Optical trapping and polarization-controlled scattering of dielectric spherical nanoparticles by femtosecond laser pulses

Optical Force-Induced Chemistry at Solution Surfaces

When an intense 1,064-nm continuous-wave laser is tightly focused at solution surfaces, it exerts an optical force on molecules, polymers, and nanoparticles (NPs). Initially, molecules and NPs are gathered into a single assembly inside the focus, and the laser is scattered and propagated through the assembly. The expanded laser further traps them at the edge of the assembly, producing a single assembly much larger than the focus along the surface. Amino acids and inorganic ionic compounds undergo crystallization and crystal growth, polystyrene NPs form periodic arrays and disklike structures with concentric circles or hexagonal packing, and Au NPs demonstrate assembling and swarming, in which the NPs fluctuate like a group of bees. These phenomena that depend on laser polarization are called optically evolved assembling at solution surfaces, and their dynamics and mechanisms are elucidated in this review. As a promising application in materials science, the optical trapping assembly of lead halide perovskites, supramolecules, and aggregation-induced emission enhancement-active molecules is demonstrated and future directions for fundamental study are discussed.

Nanoparticle Assembling Dynamics Induced by Pulsed Optical Force

Femtosecond (fs) laser trapping dynamics is summarized for silica, hydrophobically modified silica, and polystyrene nanoparticles (NPs) in aqueous solution, highlighting their distinct optical trapping dynamics under CW laser. Mutually repulsive silica nanoparticles are tightly confined under fs laser compared to CW laser trapping and, upon increasing laser power, they are ejected from the focus as an assembly. Hydrophobically modified silica and polystyrene (PS) NPs are sequentially ejected just like a stream or ablated, giving bubbles. The ejection and bubbling take place with the direction perpendicular to laser polarization and its direction is randomly switched from one to the other. These characteristic features are interpreted from the viewpoint of single assembly formation of NPs at an asymmetric position in the optical potential. Temporal change in optical forces map is prepared for a single PS NP by calculating scattering, gradient, and temporal forces. The relative contribution of the forces changes with the volume increase of the assembly and, when the pushing force along the trapping pulse propagation overcome the gradient in the focal plane, the assembly undergoes the ejection. Further fs multiphoton absorption is induced for the larger assembly leading to bubble generation. The assembling, ejection, and bubbling dynamics of NPs are characteristic features of pulsed optical force and are considered as a new platform for developing new material fabrication method.

Three-dimensional structure of polarization singularities of a light field near a dielectric spherical nanoparticle

We numerically study the structure of polarization singularity lines in a near-field of the sub-wavelength dielectric particle when it is irradiated by a monochromatic elliptically polarized plane wave. For the various values of the ellipticity degree of the incident radiation, we trace the C^T and L^T-lines on which the polarization ellipse turns into a circle or a line, respectively. The continuous variation of the isotropy parameters of the singularities is visualized along the lines. The main rules of C^T and L^T-lines deformation, emergence, and vanishing are revealed.

Time-resolved emission microscopy of light-induced aggregation of luminescent polymers

Photon pressure has been used to induce the aggregation from solution of a series of photoluminescent conjugated polyelectrolytes containing tetraphenylethene units. These polymers show steady-state and time-resolved emission properties that are dependent on the local chromophore environment that can be influenced by the degree of intra- and inter-molecular interactions, which enables the photoaggregation process to be monitored by time-resolved fluorescence imaging techniques. Structural differences in the polymer lead to variations in the photo-induced aggregation behaviour.

A Single Large Assembly with Dynamically Fluctuating Swarms of Gold Nanoparticles Formed by Trapping Laser

Laser trapping has been utilized as tweezers to three-dimensionally trap nanoscale objects and has provided significant impacts in nanoscience and nanotechnology. The objects are immobilized at the position where the tightly focused laser beam is irradiated. Here, we report the swarming of gold nanoparticles in which component nanoparticles dynamically interact with each other, keeping their long interparticle distance around the trapping laser focus at a glass/solution interface. A pair of swarms are directionally extended outside the focal spot perpendicular to the linear polarization like a radiation pattern of dipole scattering, while a doughnut-shaped swarm is prepared by circularly polarized trapping laser. The light field is expanded as scattered light through trapped nanoparticles; this modified light field further traps the nanoparticles, and scattering and trapping cooperatively develop. Due to these nonlinear dynamic processes, the dynamically fluctuating swarms are evolved up to tens of micrometers. This finding will open the way to create various swarms of nanoscale objects that interact and bind through the scattered light depending on the properties of the laser beam and the nanomaterials.

Resonance optical trapping of individual dye-doped polystyrene particles with blue- and red-detuned lasers

We demonstrate resonance optical trapping of individual dye-doped polystyrene particles with blue- and red-detuned lasers whose energy are higher and lower compared to electronic transition of the dye molecules, respectively. Through the measurement on how long individual particles are trapped at the focus, we here show that immobilization time of dye-doped particles becomes longer than that of bare ones. We directly confirm that the immobilization time of dye-doped particles trapped by the blue-detuned laser becomes longer than that by the red-detuned one. These findings are well interpreted by our previous theoretical proposal based on nonlinear optical response under intense laser field. It is discussed that the present result is an important step toward efficient and selective manipulation of molecules, quantum dots, nanoparticles, and various nanomaterials based on their quantum mechanical properties.

Enhanced optical confinement of dielectric nanoparticles by two-photon resonance transition

Two-photon resonance enhances optical confinement of nanoparticles.

Exploring the physics of efficient optical trapping of dielectric nanoparticles with ultrafast pulsed excitation

Stable optical trapping of dielectric nanoparticles with low power high-repetition-rate ultrafast pulsed excitation has received considerable attention in recent years. However, the exact role of such excitation has been quite illusive so far since, for dielectric micron-sized particles, the trapping efficiency turns out to be similar to that of continuous-wave excitation and independent of pulse chirping. In order to provide a coherent explanation of this apparently puzzling phenomenon, we justify the superior role of high-repetition-rate pulsed excitation in dielectric nanoparticle trapping which is otherwise not possible with continuous-wave excitation at a similar average power level. We quantitatively estimate the optimal combination of pulse peak power and pulse repetition rate leading to a stable trap and discuss the role of inertial response on the dependence of trapping efficiency on pulse width. In addition, we report gradual trapping of individual quantum dots detected by a stepwise rise in a two-photon fluorescence signal from the trapped quantum dots which conclusively proves individual particle trapping.

Plasmonic Optical Tweezers toward Molecular Manipulation: Tailoring Plasmonic Nanostructure, Light Source, and Resonant Trapping

This Perspective describes recent progress in optical trappings of nanoparticles based on localized surface plasmon. This plasmonic optical trapping has great advantages over the conventional optical tweezers, being potentially applicable for a molecular manipulation technique. We review this novel trapping technique from the viewpoints of (i) plasmonic nanostructure, (ii) the light source for plasmon excitation, and (iii) the polarizability of the trapping target. These findings give us future outlook for plasmonic optical trapping. In addition to a brief review, recent developments on plasmonic optical trapping of soft nanomaterials such as proteins, polymer chains, and DNA will be discussed to point out the important issue for further development on this trapping method. Finally, we explore new directions of plasmonic optical trapping.

Optical trapping of NaYF4:Er3+,Yb3+ upconverting fluorescent nanoparticles

We report on the first experimental observation of stable optical trapping of dielectric NaYF4:Er(3+),Yb(3+) upconverting fluorescent nanoparticles (~26 nm in diameter) using a continuous wave 980 nm single-beam laser. The laser serves both to optically trap and to excite visible luminescence from the nanoparticles. Sequential loading of individual nanoparticles into the trap is observed from the analysis of the emitted luminescence. We demonstrate that the trapping strength and the number of individual nanoparticles trapped are dictated by both the laser power and nanoparticle density. The possible contribution of thermal effects has been investigated by performing trapping experiment in both heavy water and into distilled water. For the case of heavy water, thermal gradients are negligible and optical forces dominate the trap loading behaviour. The results provide a promising path towards real three dimensional manipulation of single NaYF4:Er(3+),Yb(3+) nanoparticles for precise fluorescence sensing in biophotonics experiments.

Permanent fixing or reversible trapping and release of DNA micropatterns on a gold nanostructure using continuous-wave or femtosecond-pulsed near-infrared laser light

The use of localized surface plasmons (LSPs) for highly sensitive biosensors has already been investigated, and they are currently being applied for the optical manipulation of small nanoparticles. The objective of this work was the optical trapping of λ-DNA on a metallic nanostructure with femtosecond-pulsed (fs) laser irradiation. Continuous-wave laser irradiation, which is generally used for plasmon excitation, not only increased the electromagnetic field intensity but also generated heat around the nanostructure, causing the DNA to become permanently fixed on the plasmonic substrate. Using fs laser irradiation, on the other hand, the reversible trapping and release of the DNA was achieved by switching the fs laser irradiation on and off. This trap-and-release behavior was clearly observed using a fluorescence microscope. This technique can also be used to manipulate other biomolecules such as nucleic acids, proteins, and polysaccharides and will prove to be a useful tool in the fabrication of biosensors.

Optical trapping of nanoparticles by ultrashort laser pulses

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.

Laser trapping chemistry: from polymer assembly to amino acid crystallization

Laser trapping has served as a useful tool in physics and biology, but, before our work, chemists had not paid much attention to this technique because molecules are too small to be trapped in solution at room temperature. In late 1980s, we demonstrated laser trapping of micrometer-sized particles, developed various methodologies for their manipulation, ablation, and patterning in solution, and elucidated their dynamics and mechanism. In the 1990s, we started laser trapping studies on polymers, micelles, dendrimers, and gold, as well as polymer nanoparticles. Many groups also reported laser trapping studies of nanoclusters, DNA, colloidal suspensions, etc. Following these research streams, we have explored new molecular phenomena induced by laser trapping. Gradient force leading to trapping, mass transfer by local heating, and molecular reorientation following laser polarization are intimately coupled with molecular cluster and aggregate formation due to their intermolecular interactions, which depend on whether the trapping position is at the interface/surface or in solution. In this Account, we summarize our systematic studies on laser trapping chemistry and present some new advances and our future perspectives. We describe the laser trapping of nanoparticles, polymers, and amino acid clusters in solution by focusing a continuous wave 1064 nm laser beam on the molecules of interest and consider their dynamics and mechanism. In dilute solution, nanoparticles with weak mutual interactions are individually trapped at the focal point, while laser trapping of nanoparticles in concentrated solution assembles and confines numerous particles at the focal spot. The assembly of polymers during their laser trapping extends out from the focal point because of the interpolymer interactions, heat transfer, and solvent flow. When the trapping laser is focused at an interface between a thin heavy water solution film of glycine and a glass substrate, the assembled molecules nucleate and evolve to a liquid-liquid phase separation, or they will crystallize if the trapping laser is focused on the solution surface. Laser trapping can induce spatiotemporally the liquid and solid nucleation of glycine, and the dense liquid droplet or crystal formed can grow to a bulk scale. We can control the polymorph of the formed glycine crystal selectively by tuning trapping laser polarization and power. These results provide a new approach to elucidate dynamics and mechanism of crystallization and are the fundamental basis for studying not only enantioselective crystallization but also confined polymerization, trapping dynamics by ultrashort laser pulses, and resonance effect in laser trapping.