= Ashkin radiation-pressure trapping experiment
{c}
<Arthur Ashkin>'s 1970 first experiment suspended transparent latex <spheres> in <water> inside a $120\,\mathrm{\mu m}$ glass cell. A horizontal $514.5\,\mathrm{nm}$ argon <laser> beam, focused to a $6.2\,\mathrm{\mu m}$ radius, illuminated individual beads viewed through a <microscope>. An off-centre $2.68\,\mathrm{\mu m}$ bead moved toward the beam axis and downstream until reaching the cell wall; blocking the beam released it into <Brownian motion>. Transparent particles and liquid reduced competing thermal effects. This observed sequence separates transverse attraction from downstream <radiation pressure>: wall-assisted confinement is not free three-dimensional confinement by one beam.<Acceleration and Trapping of Particles by Radiation Pressure by Arthur Ashkin>[A. Ashkin, Acceleration and Trapping of Particles by Radiation Pressure, 1970, p. 156 and Fig. 1(a)]{ref}
The beam-blocking comparison is particularly informative: persistence against a wall while illuminated and release when illumination stops test an optical cause. To infer a <force> from the downstream <velocity>, one additionally needs a <linear drag> model and the bead's distance from the wall; the observed motion alone does not calibrate an unrestricted <harmonic optical trap>.
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