Significance Since the seminal work of Arthur Ashkin in 1986, optical tweezers have been extensively researched and used invarious fields, including physics, chemistry, and biomedicine.The radiation force induced by momentum exchange in light scattering and absorption is the fundamental mechanism of optical tweezers, allowing non-contact trapping and micro and nanoparticles manipulation.Continuous waves are primarily used in traditional optical trapping approaches for small particles.As an alternative, femtosecond laser pulses with high repetition rates have recently been applied to optical tweezers. The ultrashort pulse duration of femtosecond laser pulses produces minimal thermal effect during light-matter interactions, which lays the foundation for applications in biological sciences. Furthermore, the ultra-high peak power of femtosecond laser pulses excites the nonlinear response of trapped objects. In the presence of nonlinearity, some intrinsic properties of the sample (e.g., permittivity) will be changed, thereby breaking the force balance formed under a linear condition, and generating some novel phenomena with potential applications. To provide an overview and perspective on its developments, we review the research progress of femtosecond optical tweezers and discuss the nonlinear effects and applications in detail.

Progress Over the past decades,femtosecond optical tweezers have made significant progress and facilitated researches in various disciplines,particularly in the field of biological sciences.In 2005, Mao et al. reported the stable trapping of red blood cells with femtosecond optical tweezers ( Fig. 1), demonstrating of practical applications of this technology in biological research. In 2008, Zhou et al. investigated the manipulation of red blood cells and their states under different laser powers using a similar experimental setup. In addition to the spatial manipulation of cells, femtosecond optical tweezers have been used in complex operations, such as cell fusion ( Fig. 2) and cell transfection ( Fig. 3). These applications of femtosecond optical tweezers are of great significance for the analysis of gene expression and immunotherapy.

However, conventional optical tweezers are limited by the far-field diffraction and therefore are inefficient for trapping nanoscale objects (e.g., atoms, molecules, and quantum dots). Although the stiffness of optical tweezers can be enhanced by increasing the incident power, the robust optical intensity damages the sample. To improve trap precision, researchers have developed a new type of optical tweezers based on the principle of near-field optics. Among the different branches of near-field optics, surface plasmons hold the greatest potential for manipulation of objects at the nanoscale. In 2012, Roxworthy et al. reported enhanced particle trapping in a femtosecond plasmonic optical tweezers system (Fig. 4). The stiffness of the optical trap is enhanced by two times compared to the use of continuous-wave. In 2013, Shoji et al. demonstrated reversible trapping and release of λ-DNA molecules by switching femtosecond pulses in a plasmonic optical tweezers system (Fig. 5). In 2016, Kotsifaki et al.investigated the trapping efficiency of the femtosecond plasmonic optical tweezers based on gold-coated black silicon arrays. In 2021, Zhang et al. numerically calculated the force distribution for the quantum dots in a metallic bowtie structure illuminated by focused femtosecond laser pulses (Fig. 6).

The interaction of femtosecond laser pulses and materials can excite several significant nonlinear effects, such as two-photon absorption, second harmonics, and the Kerr effect. Among them, the Kerr effect can proactively modulate the physical properties of samples, thereby breaking the force balance established under linear conditions and exhibiting some novel phenomena distinct from linear optical traps.Nonlinear optical trapping is a new area of study in the field of femtosecond optical tweezers. Over the past decade, fruitful efforts to facilitate its development have been made, including both experimental and theoretical research. In 2010, Jiang et al. reported the phenomenon of “trap split” when trapping gold nanoparticles viafemtosecond laser pulses (Fig. 7).In 2018, Gong et al. presented an analytical model for calculating the nonlinear optical force in femtosecond optical tweezers (Fig. 8); Zhang et al. achieved the multiplexed trapping of gold nanoparticles using femtosecond cylindrical vector beams (Fig. 9). In 2019, Gong et al. demonstrated the generation of optical pulling force by regulating the nonlinearity of surroundings (Fig. 10). In 2020, Huang et al. demonstrated that the Kerr effect of gold nanoparticles can induce a three-dimensional shell-like potential well (Fig. 11).

Conclusions and Prospects Compared with continuous optical tweezers, femtosecond optical tweezers can significantly reduce the thermal effect in light-matter interactions because of the ultrashort pulse duration and temperature dissipation between pulses. At the same time, the ultra-high peak power of femtosecond laser pulse provides the basis for the nonlinear modulation of optical traps.These features of femtosecond optical tweezers of ferseveral distinct advantages in the manipulation of microscopic particles. However, the study of femtosecond optical tweezers are still in their early stages, particularly nonlinear optical trapping,which has received significant attention in recent years. There are still challenges to overcome in extending the capabilities and applicability of the technology. Even though we are confident that the uses of femtosecond optical tweezers will continue to expand in the near future, and more potential applications will be developed.