The principle of optical trapping is conventionally based on the interaction of optical fields with linear-induced polarizations. However, the optical force originating from the nonlinear polarization becomes significant when nonlinear optical nanoparticles are trapped by femtosecond laser pulses. Herein we develop the time-averaged optical forces on a nonlinear optical nanoparticle using high-repetition-rate femtosecond laser pulses, based on the linear and nonlinear polarization effects. We investigate the characteristics of transverse and longitudinal optical forces for particles exhibiting self-focusing and defocusing effects. It is shown that the self-focusing effect increases the trapping force strength and improves the confinement of particles, whereas the self-defocusing effect leads to the splitting of potential well at the focal plane and destabilizes the optical trap, resulting in ejections of trapped particles along the direction of the beam’s propagation. The optical forces exerted on the nonlinear optical particles are experimentally related to the trapping stiffness. It is expected that the self-focusing (or self-defocusing) effect increases (or decreases) the trapping efficiency and stiffness. Our results successfully explain the reported experimental observations and provide theoretical support for capturing nonlinear nanoparticles with femtosecond laser trapping.
Optical trapping and manipulation using focused laser beams has emerged as a powerful tool in the biological and physical sciences. However, scaling this technique to nanoparticles remains challenging. In this work, we propose a novel strategy to optically trap nanoparticles even under the most challenging situation using engineered optical field. The distribution of the optical forces can be tailored through optimizing the spatial distribution of a vectorial optical illumination to favor the stable trapping of a variety of nanoparticles. It is shown that the proposed optical tweezers has the ability of supporting stable three-dimensional trapping for nanoparticles while avoiding trap destabilization due to optical overheating. Besides, the interaction between the angular momentum of the light and the nanoparticle is also explored to control the movement behavior of the nanoparticle. The technique presented in this work offers a versatile solution for trapping nanoparticles and may open up new avenues for optical manipulation.
Theoretically, we propose an investigation of the vectorial light field interacting with the isotropic Kerr medium. We obtain the analytical expression of the focal field of the hybrid polarized beam based on the vectorial Rayleigh-Sommerfeld formulas under the paraxial condition. Then we numerically simulate the far-field vectorial self-diffraction behavior and nonlinear ellipse rotation of a hybrid polarized beam by isotropic Kerr nonlinearity. Experimentally, we observe the vectorial self-diffraction behavior of the femtosecond-pulsed hybridly polarized beam in carbon disulfide at 800 nm, which is in agreement with the theoretical predictions. Our results demonstrate that the self-diffraction intensity pattern and the distribution of state of polarization (SoP) of a hybridly polarized beam could be manipulated by tuning the magnitude of the isotropic optical nonlinearity, which may find interesting applications in nonlinear mechanism analysis, nonlinear characterization technique, and spin angular momentum (SAM) manipulation.