Using wide-field and point detection modalities, we show how optical trapping dynamics under femtosecond pulsed excitation can be explored by complementing detection of two-photon fluorescence with backscatter. Radial trajectories of trapped particles are mapped from correlated/anti-correlated fluctuations in backscatter pattern whereas temporal evolution of two-photon fluorescence is used to mark the onset of trapping involving multiple particles. Simultaneous confocal detection of backscatter and two-photon fluorescence estimates axial trap stiffness, delineating short-time trapping dynamics. When a second particle is being trapped an oscillatory signal is observed which is due to interference of backscatter amplitudes, revealing inter-particle interactions within the trap. These findings are crucial steps forward to achieve controlled manipulation by harnessing optical nonlinearity under femtosecond pulsed excitation.

We present experimental studies to measure optical trapping efficiencies for annular beams of variable diameters but of fixed widths, with aid of laser beam-shaping, to test the ‘light cone’ model in the geometric optics limit.

We present a comparative study on optical trapping dynamics for polystyrene and silica micro-particles under both continuous wave and pulsed excitations. The effect of optical nonlinearity under femtosecond pulsed excitation on trapping efficiency is investigated.

Recent theoretical and experimental studies have shed light on how laser trapping dynamics under femtosecond pulsed excitation are fine-tuned by optical and thermal nonlinearities. Here, we present experimental results of trapping of single and multiple polystyrene beads (of 1 μm diameter). We show how integration and synchronization of bright-field video microscopy with confocal detection of backscatter provide both spatial and temporal resolution required to capture intricate details of nonlinear trapping dynamics. Such spatiotemporal detection is promising to have far-reaching applications in exploring controlled laser trapping and manipulations harnessed by optical and thermal nonlinearities.

Using dipole approximation, a comparative study of trapping force/potential on different types of dielectric nanoparticles is presented. The trapping force for multilayered nanoparticles, i.e. core-shell-shell type nanoparticles, is found to be enhanced compared with both core-only type and core-shell type nanoparticles. It is shown that an appropriate choice of material and thickness of the middle layer results in tuning the polarizability, thereby playing a vital role in determining the trapping efficiency for core-shell-shell type nanoparticles. Further, the effect of optical nonlinearity under femtosecond pulsed excitation is investigated and it is elucidated that depending on the specific need, the nature of excitation (i.e. pulsed excitation or continuous-wave excitation) can be judiciously chosen. These findings are promised to open up new prospects for controlled nanoscale optical trapping and manipulation across different fields of nanoscience and nanotechnology.

Here, we present experimental results of optical trapping of dielectric microparticles with structured laser beam created by computer-generated holograms using a spatial light modulator. We compared the trapping efficiencies with converging obstructed Gaussian and Gaussian beams.

By simultaneously detecting backscatter signal and recording bright-field video-microscopy, we spatiotemporally track optical trapping dynamics of dielectric micro-particles in a nonlinear laser trap generated by high-repetition-rate femtosecond pulse-train.

Under short pulsed excitation, depending on the nonlinear refractive index of the particle and the surrounding medium, optical nonlinearity plays a significant role in modulating optical forces on particles. Here, we explore the trapping forces experienced by dielectric nanoparticles in a highly nonlinear medium taking into account nonlinear propagation of Gaussian beam.

We present a comparative study on optical trapping dynamics for particles of different sizes. . The trapped particles’ confinement time is found to be nicely correlated with the axial trap stiffness.

Crucial to effective optical trapping is the ability to precisely control the nature of force/potential to be attractive or repulsive. The nature of particles being trapped is as important as the role of laser parameters in determining the stability of the optical trap. In this context, hybrid particles comprising of both dielectric and metallic materials offer a wide range of new possibilities due to their tunable optical properties. On the other hand, femtosecond pulsed excitation is shown to provide additional advantages in tuning of trap stiffness through harnessing optical and thermal nonlinearity. Here we demonstrate that (metal/dielectric hybrid) core/shell type and hollow-core type nanoparticles experience more force than conventional core-type nanoparticles under both continuous-wave and, in particular, ultrafast pulsed excitation. Thus, for the first time, we show how tuning both materials properties as well as the nature of excitation can impart unprecedented control over nanoscale optical trapping and manipulation leading to a wide range of applications.

Optical trapping efficiency for silver nanoparticles is theoretically estimated using dipole approximation and generalized Lorenz-Mie theory, including higher-order optical nonlinear effects. Here, we show a reversal in asymmetry of axial trapping potential due to Fano resonance is observed, along with a splitting of trapping potential well due to optical nonlinearity. Further, it is shown that there exists a limit for particle size beyond which the particle cannot be trapped and how this limit can be extended by harnessing optical nonlinearity under femtosecond pulsed excitation.

Recent theoretical and experimental results have shown how the trapping force/potential can be dramatically modulated due to optical and thermal nonlinearity. Compared with dielectrics, metals show even more interesting behavior (for example, trap-splitting, enhanced forward scattering, etc.) owing to higher-order optical nonlinearities. Hence, we present a comparison study for dielectric and metallic nanoparticles using generalized Lorenz-Mie theory.

Recently, the effect of optical nonlinearity in laser trapping has been investigated under pulsed excitation, and it was observed that the inclusion of nonlinearity significantly modulates trapping potential for metallic nanoparticles using dipole approximation. In this paper, we present theoretical studies on nonlinear laser trapping for silver nanoparticles using generalized Lorenz-Mie theory. We observe a reversal in the direction of asymmetry of potential well and splitting of the potential well due to nonlinear effects which is further modulated with an increase in laser power.

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.