This article reviews the studies of light-matter mechanical interaction and its applications. It focuses on the recent trend where the targets of optical manipulation have been shifting to nanoscale objects, where the microscopic light-matter interaction plays an important role. We discuss optical manipulations using light that is resonant with the electronic transitions of nano-objects. The resonant effect is the key mechanism not only for enhancing the exerted force but also for linking the quantum mechanical properties of individual nano-objects to their macroscopic motions.

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This paper presents a summary and consideration of recent advances in optical trapping-induced formation of a millimeter-scale domain composed of clusters of amino acids or proteins. Molecules and/ or clusters in solution are gathered by optical trapping into a one-micrometer focal volume, where local concentration is increased. The increase in local concentration overcomes an energy barrier leading to liquid-liquid phase separation, and then the local resultant domain extends into a millimeter-scale due to interactions with molecules/clusters of surroundings, eventually forming a millimeter-scale cluster domain characteristic of optical trapping. Crystallization is induced through the domain formation, and growth rate and polymorphism are governed by concentration and orientation of clusters in the domain depending on laser polarization direction. This domain represents a new material state with high potential for application in laser trapping.

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We introduce a novel technique for the quantitative analysis of plasmonic trapping potentials experienced by a nanometer-sized particle. Our experimental results show that these potentials have nanoscale spatial structures that reflect the near-field landscape of the metal nanostructure. The trap stiffness of plasmonic trapping can be enhanced by three orders of magnitude compared to conventional far-field trapping. We also demonstrated super-resolution optical trapping by observing double potential wells with 80-nm separation, which was realized by a gold double-nonogap structure. In addition, we analyzed the nanoscale spatial profiles of plasmonic fields within a nanogap, which exhibit complicated fine structures created by the constructive and destructive interferences of dipolar, quadrupolar, and higher-order multipolar plasmonic modes. The nanoprofile can be drastically changed by controlling the excitation optical system, which is applicable to the dynamic nanomanipulation of single molecules and molecular assemblies.

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Localized surface plasmon generates a strong radiation force on nanoparticles in the vicinity of noble metallic nanostructures, resulting in efficient and stable optical trapping. Such plasmonic optical trapping is a hot topic in nanophotonics, and can be applied to molecular manipulation techniques. We review plasmonic optical trappings of thermoresponsive polymer microgels and DNA. Discussion on trappings of these soft nanomaterials provides us a crucial important issue for achieving molecular manipulation based on plasmonic optical trapping. Finally, we will describe future outlook for this trapping method.

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A target object can be manipulated by radiation force. Here we explain the optical transportation of semiconductor nanoparticles using its resonant enhancement. Since semiconductor nanoparticles have discrete energy level structures owing to the quantum confinement effect, the radiation force is strongly enhanced when the incoming light resonantly excites the semiconductor nanoparticles. This enhancement enables us to efficiently manipulate and sort the nanoparticles with resonant radiation force.

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The principle of optical trapping is based on the interaction between optical electric fields and induced polarizations (gradient force). Here we describe a novel trapping phenomenon that arises from nonlinear polarization using ultrashort near-infrared laser pulses. A stable trap site is split into two equivalent positions, in contrast to conventional (linear) optical trapping where the trap site is at the center of the focus. The trapping behavior with ultrashort pulses is successfully interpreted in terms of the nonlinear polarization induced on the trapped particles. Nonlinear optical trapping might greatly expand the potential of optical trapping.

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Chiral copper nano-needles with a high aspect ratio were fabricated by the irradiation of an optical vortex pulse onto a copper substrate. The angular momentum of the optical vortices forces the constituent elements of the irradiated material to rotate around the axial core of the optical vortex, forming a chiral copper nano-needle. The length and tip curvature radius of the copper nano-needle were measured to be approximately 12.5-μm and 48-nm, respectively.

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We developed an Integrated Simulation code for Laser Ablation Peening (ISLAP) to estimate the plume pressures produced by laser ablation in a water confinement regime, and to calculate the stress and strain formations in solid metals. Since controlling laser-ablation-produced-plumes is essential in laser peening, laser ablation phenomena must be studied in a water confinement regime by computer simulations. In ISLAP, the phase transitions from solid to liquid and from liquid to gas are modeled by the improved Anisimov formula. We evaluated the formation of stress and strain in solid metals by solving hydrodynamics equations, the strain-displacement relation, and the stress-strain relation. The input physical parameters are the laser peak intensity, the laser pulse duration, the laser wavelength, the temporal pulse shape, the material type, and the type of confinement material (vacuum, air, water, or glass). Using ISLAP, we can estimate the pressures of the plumes, the stress and the strain in solid metals, the plume lengths, and the ratio between the thermal energy and the internal energy of the plumes.

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