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Pulling and Lifting Macroscopic Objects by Light

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Pulling and Lifting Macroscopic Objects by Light Pulling and lifting macroscopic objects by light Gui-hua Chen1,2, Mu-ying Wu1, and Yong-qing Li1,2* 1School of Electronic Engineering & Intelligentization, Dongguan University of Technology, Dongguan, Guangdong, P.R. China 2Department of Physics, East Carolina University, Greenville, North Carolina 27858-4353, USA (Received XX Month X,XXX; published XX Month, XXXX) Laser has become a powerful tool to manipulate micro-particles and atoms by radiation pressure force or photophoretic force, but optical manipulation is less noticeable for large objects. Optically-induced negative forces have been proposed and demonstrated to pull microscopic objects for a long distance, but are hardly seen for macroscopic objects. Here, we report the direct observation of unusual light-induced attractive forces that allow pulling and lifting centimeter-sized light-absorbing objects off the ground by a light beam. This negative force is based on the radiometric effect on a curved vane and its magnitude and temporal responses are directly measured with a pendulum. This large force (~4.4 μN) allows overcoming the gravitational force and rotating a motor with four-curved vanes (up to 600 rpm). Optical pulling of macroscopic objects may find nontrivial applications for solar radiation-powered near-space propulsion systems and for understanding the mechanisms of negative photophoretic forces. PACS number: 42.50.Wk, 87.80.Cc, 37.10.Mn In Maxwell’s theory of electromagnetic waves, light carries rotation of the objects, they are insufficient to manipulate the energy and momentum, and the exchange of the energy and objects in vertical direction. momentum in light-matter interaction generates optically-induced Here we report a direct observation of a negative optically- forces acting on material objects. Can a centimeter-sized or larger induced force on centimeter-sized light-absorbing objects on object that can be viewed by naked eye be lifted up by a light beam the order of μN that allows lifting up the object over a and pulled towards the light source? Although optical forces have longitudinal distance of >5 mm in ~40 ms towards the light been widely applied for the manipulation of microscopic objects [1– source by a light beam of 1-W power. This indicates that the 6], they are usually unnoticeable on macroscopic objects because optically-induced force acting on macroscopic objects could be the magnitude of optical forces is generally much weaker than the larger than the object’s weight by changing the surface geometry. gravitational force (FG) of the large objects. Generally, when an Thus, our experiment increases the force per object’s weight object immersed in a gaseous environment is illuminated by by three orders of magnitude compared to previous works. Our light, two types of optically-induced forces are generated by experiment relies on radiometric force FRM that acts on a thin the light-matter interaction. One is radiation pressure force vane with a curved surface structure immersed in a rarefied (FRP) arising from direct momentum transfer between the gas, on which a layer of black absorber is coated to increase object and the incident light [1,2], which is in pN or nN range the absorption. FRM could be classified into two types based and is not enough to lift large objects. The other is on the variation in surface temperature or in heat exchange photophoretic or radiometric force (FRM) due to photo-heating coefficient of the vane, like photophoretic forces acting on effect, in which the photon energy of the incident light is first micro-particles [7,8]. The first type of radiometric force (FΔT) converted into the thermal energy of the object due to light is caused by the temperature difference between two sides of absorption and then asymmetrical momentum transfer the vane, pointing from the hot to the cold side and being between the heated object and the surrounding gas molecules repulsive from the illumination light. The second type of produces FRM [7,8], which can be several orders of magnitude radiometric force (FΔα) is caused by the unbalanced heat larger than the radiation force FRP [9,10]. Optically-induced exchange between two sides of the vane with the gas molecules negative forces have been demonstrated to pull microscopic even if two sides are heated at the same temperature, due to the objects for a long distance [11,12]. The generation of optically- difference in surface geometry or accommodation coefficient. The induced forces on macroscopic objects could trigger a new era of FΔα-force could be attractive or repulsive in the light propagation optomechanical applications. direction [7–9]. Since Crookes’ pioneering work in Surprisingly, only a few experiments reported on the 1870s [15,16], extensive efforts have been made to understand observation of optically-induced forces on centimeter-sized the mechanisms of radiometric forces including Maxwell, objects [13–16], involving force magnitude of up to 1–102 nN Reynolds, and Einstein’s work [18–20], but almost all the which was three or more orders of magnitude less than the efforts were focused on the FΔT-force acting on flat vanes that gravity of the target objects. For examples, Magallanes et al requires a temperature difference across or along the recently observed a lateral optical force of ~1 nN on a vane [14,21–23]. There are still some confusions about the birefringence object illuminated with a 1-W incident light, dominant mechanisms in various regimes and causing a lateral displacement of 0.5 mm in ~100 s [13]. Wolfe configurations [16]. Recent applications in near-space et al reported a light-induced transverse force of ~400 nN propulsion systems driven by solar radiation [24–26], acting on a horizontal vane radiometer due to thermal creep micromachines [13,21], and computational techniques [27–29] effect [14]. Crookes demonstrated a radiometric force on black- renewed the interest in FRM. Our experiment deals with the white flat vanes or cup-shaped vanes when illuminated by FΔα-force acting on a curved vane, which doesn’t require a light [15,16], but the magnitude and dynamic properties of Crookes temperature gradient on the vane and is seldom considered radiometric forces have never been directly measured [17]. previously. Although these forces may cause the horizontal motion or 1 The experimental setup is described in detail in the cylinder surface, where Θ0=W0/R in radian, and W0 is the arc supplementary materials (SM). In FIG. 1, we show direct length and R is the radius. FRM is attractive for the illumination observation of a light-induced pulling or pushing force acting on concave side (Θ0>0). FRM is repulsive for the illumination on a thin absorbing curved metal vane. To determine the on convex side (Θ0<0). For a flat vane (Θ0=0), FRM is zero so magnitude and direction of the radiometric force FRM, a single that it is not deflected by the laser beam, and this is a proof of cylindrical vane is suspended with an ultrafine copper wire as no temperature variation between the opposite sides of the a pendulum in a vacuum chamber and illuminated by a light aluminum vane. beam (see SM). The photon energy of the illumination light is In FIG. 2, we show that the attractive radiometric force is absorbed by Black 2.0 paint on one side and transferred by large enough to overcome the gravitational force FG and lift an thermal conduction to metal vane, resulting in both sides of the ultrathin curved gold vane off the ground by the laser vane heated to an identical temperature since the used illumination. FG of the curved gold leaf is estimated to be 1.85 aluminum or gold vanes are excellent conductors. Therefore, μN from the known density and volume of the vane. The vane FRM acting on a curved metal vane is the second type of can be quickly pulled off from the supporting surface and radiometric force caused by unbalanced heat exchange due to lifted up until the top of the vacuum chamber in ~40 ms by the the curved surface geometry. The magnitude of FRM is attractive FRM, see FIG. S1 and Supplementary Video S2. determined by FRM=mgsinθ, where m is the mass of the vane, g is the gravitation acceleration, and θ is the angle displacement. When a laser beam is incident on the concave side of the vane, it is pulled towards the light source (FIGS. 1b and 1c, and Supplementary Video S1). Once the laser is turned off, the vane returns to its equilibrium position. However, when the laser beam is incident on the convex side, the vane is pushed along the light propagation direction (FIG. 1e). Figure 1d shows the dependence of the force magnitude on the gas pressure with a constant laser power. The maximum angle displacement is 0.172 rad at 0.1~0.2 Torr corresponding to FIG. 2. Lifting of a curved vane. (a) An ultrathin curved vane is lifted Knudsen number Kn~0.1, and thus the maximum pulling force when the concave side is illuminated. (b) Time-lapsed images of a is ~4.4 µN from the known density and volume of the vane, cylindrical gold leaf (with 7×7 mm in size and 0.2 µm in thickness) when the laser beam is turned on. The laser power is 1 W at 650 nm λ where Kn= /W is defined as the ratio of the molecular mean and the pressure is 0.3 Torr. free path λ of the surrounding gas to the characteristic length W of the vane [17]. It should be noted that the radiation We interpret the observed negative FRM acting on a curved pressure force (=P/c) is ~2.3 nN, which is smaller than FRM by vane by the combination of pressure force (Fn) and shear force about three orders of magnitude, where P is the absorbing (Ft), in contrast to the area force, edge force, and shear force power of incident laser, and c is the speed of light.
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