Light-Induced Pulling and Pushing by the Synergic Effect of Optical Force and Photophoretic Force

Optical force, coming from momentum exchange during light-matter interactions, has been widely utilized to manipulate microscopic objects, though mostly in vacuum or in liquids. By contrast, due to the light-induced thermal effect, photophoretic force provides an alternative and effective way to transport light-absorbing particles in ambient gases. However, in most cases these forces work independently. Here, by employing the synergy of optical force and photophoretic force, we propose and experimentally demonstrate a configuration which can drive a micron-size metallic plate moving back and forth on a tapered fiber with supercontinuum light in ambient air. Optical pulling and oscillation of the metallic plate are experimentally realized. The results might open exhilarating possibilities in applications of optical driving and energy conversion.

Figure 1

Numerical modeling of light-induced driving by synergy effect of optical force and photophoretic force. (a),(b) Schematic of pulling a hexagonal gold plate (side length, $5.4\mu \mathrm{m}$; thickness, 30 nm) up on a tapered fiber (cone angle, 6°) near the tapered fiber end (a) and pushing the same gold plate back in the middle section of the tapered fiber (b) in ambient air. The tapered fiber is guided with unpolarized supercontinuum light (power, 1.3 mW). The distance between the center of the gold plate and the tapered fiber end is $D$. (c),(d),(e) Calculated (c) photophoretic force ($F_x^{\mathrm{PPF}}$), (d) optical force ($F_x^{\mathrm{OF}}$), and (e) total light-induced force ($F_x^{\text{TOTAL}}$) along the tapered fiber acting on the gold plate. The inset in (e) shows an enlarged view of the part enclosed by the dashed box. The total light-induced force is calculated with the sum of the optical force and the photophoretic force, and both the TE and TM mode account for 50 percent. The red curve in (d) represents regions with optical pulling force ($F_x^{\mathrm{OF}}<0$). Non-zero electromagnetic field components are $E_z$, $H_x$, $H_y$ for the TE mode and $E_x$, $E_y$, $H_z$ for the TM mode. Here, we use the piecewise cubic hermite interpolating polynomial (PCHIP) to connect the data points with a smooth curve.

Figure 2

Experimental configuration. The supercontinuum light is delivered into a single mode fiber connected with a tapered fiber. The optical microscope including a $10\times$ microscope objective, and a camera is used for observation. The tapered fiber is slightly tilted down ($\alpha$) around 9° and the cone angle ($\beta$) of the tapered fiber is about 6°. The tapered fiber is mounted on a three dimensional console and suspended in ambient air.

Figure 3

Sequencing optical microscopy images of a back-and-forth moving gold plate driven by unpolarized supercontinuum light on a tapered fiber in ambient air. The cone angle of tapered fiber used in experiment is about 6°. A time series of images shows that the gold plate is first pulled backward at an average speed of $35.60\mu \mathrm{m}/\mathrm{s}$ after the light is switched on, next, it is pushed forward at an average speed of $28.52\mu \mathrm{m}/\mathrm{s}$, thereafter, it is pulled back again at an average speed of $37.11\mu \mathrm{m}/\mathrm{s}$, and finally, it stops at a location $43\mu \mathrm{m}$ away from the tapered fiber end (see Supplemental Material [32], video 1). The total power of supercontinuum light guided into the tapered fiber is 1.3 mW.

Figure 4

Calculated optical force ($F_x^{\mathrm{OF}}$) and photophoretic force ($F_x^{\mathrm{PPF}}$) with respect to the wavelength of light source and the distance ($D$) between the center of gold plate and the tapered fiber end. (a),(b) Optical force.(c), (d) Photophoretic force. (a), (c) TE mode. (b), (d) TM mode.