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Joined 2026-09-21 Message
Incoming links: Arthur Ashkin
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.
Nobel Prize in Physics in 2018 2026-10-18
One half went to Arthur Ashkin for optical tweezers and their biological applications. One quarter each went to Gérard Mourou and Donna Strickland for a method of generating intense, ultrashort optical pulses.[ref]
The Nobel Prize in Physics 2018 by Nobel Prize Outreach. Individual official summary: Ashkin one half for optical tweezers and biological applications; Mourou and Strickland one quarter each for intense ultrashort optical pulses.
Single-beam gradient trapping experiment 2026-10-18
In 1986, Arthur Ashkin, Joseph Martin Dziedzic, John Ernst Bjorkholm and Steven Chu used spatially filtered argon laser light, a water-immersion objective of numerical aperture , and a water-filled glass cell. An auxiliary beam loaded large glass beads; dispersed small particles entered by Brownian motion. A side microscope and scattering of light detector provided readout. Their Fig. 1 shows a bead below the focus with about . On p. 290, silica size was inferred by scattering against a latex standard; translating the cell imposed fluid drag. These observations support single-beam confinement, while inferred size depends on the scattering model.[ref]
Observation of a single-beam gradient force optical trap for dielectric particles by Arthur Ashkin and others. A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm and Steven Chu, Observation of a single-beam gradient force optical trap for dielectric particles, 1986, pp. 288–290, Figs. 1–2 and final experimental paragraphs
A loading beam can help bring a bead into the capture region without supplying its subsequent confinement. Distinguish loading, holding and sensing when interpreting an apparatus drawing. The following optical trap position readout is a proposed calibration arrangement; its detector layout is not a reconstruction of this historical experiment.