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Abstract
Optical trapping using laser tweezer has revolutionized the field of force spectroscopy having enormous applications in biological manipulation. While a number of theories were developed for particles of different sizes to estimate trapping force under continuous-wave excitation, they were not under short pulsed excitation which leads to nonlinear optical force. Here, we present a comparative study of various theories and provide a unified description for laser trapping under femtosecond pulsed excitation. Numerical results show that exact Mie theory (EMT) can provide a precise qualitative and quantitative prediction of trapping force when optical Kerr effect is included. Moreover, we also show how Mie interference phenomena, leading to observation of Fano resonance, are naturally captured within EMT. Thus, our findings pave the way for potential far-reaching applications in the accurate numerical estimation of nonlinear optical force on arbitrary-sized spherical dielectric particles.
Highlights
Optical trapping using laser tweezer has revolutionized the field of force spectroscopy having enormous applications in biological manipulation
We present an accurate method of calculating force acting on the arbitrary size particle by comparing exact Mie theory (EMT) results with dipole, generalized Lorenz Mie theory (GLMT), and geometric optics (GO) approximations under pulsed excitations taking the Kerr effect into account
In the literature, it is proposed that GLMT using localized approximation is a general theory holding ground for all size limits [34], yet we found that according to the nature of the force curve, it is only applicable for small-sized particles
Summary
We choose the central wavelength of the trapping beam to be 800 nm having Gaussian beam profile under both cw and pulsed excitation. The τ and RR are 120 fs and 76 MHz, respectively, for a commercially available Ti:sapphire oscillator for which. The numerical apertures (NA) for commercial oil-immersion objectives are NA = 1.4 and 1.3
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