Optical trapping is a technique that uses focused laser light to confine, manipulate, and apply optical forces to a variety of microscopic objects including dielectric, metallic, and biological samples. In this paper, we review the basic principles of optical trapping and summarize some of the key results derived from ray-optic and wave-optic theories for trapping forces and their parametric dependence. The application of these theories to the study of low-index microparticles and different trapping geometries are also described.

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Micromanipulation of biological materials by laser trapping and laser scalpel has now become indispensable for biological studies under observation by high-resolution microscope. Piconewton-scale controlled forces given by infrared laser trapping enable us not only to analyze mechanical rigidity of filamentous macromolecules to support cellular structure but also to measure working forces to promote motor protein molecules. In contrast to such invasive infrared laser trapping, weak focused laser beam of ultraviolet or visible radiation transects biological materials or structures instantaneously and thus can serve as a micro-surgical scalpel for in-situ characterization of living cells. This article reviews recent progress in biological studies applied by these laser techniques, and also discusses the effect of laser irradiation to biological samples.

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Radiation pressure exerted on particles can be used as an optical spring for characterizing physical and chemical interactions between particles and solid surfaces and for analyzing atomic and molecular forces such as van der Waals and electric-double-layer forces. A three-dimensional potential produced by radiation pressure of a focused laser beam is observed on the basis of the Boltzmann distribution for thermal energy of an optically-trapped particle. The surface charge density of a single particle can be analyzed by the balance of radiation pressure with an electrostatic force that acts on the particle placed between microelectrodes. A noncontact and nondestructive technique for measuring an adhesion force between a particle and a solid surface is proposed, which utilizes strong radiation pressure of a nanosecond pulsed laser. Applications of these methods are also discussed.

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A laser manipulation technique has been applied to tweeze individual fine particles. As a new application of a laser manipulation technique, phase transition of a polymer solution and a polymer gel induced by the radiation pressure was demonstrated. The characteristic features of the phenomena are discussed.

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Techniques to optically manipulate micrometer-sized dielectric particles have been used as optical tweezers in biological and chemical applications. We have extended the use of optical manipulation to study the rotational manipulation of dissymmetrically shaped micro-objects fabricated by micromachining. Principles and various characteristics of the optically induced rotations are reviewed and discussed.

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Optical trappings of metallic oxide particles with various optical constants, GeO₂, MgO, Al₂O₃, ThO₂, Nd₂O₃, CeO₂, TiO₂ and UO₂ have been experimentally performed in water using two kinds of visible laser lights and an objective lens of N.A. = 1.3. Two-dimensional optical trappings of all particles except UO₂, which has a large extinction coefficient, were possible for either of the laser lights. On the other hand, while three-dimensional optical trapping was possible for CeO₂ having a refractive index of -2.2 by a He-Ne laser light at 633nm, it was observed for particles having refractive index of ≤ 1.7 by an Ar⁺ laser light at 488nm. The trapping efficiencies of absorbing particles were calculated based on a geometrical optics model to determine the range of the complex index of refraction for which three-dimensional optical trapping is possible to take place. Calculational results have a fairly good agreement with experimental results.

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A newly designed cylindrical optical micro-rotor which has slopes for trapping and rotation on its top is proposed. The cylindrical shape is effective in decreasing medium viscosity. Rotation is induced not only by a focused beam but also by a parallel (unfocused) beam. A ray-tracing method considering beam waist is employed for analyzing the radiation pressure exerted on the top and the bottom of the rotor. We theoretically demonstrate optical trapping and high speed rotation for various rotor shape parameters such as oblique angle, height and diameter.

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Optical trapping for monodisperse hematite particles of uniform in shape and size as peanut-type, pseudo-cubic-type and ellipsoid-type, in micrometer range has been demonstrated using a Nd: YAG laser beam. Standing up of the trapped particle was observed for the peanut-type and the ellipsoid-type by a point focusing. When the laser beam focused into line-shape, the trapped particles appeared to be lined up. Unique particle arrangement which is not caused by flocculation was possible.

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A negative input-output characteristic was obtained in an erbium doped yttrium aluminum garnet crystal which has multiple-level energy structure: absorption can occur between a ground level and an excited level (⁴I_15/2-⁴I_9/2) and between two other excited levels (⁴I_13/2-²H_11/2) in Er³⁺. According as modulation degrees of the incident laser decreased, the transmitted laser waveform was distorted and eventually inverted against the incident one in case that the modulation degree was less than 0.5. It is suggested that an enhanced absorption phenomenon takes place since the excited state absorption from ⁴I_13/2 to ²H_11/2 and that a ⁴S_3/2 level plays an important role. Because the time-dependence of this phenomenon matches a life time of the ⁴S_3/2 level. By considering multiple excited state absorption in detail, active functions such as an optical signal inverter could be derived from the negative nonlinear absorption effect.

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New design of diode-end-pumped solid-state laser rods using composite structured crystals, diffusion-bonded with undoped end for improvement of thermal conduction is presented. In this paper, we discuss a analytical and experimental study on the characteristics of the composite laser rod in detail using Nd:YAG rods as example. The undoped end section is calculated to act as an effective heat diffuser and then peak temperature rise is reduced to <70% when it is compared with the non-composite crystals. We have also demonstrated high power operation of a diode-end-pumped Nd:YVO₄ laser using composite rod. A maximum CW output power of 9.3W was achieved, which is 1.5 times higher than in the non-composite Nd:YVO₄ rod. This high power performance of the composite rod is achieved owing to a decrease of thermally induced stress.

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Penetration depth is difficult to maintain constant in production, since it is affected by many parameters in long term operation. Present study is aiming at developing a method which can predict a welding parameter to decrease the penetration depth. An in-process monitoring system has been developed, which can separate the light emission of the keyhole plasma, P_K, from that of the plasma plume, P_P, by using two photo sensors set at 5 and 75 degrees with respect to the work surface. The effects of welding parameters including flow rate of Ar assist gas, laser power and focal position have been determined on the penetration depth as well as time-averaged values of P_P and P_K in CO₂ laser welding mild steel. In the analysis in P_K-P_P coordinate, one cane determine which welding parameter has decreased the penetration depth from the detected change in the emission from the laser-induced plasma.

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We have developed a set of simulation codes for recombination x-ray lasers. The simulation consists of a hydrodynamics code and an atomic kinetics postprocessor including the effect of line trapping of the hydrogenlike Lyman-α transition. The code was primarily designed to clarify the multidimensional effect of expansion cooling of the plasma as well as transfer of x-ray radiation, which plays a critical role in the production of soft x-ray gain. Using this code, temporal and spatial development of soft x-ray gain was calculated and excitation condition for high gain was examined.

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We newly adopted xenon excimer lamp as a light source of material processing such as photo-etching of polymers. Sheet shaped polymethylmetacrylate (PMMA) and polyimide (PI) resin have been used as samples. The samples were irradiated at a intensity of 12mW/cm². After irradiation, both kinds of polymers were confirmed to be etched by means of scanning electron microscopy, surface roughnessmetry and so on. The etchrates were found to be varied by changing the species and pressures of circumstance gases. The maximum etchrate was 10nm/min.

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Bacteriologic cultivation on a fluorocarbon-resin FEP surface has been successfully demonstrated by a photochemical modification method using an ArF excimer laser and a boric-acid aqueous solution. The resin surface was photochemically defluorinated with boron atoms which were produced by the photodissociation of the boric-acid aqueous solution; dangling bonds of carbon atoms in the surface were bonded with OH radicals which were also produced by the photodissociation. The sample surface was area-selectively modified into hydrophilic property by the exposure of ArF excimer laser with a mask-pattern image. A culture medium of the Czapeck-dox agar was fabricated on the hydrophilic parts, and a mold was cultured in the agar at 25°C and 7 days.

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