Optical forces of focused femtosecond laser pulses on nonlinear optical Rayleigh particles

Abstract

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 ultrafast laser pulses. Herein we establish the time-averaged optical forces on a nonlinear optical nanoparticle using highrepetition-rate ultrafast laser pulses, based on the linear and nonlinear polarization effects. We investigate the dependence of the optical forces on the magnitudes and signs of the refractive nonlinearities. It is found that the self-focusing effect enhances the trapping ability, whereas the selfdefocusing effect leads to the splitting of potential well at the focal plane and destabilizes the optical trap. Our results show good agreement with the reported experimental observations and provide a theoretical support for capturing nonlinear optical particles. PACS numbers: 42.65.Jx, 78.20.Bh, 78.67.Bf, 62.65.Re Optical trapping, also known as optical tweezers, is a useful technique for noncontact and noninvasive manipulation of small particles using a focused laser beam [1]. This technique has wide applications in physics, chemistry, biology, and other disciplines [2, 3]. Up to now, the stable optical trapping of micro-and nanoparticles have been extensively demonstrated by the use of continuouswave (CW) Gaussian laser beam [1-3], cylindrical vector beam [4], evanescent field [5], plasmonic field [6], spinning light fields [7], etc. Lots of efforts have been devoted to trap a variety of small objects, such as dielectric particles [1], metallic Rayleigh nanoparticles [4], semiconductor quantum dots [8], and biological cells [9].Recently, optical trapping technique has been extended by substituting CW laser with high-repetition-rate ultrafast laser pulses [10][11][12]. With the ultrafast laser pulses, several novel phenomena have been observed, including the trapping split behavior in the process of capturing gold nanoparticles by ultrafast near-infrared laser pulses [13], a controllable directional ejection of optically trapped nanoparticles [14], and the immobilization dynamics of a single polystyrene sphere [15]. It should be noted that the optical force originating from the nonlinear polarization becomes significant and cannot be neglected if the trapped particles exhibit nonlinear optical effects. Moreover, the experimental observations have revealed that the nonlinear optical effects could enhance the optical force [8,16] or modify the optical trapping potential [13].To quantitatively appraise the trapping ability, the optical force exerted on a spherical nanoparticle arising from the linear polarization has been calculated using various approaches, such as Rayleigh scattering formulae [1], Maxwell's stress tensor [17], and discrete dipole approximation [18]. For a nonlinear optical Rayleigh particle, however, the optical force unambiguously originates from the contribution of both the linear and nonlinear induced polariz...

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“…With the control of multi-singularity VBs, many new techniques were designed and applied to trap multiple particles 82,217,218 , including the fractional optical VB for optical tweezers 219 . With femtosecond VBs, the tweezers carrying special nonlinear properties can be used to manipulate optical Rayleigh particles 220 . Furthermore, with femtosecond vector VBs, nonlinearity-induced multiplexed optical trapping and manipulation was designed 221 , where the number of traps and their orientations could be flexibly controlled.…”

Spheroidal trap shell beyond diffraction limit induced by nonlinear effects in femtosecond laser trapping

“…It is worth noting that we analyse the nonlinear effect based on the perturbation expansion of the polarization in the paper [32][33][34]. The theoretical analysis qualitatively illustrates the basic concept and main characteristics in this novel nonlinear trapping.…”

“…For a particle trapped in a tightly focused laser pulse, the transverse scattering force is several orders of magnitude smaller than the gradient force [33], the scattering force as shown in Figure S5 in the Supplementary material. The longitudinal scattering force will become significant or even dominant in the case of GNPs trapping because of the strong scattering and absorption of light by the particles, but it does not account for the generation of trap split, as while as the 3D structure of potential well because the scattering is axisymmetric and monotonically along the light propagation.…”

Optical levitation using broadband light

“…Such sources have also been used to transfer orbital angular momentum to the trapped particles [17]. In addition, the high peak intensities of these sources have been shown to be useful for inducing nonlinear optical properties within the trapped particle [18].…”