We demonstrate photophoretic force-based optical trapping of multiple absorbing particles in air by loosely focusing a Gaussian beam with a series of convex lenses of different focal lengths, and investigate the dependence of the number of trapped particles and their sizes on the focal length. We observe the formation of particle chains at a particular focal length, and measure the dynamic range of optical trapping for each lens system. We then develop a numerical simulation to explain the observed dynamic range of trapping by estimating the temperature distribution across a particle surface, and determining the photophoretic force. Our simulation results are in reasonable agreement with experimental results. Interestingly, we also observe that the average size of trapped particles reduces as we increase the lens focal lengths, which suggests that intensity gradients may somehow be involved in the mechanism of photophoretic trapping.
Photophoretic forces - which are of thermal origin - have defined an alternative route of optical trapping of absorbing microparticles in air. Here, we show that a single multi-mode fiber facilitates significantly more robust optical traps compared to a pure Gaussian beam emanating from a single mode fiber for the trapping and manipulation of absorbing particles using photophoretic forces. We carefully study the dependency of trapping on speckle patterns generated from different modes from a multimode fiber, and experimentally observe that maximum trapping force can be obtained when the mean s…
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 cu…
Optical vortex trapping and annihilation by means of nonlinear Bessel beams in nonlinearly absorbing media
在非线性克尔介质中，当多光子吸收显著时，高阶非线性贝塞尔光束（NBB）中心的拓扑电荷m的涡旋可以稳定存在，并且无休止地存在。我们表明，m-充电NBB不仅是稳定的，但自发地形成从任何其他n-充电NBB和N“外国”旋涡的总电荷s随机嵌套在束截面，如果n + s = m。所有嵌套的旋涡合并在中心的原始NBB，这经历了模式转换的NBB，保持拓扑电荷和向内指向的功率电流，维持无衍射和无衰减的传播介质中的非线性吸收。我们预见到不同的应用，如创建稳定的，多电荷的旋涡没有严格的对齐要求，但通过自发的旋涡组合，混合波或粒子的旋涡可以引导，快速湮灭的旋涡偶极子，和清洁斑点光束的大量湮灭的旋涡。
The need for precise manipulation of nanoparticles in gaseous or near-vacuum environments is encountered in many studies that include aerosol morphology, nanodroplet physics, nanoscale optomechanics, and biomolecular physics. Photophoretic forces, whereby momentum exchange between a particle and surrounding gas is induced with optical light, were recently shown to be a robust means of trapping and manipulating nanoparticles in air. We previously proposed a photophoretic "optical funnel" concept for the delivery of bioparticles to the focus of an x-ray free-electron laser (XFEL) beam for femtos…
The intensity distribution of a holographically-projected optical trap can be tailored to the physical properties of the particles it is intended to trap. Dynamic optimization is especially desirable for manipulating dark-seeking particles that are repelled by conventional optical tweezers, and even more so when dark-seeking particles coexist in the same system as light-seeking particles. We address the need for dexterous manipulation of dark-seeking particles by introducing a class of "dark" traps created from the superposition of two out-of-phase Gaussian modes with different waist diameters…
