Metasurface-enhanced light detection and ranging technology

The study addresses the challenge of achieving high-frame-rate, high-resolution LiDAR with large field of view for robotic and autonomous systems. Conventional LiDARs rely on time-of-flight (ToF) or continuous-wave modulation schemes and require fast beam steering across wide angles. Industrial scanners (macro-mechanical) reach large FoV but have limited imaging rates (tens of Hz), while MEMS offer kHz speeds but small FoV (~25°×15°). Other approaches like optical phased arrays provide speed with ~60° FoV but face manufacturing hurdles; liquid crystal modulators have limited FoV (<20°) and kHz speed; acousto-optic deflectors (AODs) provide MHz scanning but very narrow FoV (~2°). The research question is whether passive metasurfaces, combined with ultrafast low-FoV deflectors, can expand the FoV while preserving ultrafast scanning, enabling simultaneous peripheral and foveal (central) imaging akin to human vision. The purpose is to demonstrate a metasurface-augmented LiDAR achieving up to 150° FoV with MHz-rate beam steering and multizone (high- and low-resolution) imaging for ADAS and robotics.

The paper reviews LiDAR modalities (ToF, AMCW, FMCW, SFCW) and applications (mapping, atmospheric sensing, wind speed, object tracking, AR/VR). It surveys scanning technologies: macro-mechanical systems with 360° FoV but low frame rates; MEMS scanners reaching kHz but limited FoV; OPAs with fast steering and ~60° FoV but fabrication complexity; liquid crystal beam deflectors with limited FoV and kHz switching; and AODs with MHz speeds but very narrow FoV (~2°). Metasurfaces are introduced as flat optical components enabling wavefront control using various phase mechanisms (resonant scattering, geometric phase, effective-index pillars, topological phase). Dynamic metasurfaces can provide tunability via external stimuli (e.g., liquid crystals, phase-change, electro-optic effects). Prior work demonstrated a liquid-crystal-infiltrated resonant metasurface scanner with ~120° FoV, but with complex electronics and potential losses with metallic elements. This context motivates a hybrid approach using passive metasurfaces with ultrafast AODs to overcome speed-FoV trade-offs.

System architecture: A modulated laser at 633 nm (TOPTICA i-beam smart) emits single pulses up to 250 MHz repetition rate. For single-pulse ToF LiDAR, the maximum unambiguous distance d_max = c/(2 f_rep). The beam is steered by an acousto-optic deflector (AOD) with ~2°×2° FoV and relayed to a metasurface (MS) using a scanning lens (or directly, in the cm-scale MS configuration). The MS converts the small-angle AOD scan into a large-angle output, achieving overall FoV of approximately −75° to +75° per axis. Metasurface design: An effective refractive index (ERI) multibeam deflecting transmissive metasurface with radial symmetry is designed to impart a spatially varying phase gradient. The phase function follows ∂φ/∂r = −k0 (r/r_max), resulting in a parabolic phase retardation from 0 (center) to π (edge). Generalized Snell’s laws relate in-plane phase gradients to output k-vectors, enabling mapping from the AOD impact point (in polar coordinates r, θ_MS) to far-field angles (θ, φ). For small incident angles from the AOD, linear relationships between AOD drive voltage and output deflection angles are obtained. Devices of 1–3 mm diameter were first used with a scanning lens to maintain small beam spots (~50 μm) and limit divergence; a 1 cm-diameter device was later fabricated via nanoimprint lithography (NIL) for collimated-beam operation and reduced divergence. Fabrication: Dielectric nanopillar metasurfaces with laterally varying pillar sizes (varying effective index) were fabricated. A large-area (1 cm) ERI MS was produced using NIL. SEM confirms nanopillar arrays of varying geometry. Two-axis scanning: A second AOD, oriented orthogonally, was added to provide elevation control, enabling 2D scanning over the MS to realize large-FoV 3D imaging. Detection and ToF processing: Backscattered light is collected by fast detectors. For standard operation, a single detector captures echoes for ToF extraction and 2D/3D ranging. For multizone imaging, a dual-detector scheme is employed: one detector with a blocked central NA collects peripheral returns (1st-order MS beam), and a second detector with a spatial filter collects narrow-FoV central returns (0th-order AOD-only beam). Data are processed to detect single echoes and compute distances. Experiments: 1) 1D ranging of three reflective targets on a table to form 2D (angle-depth) images; 2) 2D line scans across azimuth and elevation to demonstrate ~150° FoV per axis; 3) 3D imaging of three actors at 1.2, 2.7, and 4.9 m; 4) Multizone imaging using the cm-scale NIL MS with ~40% first-order deflection efficiency, simultaneously capturing peripheral low-resolution and central high-resolution scenes; 5) High-speed performance characterization by measuring reflected signal amplitude versus scan frequency to determine −3 dB cutoff; 6) Time-series velocimetry using a rotating mechanical chopper (~100 Hz) with reflective tape, acquiring sequences at 741, 1020, and 3401 fps and tracking angular position over time.

• Metasurface-augmented scanning expands an AOD’s small ~2° FoV to a large ~150° FoV (−75° to +75°) while preserving ultrafast beam steering (MHz rates).
• 1D ToF imaging correctly localized three targets at approximately: square reflector at x ≈ −0.4 m, z ≈ 1.5 m; round reflector at x ≈ −0.1 m, z ≈ 2.4 m; box reflector at x ≈ 0.6 m, z ≈ 3.5 m, validating short-range (~5 m) operation.
• Two-axis scanning (dual orthogonal AODs) achieved line scans across elevation and azimuth over ~150° in each axis, enabling wide-angle 3D imaging of actors at 1.2, 2.7, and 4.9 m.
• Multizone imaging demonstrated simultaneous acquisition of: (i) a high-resolution, narrow-FoV central (0th-order) scan and (ii) a low-resolution, large-FoV peripheral (1st-order metasurface) scan using a dual-detector scheme. The large-area metasurface provided ~40% first-order deflection efficiency; a 3 mm device exhibited <1.5° divergence.
• High-speed performance: measured −3 dB amplitude loss up to ~6 MHz for single-axis scanning and ~10 MHz for two-axis operation; correct imaging maintained up to 6.25 MHz scanning frequency. This is roughly two orders of magnitude faster than other beam-pointing technologies.
• Time-series velocimetry: the rotating chopper’s speed was measured across multiple frame rates (741, 1020, 3401 fps), yielding an average of 92.71 Hz (vs. 100 Hz nominal), and revealing small periodic wobble correlated with the reflective tape position. Estimated reflective tape size from ranging data was ~4 cm (within 1 cm of actual considering nearby screw reflections).
• Analytical frame rate relation f_rate = c/(2 n d_max) highlights trade-offs between pixel count and maximum unambiguous distance. With code-division multiplexing (CDMA), the approach could reach ~125 fps at 200×200 pixels while maintaining large FoV and speed.

The work demonstrates that passive metasurfaces, when cascaded with ultrafast AODs, overcome the classic speed–FoV trade-off in LiDAR. The metasurface maps small-angle AOD deflections to large-angle output, enabling up to 150° FoV with MHz-rate scanning. This capability enables simultaneous peripheral and foveal imaging, mimicking human vision, which is valuable for ADAS and robotics: peripheral, low-resolution scanning supports situational awareness and object detection, while central, high-resolution scanning addresses fine recognition tasks. The experiments validate ranging accuracy at short to mid distances, wide-angle 2D/3D scans, multizone operation via 0th/1st orders, and real-time time-series tracking of fast events. The significance lies in five orders of magnitude improvement in wide-angle scanning rate over mechanical systems, with flexible implementation across wavelengths and materials. Practical deployment will need to manage trade-offs between maximum range and spatial resolution and integrate signal encoding (e.g., CDMA) and high-throughput data processing to fully exploit the achievable scan rates. Beyond automotive, the approach is relevant to microscopy and wide-angle OCT where short ambiguity distances suffice.

A metasurface-enhanced LiDAR architecture was realized by cascading ultrafast AODs with passive ERI metasurfaces, achieving beam steering at MHz speeds over ~150×150° FoV. The system supports 1D/2D/3D ToF imaging, demonstrates simultaneous multizone (peripheral and foveal) imaging using 0th and 1st diffraction orders, and enables time-series measurements of rapid events. The approach substantially increases wide-angle scanning rates relative to mechanical scanners and offers a compact, flexible path toward high-performance LiDAR for ADAS and robotics. Future work should focus on increasing ranging distance while maintaining resolution (e.g., higher power sources, improved optical efficiency), advanced encoding (CDMA) to boost frame rates at high pixel counts, larger-area low-divergence metasurfaces, and hardware-accelerated, real-time data processing pipelines to handle the data rates inherent to MHz scanning.

• Data throughput and processing: real-time computation and storage could not keep pace with acquisition using CPU/LabVIEW; dedicated hardware acceleration will be required for full-rate operation.
• Trade-off between maximum unambiguous distance and spatial resolution per f_rate = c/(2 n d_max); increasing pixel count or range reduces frame rate.
• Demonstrations employed relatively low laser peak power (~10 mW) and high-reflectivity targets indoors; longer-range outdoor performance would require higher power and improved efficiency.
• Beam divergence and resolution depend on metasurface size; small MS require focusing optics and exhibit higher divergence. Larger, more complex MS reduce divergence but add fabrication complexity.
• First-order deflection efficiency was moderate (~40%); higher efficiency would improve SNR and range.
• AOD intrinsic narrow input FoV necessitates the metasurface mapping; alignment and optical losses in cascaded elements can impact overall performance.