Dynamics of a dielectric microsphere inside a nonlinear laser trap

Abstract

The 2018 Nobel Prize in Physics was awarded for the invention of optical trapping and generation of ultrashort pulses, which revolutionized many areas of modern science and technology. However, physics of optical trapping under ultra-short pulsed excitation has not been explored much. The nonlinear nature of optical trapping force/potential under ultra-short pulsed excitation was theoretically investigated, however, without any direct experimental demonstration and development of any generalized theory independent of the particle size. In this work, we present a methodology to numerically estimate trapping force/potential including optical as well as thermal nonlinearity under ultra-short pulsed excitation and implement a variety of detection modalities to capture the particle's real-time trajectories. We show how highly asymmetric nonlinear axial potential, created by a femtosecond pulse-train, can be mapped from the dynamics of the trapped particle. Considering fine-tuning of trap-stiffness by changing nonlinearity, we envision far-reaching applications of using ultra-short pulsed excitation in laser trapping and manipulation.

Deciphering single- and multi-particle trapping dynamics under femtosecond pulsed excitation with simultaneous spatial and temporal resolution

“…In section S1 in the Supplementary Information (SI), we provide a detailed discussion on single-particle dynamics. Earlier, the single-particle dynamics of uorescent dye coated particle was shown to proceed through sequential steps inside a nonlinear laser trap [10]. These steps are further con rmed here for the uncoated particles.…”

“…3) indicates the dragging of second particle. Noticeably, unlike the singleparticle case, this rise is not followed by any drop in the signal that due to adjustment dynamics ( [10], section S1 in SI) because the shift in the equilibrium position (due to thermal nonlinearity) is not signi cant when second particle is dragged. This is because trapping potential is already modulated by the presence of rst particle.…”

“…Consequently, trap stiffness is less than CW excitation. In addition to mapping this escape potential, experiments reveled a quick 'adjustment' of the initially dragged particle characterized by a movement in forward direction which is a result of laser-induced thermal effects in the presence of the particle [10,11].…”

“…One crucial aspect to disentangle the intricate events in these experiments [10][11] was to integrate various detection methods (for example, two-photon uorescence and backscatter detection) using a multimodal optical tweezers set-up [12] since different methods furnish complementary information.…”

“…For trapping of particles with a wide range of sizes, mostly continuous-wave (CW) lasers were used with a few notable exceptions where femtosecond lasers were used [3][4][5][6] which reported that micrometer-sized silica or polystyrene particles show similar (at less power) [3][4] or less (at high power) [5][6] trap stiffness compared with CW excitation. In a quest of understanding the fundamental physics underlying these puzzling observations, optical and thermal nonlinearity was demonstrated to modulate the trapping force under femtosecond pulsed excitation both theoretically as well as experimentally [7][8][9][10][11]. As delineated in these works focused on optical trapping dynamics under pulsed excitation, at low average power, nonlinear effects are insigni cant, hence, trap stiffness is equivalent to CW excitation.…”