A fine particle (which hereinbelow will also be referred to as a “nanoparticle”) having a nanometer-order particle diameter has properties that cannot be obtained from a bulk material in, for example, a quantum size effect and a characteristic physical and chemical phenomenon occurring on a surface or an interface. Therefore the fine particle has been attracting attention in various industrial fields. Various microforces, such as forces caused by Van der Waals interactions, electric double layers, thermal motions, and force of gravity, act on the fine particle having a nanometer-order particle diameter. Analyzing these microforces acting on the fine particle is very important to clarify mechanisms of, for example, adsorption and adhesion of the nanoparticle onto a solid surface as well as aggregation and association among a plurality of fine particles. Since the clarification of these mechanisms significantly contributes to the design and the creation of functional materials using nanoparticles, the realization thereof is expected.
As a method for performing three-dimensional measurement of potential of a microforce acting on a single fine particle in a solution, the inventors for the present patent application have proposed an optical-trapping potential measuring method developed by combining laser trapping and nanometer position detection techniques (K. Sasaki et. al., Appl. Phys. Lett., 71, 37 (1997)). The optical-trapping potential measuring method enables a high-speed and high-accuracy three-dimensional analysis of a picoNewton-femtoNewton-order force acting on a single fine particle. Hitherto, while the measurement of a microforce such as a radiation pressure or electrostatic force has been successfully achieved, the measurable sizes of fine particles by the method have still been limited to those of micrometer-submicrometer order. When the particle diameter of a fine particle set as a measurement object is reached to a nanometer order, high-speed fluctuations occur in the position of the fine particle according to a Brownian motion. Therefore a position detecting system in the measurement position cannot follow the speed, thereby disabling the implementation of an accurate measurement. This problem is attributed to the fact that since a continuous-wave laser is used as an observation light source in a measuring apparatus used with the method, the response time in the overall system is limited to a time of microsecond order corresponding to the order of a response time of a position-detecting electronic circuit.