No abstract

Optical trapping with continuous-wave lasers has been a fascinating field in the optical manipulation. It has become a powerful tool for manipulating micrometer-sized objects, and has been widely applied in physics, chemistry, biology, material, and colloidal science. Replacing the continuous-wave- with pulsed-mode laser in optical trapping has already revealed some novel phenomena, including the stable trap, modifiable trapping positions, and controllable directional optical ejections of particles in nanometer scales. Due to two distinctive features; impulsive peak powers and relaxation time between consecutive pulses, the optical trapping with the laser pulses has been demonstrated to have some advantages over conventional continuous-wave lasers, particularly when the particles are within Rayleigh approximation. This would open unprecedented opportunities in both fundamental science and application. This Review summarizes recent advances in the optical trapping with laser pulses and discusses the electromagnetic formulations and physical interpretations of the new phenomena. Its aim is rather to show how beautiful and promising this field will be, and to encourage the in-depth study of this field.

We demonstrate that laser pulse duration, which determines its impulsive peak power, is an effective parameter to control the number of optically trapped dielectric nanoparticles, their ejections along the directions perpendicular to polarization vector, and their migration distances from the trapping site. This ability to controllably confine and eject the nanoparticle is explained by pulse width-dependent optical forces exerted on nanoparticles in the trapping site and ratio between the repulsive and attractive forces. We also show that the directional ejections occur only when the number of nanoparticles confined in the trapping site exceeds a definite threshold. We interpret our data by considering the formation of transient assembly of the optically confined nanoparticles, partial ejection of the assembly, and subsequent filling of the trapping site. The understanding of optical trapping and directional ejections by ultrashort laser pulses paves the way to optically controlled manipulation and sorting of nanoparticles.

The development in optical trapping and manipulation has been showing rapid progress, most of it is in the small particle sizes in nanometer scales, substituting the conventional continuous-wave lasers with high-repetition-rate ultrashort laser pulse train and nonlinear optical effects. Here, we evaluate two-photon absorption in optical trapping of 2.7 nm-sized CdTe quantum dots (QDs) with high-repetition-rate femtosecond pulse train by probing laser intensity dependence of both Rayleigh scattering image and the two-photon-induced luminescence spectrum of the optically trapped QDs. The Rayleigh scattering imaging indicates that the two-photon absorption (TPA) process enhances trapping ability of the QDs. Similarly, a nonlinear increase of the two-photon-induced luminescence with the incident laser intensity fairly indicates the existence of the TPA process.

Understanding of optical trapping dynamics of a single particle in the trapping site is important to develop its optical manipulation for molecular assembly and chemical application. For micrometer-sized Mie particles, similar trapping efficiency of the conventional continuous wave (cw) laser or high-repetition-rate femtosecond (fs) laser pulse train has been established [Dholakia et al., Opt. Express 2010, 18, 7554–7568], in contrast to higher efficiency of the laser pulses to trap dielectric Rayleigh particles. To further explore and clarify the switching phenomena of optical trapping efficiency with cw laser and fs laser pulse and to elucidate its nature, we study the immobilization dynamics of a single polystyrene sphere with 500 nm in diameter (which is comparable to focal beam size) in shallow potential well. By observing trapping events and immobilization time of the particle with a size in Lorenz–Mie regime, distinct from well-known Rayleigh particle and ray optics approximations, we found that immobilization time is only linearly related to the incident laser power ≤40 mW, and at higher laser powers cw laser is more efficient than fs laser pulses to immobilize the particle. This finding means that the dynamics of the particle in this size region is still affected by the strong transient force fields induced by high-repetition-rate ultrashort pulse train as usually observed for Rayleigh particles. This may provide an understanding that the dynamics of the target particle in the trapping site is size- and laser mode-dependent.

Direct visualization of ultrafast coupling between charge carriers and lattice degrees offreedom in photo-excited semiconductors has remained a long-standing challenge and is critical for understanding the light-induced physical behavior of materials under extreme non-equilibrium conditions. Here, we obtain a direct visual of the structural dynamics in monocrystalline 2D perovskites. We achieve this by monitoring the evolution of the wavevector resolved ultrafast electron diffraction intensity following above-bandgap high-density photoexcitation. Our analysis reveals a light-induced ultrafast reduction of antiferro-2 distortion resulting from a strong interaction between the electron-hole plasma and the perovskite lattice, which induces an in-plane octahedra rotation towards a more symmetric phase. Correlated ultrafast spectroscopy performed at the same carrier density as ultrafast electron diffraction reveals that the creation of a dense electron-hole plasma triggers the relaxation of lattice distortion at short timescales by modulating the crystal cohesive energy.Finally, we show that the interaction between the carrier gas and the lattice can be altered by tailoring the rigidity of the 2D perovskite by choosing the appropriate organic spacer layer.Organic-inorganic (hybrid) two-dimensional (2D) halide perovskites (2DP) are constructed by a superlattice of interlocking organic and inorganic nanometer-thick layers and have demonstrated unique and non-classical behaviors and are being extensively explored for a wide range of technologies such as photovoltaics, photodetectors, photocatalysts, light emitting diodes, lasers, and quantum emitters. [1][2][3][4][5][6][7][8][9] The underlying design principles for each of these devices are strongly correlated to the exact details of how photoexcited or electronically injected charge carriers dissipate their energy via electron-phonon coupling. For example, it has been shown recently, that unusual electron-phonon coupling mechanisms are likely to promote emission of single photons or correlated photon pairs from perovskite quantum sources. 10 There have been only handful experimental studies based on ultrafast or temperature dependent optical spectroscopies to elucidate the carrier dynamics in 2D perovskites. These studies reveal different facets of electron-phonon coupling, which could strongly govern the exciton polaronic effects, 11 hot-carrier dynamics, 12 vibrational relaxation dynamics, 13 and carrier trapping and re-combination rates. 5 Moreover, these measurements also indicate that presence of an organic cation in close proximity to the inorganic perovskite lattice strongly modulates the nature of electron-phonon interactions, 5,11,[13][14][15][16][17][18][19] and suggest that electron-phonon scattering in 2D perovskites occurs via, local dynamic disorder. 6 These short-range carrier-lattice interactions modulate the quantum-well thickness and octahedral tilts, leading to exciton self-trapping and broadband emission, thanks to the lattice softness, as well a...

Repetitive drag and release dynamics by impulsive force is characteristic of optical trapping by femtosecond laser pulses. We studied the dynamics utilizing double pulse train and found that trapped polystyrene particles are ejected repetitively from the focal spot and its frequencies become less for longer interval of the pulse trains. The ejection changes drastically in a few-ps interval region, although particles cannot move appreciable distance in such a short time. It means that displacement of particles by a conventional diffusive motion is not dominant and another fast process has an important role in femtosecond pulse trapping. We also revealed that the silica nanoparticles shows a decay at few-ps, indicating that the picosecond decay is not due to a material property but considered to be a general dynamics. We propose that a picosecond relaxation process of inertia force of particles is important for understanding laser trapping dynamics by femtosecond laser pulses.

We investigated femtosecond laser trapping dynamics of silica nanoparticles with different hydrophobic surface properties. We demonstrated that the hydrophobic surface on the silica nanoparticles facilitates mutual association of the nanoparticles in the optical trapping site. Such association of optically trapped nanoparticles is a prerequisite to induce their directional ejection away from the trapping site. The directional ejection of the optically trapped nanoparticles is most probably due to asymmetric three-dimensional ejecting forces generated by the electromagnetic interaction between transient assembly in the focal spot and the incident pulses. These findings provide important insights into the directional ejection of nanoparticles from the trapping site in the femtosecond laser trapping, and this physicochemical phenomenon is controlled by both the trapping laser and material properties.