Light scattering radiation force

A light beam propagating in matter interacts with the electrons of the medium. In insulators, the electrons are bound and the incident radiation induces a local polarization. Therefore, a scattering centre is a small polarizable element with the size of a monomer this element can be assimilated to a dipole in forced oscillation regime. The radiation produced by this dipole is the scattered radiation. This is Rayleigh scattering. 

We note that since Q involves the scattering coefficients, the radiation pressure force has resonance or near-resonance behavior. This first was observed and analyzed by Ashkin and Dziedzic (1977) in their study of microparticle levitation by radiation pressure. They made additional measurements (Ashkin and Dziedzic, 1981) of the laser power required to levitate a microdroplet, and Fig. 19 presents their data for a silicone droplet. The morphological resonance spectrum for the 180° backscattered light shows well-defined peaks at wavelengths corresponding to frequencies close to natural frequencies of the sphere. The laser power shows the same resonance structures in reverse, that is, when the scattered intensity is high the laser power required to levitate the droplet is low. 

A vertical laser beam has been used by Ashkin (1970) and Ashkin and Dziedzic (1971) to levitate weakly absorbing spherical particles by radiation pressure. Lateral stability results from the dominance of refracted over reflected components of the scattered light (see Table 7.1). Unequal reflection on opposite sides of the particle, which is caused by beam nonuniformity, produces a net force that drives the particle toward lower light levels this instability is countered by refraction, which produces a reaction that drives the particle toward higher light levels. The particle is thus laterally stabilized in the most intense part of the beam. Laser levitation has the disadvantage that it... 

In this section, we present a few examples of instruments available for visual observation and imaging of colloids and surfaces, for measurement of sizes and for surface force measurements. Such a presentation can hardly be comprehensive in fact, that is not our purpose here. Throughout the book, we discuss numerous other techniques such as osmotic pressure measurements, light and other radiation scattering techniques, surface tension measurements,... 

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