Microscopic geared metamachines

Abstract

The miniaturization of mechanical machines is critical for advancing nanotechnology and reducing device footprints. Traditional efforts to downsize gears and micromotors have faced limitations at around 0.1 mm for over thirty years due to the complexities of constructing drives and coupling systems at such scales. Here, we present an alternative approach utilizing optical metasurfaces to locally drive microscopic machines, which can then be fabricated using standard lithography techniques and seamlessly integrated on the chip, achieving sizes down to tens of micrometers with movements precise to the sub-micrometer scale. As a proof of principle, we demonstrate the construction of microscopic gear trains powered by a single driving gear with a metasurface activated by a plane light wave. Additionally, we develop a versatile pinion and rack micromachine capable of transducing rotational motion, performing periodic motion, and controlling microscopic mirrors for light deflection. Our on-chip fabrication process allows for straightforward parallelization and integration. Using light as a widely available and easily controllable energy source, these miniaturized metamachines offer precise control and movement, unlocking new possibilities for micro- and nanoscale systems.

Fig. 1. Metarotors.

a Schematic illustration of the optically powered micromotor, featuring a ring-shaped metarotor containing a metasurface, anchored to a glass chip using a capped pillar. b–e Scanning electron microscopy (SEM) images documenting the micromotor fabrication process (scale bar: 5 μm): b A Si metasurface is nanofabricated via electron beam lithography; the inset zooms in on the meta-atoms constituting the metasurface. c A SiO₂ ring containing the metasurface is etched. d The central pillar and e the cap are fabricated with SU-8 microlithography. f Illustration of the deflection of light by the metasurface, resulting in a force acting on the metasurface in the opposite direction. g The metasurface consists of four segments with different orientations of the meta-atoms. The white arrows indicate the forces they exert onto the metarotor upon illumination with linearly polarized light. The black arrow represents the resulting counterclockwise rotation. h Optical microscopy images (see Supplementary Video 1) of the rotation of a metarotor under a linearly polarized light beam with an intensity of 35 μW μm⁻². The red line indicates the tracking of a protrusion on the metarotor outer border. Scale bar: 10 μm. Average angular velocities of metarotors with equal diameters (16 μm) illuminated by a linearly polarized plane light beam as a function of the number of i meta-atoms and j laser light intensity. The shaded regions indicate the standard deviation. k Independence of the angular velocity from the gap size between the ring and the pillar. Error bars represent standard deviation from three measurements for each condition. l Probability distributions of the metarotor position along the x-axis for varying gap sizes. Smaller gap sizes lead to a higher confinement.

Fig. 2. Gear trains powered by metarotors.

a–d SEM images (top panels) and optical microscopy images (bottom panels) of metarotors acting as driving gears that propel passive gears with varying diameters (b–d). The rotation of the metarotors is indicated by the red arrows, while the rotation of the driven gear is indicated by the white arrows. e The average ratio of angular velocity between the driving gear (ω_m) and the driven gear (ω_p) depends on the ratio of their diameters. f–h SEM images (top panels) and optical microscopy images (bottom panels) of a single driving gear actuating a train of driven gears with the same diameter: f N = 3, g N = 4, and h N = 5 total gears (including the driving gear) with the same diameter D_m = D_p. i The angular velocity of the driven gears ω = ω_m = ω_p versus the number N of gears in the gear train. The dashed line represents ω/N. The optical microscopy images are from Supplementary Video 6. Error bars in e and i represent standard deviation from three measurements for each condition. Scale bars: 10 μm.

Fig. 3. Metarotor control by metasurface design and light polarization.

a–d Metarotor with a metasurface design that generates clockwise rotation as a function of light polarization. Schematic illustration of the metasurface design (top) and optical microscopy images (bottom) under illumination with a linear polarization, b right-hand circular polarization, and c left-hand circular polarization. The metasurface segments are indicated by the red-shaded areas and the resulting forces by the corresponding white arrows. The global rotation direction and speed of each motor is indicated by the direction and length of the black arrows. The experimentally measured rotation of the metarotor is overlaid on the optical images. d Angular velocity of the motor with changing light polarization. e–h Metarotor with a metasurface design that generates counterclockwise rotation as a function of the light polarization. i, j Metarotor with a metasurface design combining the designs in (a–c) and (e–g) as a function of the light polarization. The metarotor remains stationary under linear polarization, rotates counterclockwise under left-hand circular polarization, and clockwise under right-hand circular polarization. The optical microscopy images are taken from Supplementary Video 8. Scale bars: 5 μm.

Fig. 4. Microscopic rack and pinion metamachines.

a, e, i Schematic illustrations of three designs of rack and pinion metamachines to convert the rotational motion produced by a metarotor into linear motion. The movable rack and pinion are shown in blue, while the immobile parts are in turquoise. Metasurface segments are highlighted in red and yellow. The forces generated on the driving gear and rack under illumination are depicted by white and black arrows, respectively. b, f, j Corresponding SEM images, and c, g, k optical microscopy images from Supplementary Videos 10, 12, 13. d The pinion metarotor is equipped with a metasurface designed so that its rotation direction is different for left and right-handed circularly polarized light, enabling forward motion of the rack under right-handed circular polarization and backward motion under left-handed circular polarization, allowing dynamic back-and-forth motion by changing the circular polarization of light. h Equipping both the rack and the pinion with metasurfaces permits oscillatory rack movement under constant linearly polarized light. The pinion has a single tooth that periodically moves the rack leftward when engaged, while the rack’s metasurface moves it rightward, mimicking a macroscopic spring. Balancing the forces of both metasurfaces achieves oscillatory rack movement under linearly polarized light. l The same rack and pinion design can periodically move a gold mirror (illustrated in golden yellow in i), changing the average amount of transmitted light l through the white box shown in i and k. Scale bars: 10 μm.