the nanoantennas through metallic connectors and leveraging the carrier refraction effect in the MOS capacitors, our measurements demonstrate phase control in reflection over a range of about 30°, with a constant magnitude of reflection coefficient of 0.5, and the absence of secondary lobes. Comprehensive electromagnetic and quantum carrier models of the structure are developed and are in excellent agreement with the measurements. The metasurface holds promise for use as an optical phased array.

Highly Tunable Cascaded Metasurfaces for Continuous Two-Dimensional Beam Steering

Cascaded metasurfaces can exhibit powerful dynamic light manipulation by mechanically tuning the far-field interactions in the layers. However, in most current designs, the metasurfaces are separated by gaps smaller than a wavelength to form a total phase profile, representing the direct accumulation of the phase profiles of each layer. Such small gap sizes may not only conflict with the far-field conditions but also pose great difficulties for practical implementations. To overcome this limitation, a design paradigm taking advantage of a ray-tracing scheme that allows the cascaded metasurfaces to operate optimally at easily achievable gap sizes is proposed. Enabled by the relative lateral translation of two cascaded metasurfaces, a continuous two-dimensional (2D) beam-steering device for 1064 nm light is designed as a proof of concept. Simulation results demonstrate tuning ranges of ±45° for biaxial deflection angles within ±3.5 mm biaxial translations, while keeping the divergence of deflected light less than 0.007°. The experimental results agree well with theoretical predictions, and a uniform optical efficiency is observed. The  generializeddesign paradigm can pave a way towards myriad tunable cascaded metasurface devices for various applications, including but not limited to light detection and ranging (LiDAR) and free space optical communication.

Tuning the infrared resonance of thermal emission from metasurfaces working in near-infrared

We simulated numerically and demonstrated experimentally that the thermal emittance of a metasurface consisting of an array of rectangular metallic meta-atoms patterned on a layered periodic dielectric structure grown on top of a metallic layer can be tuned by changing several parameters. The resonance frequency, designed to be in the near-infrared spectral region, can be tuned by modifying the number of dielectric periods, and the polarization and incidence angle of the incoming radiation. In addition, the absorbance/emittance value at the resonant wavelength can be tuned by modifying the orientation of meta-atoms with respect to the illumination direction.

Toward a universal metasurface for optical imaging, communication, and computation

In recent years, active metasurfaces have emerged as a reconfigurable nanophotonic platform for the manipulation of light. Here, application of an external stimulus to resonant subwavelength scatterers enables dynamic control over the wavefront of reflected or transmitted light. In principle, active metasurfaces are capable of controlling key characteristic properties of an electromagnetic wave, such as its amplitude, phase, polarization, spectrum, and momentum. A 'universal' active metasurface should be able to provide independent and continuous control over all characteristic properties of light for deterministic wavefront shaping. In this article, we discuss strategies for the realization of this goal. Specifically, we describe approaches for high performance active metasurfaces, examine pathways for achieving two-dimensional control architectures, and discuss operating configurations for optical imaging, communication, and computation applications based on a universal active metasurface.

Inverse design of perimeter-controlled InAs-assisted metasurface for two-dimensional dynamic beam steering

The current commercially viable light detection and ranging systems demand continuous, full-scene, and dynamic two-dimensional point scanning, while featuring large aperture size to ensure long distance operation. However, the biasing architecture of large-area arrays with numerous individually controlled tunable elements is substantially complicated. Herein, inverse design of a perimeter-controlled active metasurface for two-dimensional dynamic beam steering at mid-infrared regime is theoretically presented. The perimeter-control approach simplifies biasing architecture by allowing column-row addressing of the elements. The metasurface consists of a periodic array of plasmonic patch nanoantennas in a metal-insulator-metal configuration, wherein two active layers of indium arsenide are incorporated into its building block. The metasurface profile facilitates wide phase modulation of ≈ 355 ° on the reflected light at the individual element level through applying independent voltages to its respective columns and rows. The multi-objective genetic algorithm (GA) for optimizing user-defined metrics toward shaping desired far-zone radiation pattern is implemented. It is demonstrated that multi-objective GA yields better results for directivity and spatial resolution of perimeter-controlled metasurface by identifying the design tradeoffs inherent to the system, compared to the single-objective optimizer. A high directivity and continuous beam scanning with full and wide field-of-view along the azimuth and elevation angles are respectively maintained.