Arthur Ashkin's 1970 first experiment suspended transparent latex spheres in water inside a glass cell. A horizontal argon laser beam, focused to a radius, illuminated individual beads viewed through a microscope. An off-centre 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.[ref]
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)
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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