The interaction between laser pulses and materials has been extensively studied in recent decades as a common physical mechanism, such as laser propulsion (LP) and laser-induced breakdown spectroscopy. LP has gained widespread attention due to its inherent advantages of reducing launch costs and increasing payload. With the development of LP, the research field has gradually transitioned from macroscopic to microscopic fields. However, during the propulsion process, the direct irradiation of high-energy laser pulses on particles can cause permanent damage to the particle surface, and the large laser spot size can lead to deviations in the particle’s trajectory. Therefore, a device that can control the spot diameter and reduce surface damage to particles needs to be proposed. In this work, we propose LP based on a tapered fiber to realize the propulsion of microscale microspheres and analyze the mechanism of LP based on the motion of microspheres. We study the effects of laser energy and microsphere size on the distance of the microsphere. In addition, we analyze the influence of laser energy emitted from the fiber tip on the fiber tip size and discuss the relationship between laser energy density and fiber tip diameter, revealing the nonlinear increase in laser energy and the decrease in scattering loss as the fiber diameter increases. Our research may provide further support for the precise manipulation of colloids and biomaterials at the micrometer level.

1) Experimental setup for LP. A tapered fiber structure is prepared through flame heating. A Nd∶YAG laser is coupled into the fiber using a 40× objective lens and emitted from the fiber tip. The tip and microspheres are placed on three-dimensional translation stages. By the combination of vertical CCD1 and horizontal CCD2, the driven microspheres can be flexibly and precisely controlled. The dynamics of the microspheres are captured by the CCD1 camera. After the propulsion experiment is completed, the laser energy emitted from the fiber tip is measured by an energy meter. 2) Formation of plasma shock wave. During the interaction between the laser emitted from the fiber tip and the atoms, the electrons in the atoms are excited or transitioned to higher energy states. These high-energy electrons are accelerated and further excite electrons in the atoms. The high-energy electrons collide with other atoms, generating additional electrons. When the number of electrons reaches a certain number (∼10¹⁶ /cm³), a high-temperature and high-pressure plasma is formed. Subsequently, the shock wave generated by the expansion of the plasma propels the microsphere forward through the recoil effect. 3) Calculation of microsphere movement distance and velocity. The dynamics of the microspheres are recorded by a CCD1 camera with a frame rate of 1000 frame/s. The time interval between adjacent images is 1/1000 s. By analyzing the movement of the microsphere between two images taken at t=1/1000 s as the initial state, the displacement s within the time range of 0-1/1000 s is determined. Then the average velocity v=s/t is calculated. We consider the average velocity as the initial velocity due to the short interaction time between the laser pulse and the microsphere. To reduce experimental errors, the experiment is repeated three times under the same conditions.

In the experiment of propelling microspheres with a diameter of ~80 μm using a laser with an energy of ~9.6 μJ through a ~8 μm fiber tip, the microsphere moves a distance of 547 μm within a time range of 6/1000 s. The maximum velocity is calculated to be 12.4 cm/s, and the momentum is determined to be P=8.3×10^-11 Ns. The calculated value P differs from the theoretical value P_M by three orders of magnitude. By adjusting the relative position between the fiber tip and the microsphere, we observe that the microsphere can move in the direction of the fiber as well as diagonally. These findings indicate that the ejection mechanism of shock waves plays a dominant role in the propulsion of the microspheres (Fig. 2). In the qualitative study of the effects of laser energy and microsphere size on microsphere movement, we find that the movement distance of the microsphere increases with increasing laser energy. It can be explained that with the increase in laser energy, the energy carried by the shock wave formed by the expansion of plasma increases, resulting in a greater force exerted on the surface of the microsphere. On the other hand, as the size of the microsphere increases, the movement distance decreases. This can be attributed to the increased resistance between the microsphere and the substrate surface due to the larger size. The above experimental results further illustrate the propagation characteristics of shock wave (Fig. 3). After investigating the relationship between laser energy and fiber tip diameter [Fig. 5(b)], we discover that the laser energy emitted from the fiber tip exhibites nonlinear increases, which is attributed to declining scattering loss with increasing fiber diameter. The calculated limit of the output energy density at the fiber tip is ~1.15 μJ/μm². For a fiber tip diameter of approximately 2 μm, the energy density is ~1.25 μJ/μm² [Fig. 5(c)], indicating that the fiber tip has been damaged.

We present a straightforward solution that makes LP of microspheres feasible using a tapered fiber structure. In the experiment, a laser with an energy of ~9.6 μJ is emitted from the fiber tip, driving the movement of a ~80 μm diameter microsphere. Within a time range of 6/1000 s, the microsphere moves a distance of 547 μm. The fact that P_M≪P indicates that the motion of the microsphere is primarily governed by the ejection mechanism of the shock wave, similar to the launch of a bullet. In the qualitative study of the influence of laser energy and microsphere size on microsphere movement, we observe that the movement distance of the microsphere increases with increasing laser energy, while it decreases with increasing microsphere size. We analyze the relationship of pulse energy and energy density with fiber tip diameter, revealing a nonlinear increase in energy, which is attributed to declining scattering loss with increasing fiber diameter. In terms of propulsion, the laser emitted from the fiber tip exhibits the characteristics of low energy and longer propagation distances. The observed interaction between light and microspheres in this experiment may provide valuable insights for future research on manipulating colloids and biomaterials at the microscale.