An optical levitation system for a physics teaching laboratory

Abstract

We describe an experimental system based on optical levitation of an oil droplet. When combined with an applied electric field and a source of ionizing radiation, the setup permits the investigation of physical phenomena such as radiation pressure, light diffraction, the motion of a charged particle in an oscillating electric field, and the interaction of ionizing radiation with matter. The trapping occurs by creating an equilibrium between a radiation pressure force and the force of gravity. We have found that an oil droplet can be trapped for at least nine hours. The system can be used to measure the size and total electric charge on the trapped droplet. The intensity of the light from the trapping laser that is scattered by the droplet is sufficient to allow the droplet to be easily seen with the naked eye, covered by laser alignment goggles. When oscillating under the influence of an ac electric field, the motion of the droplet can be described as that of a driven, damped harmonic oscillator. The magnitude and polarity of the charge can be altered by exposing the droplet to ionizing radiation from a low-activity radioactive source. Our goal was to design a hands-on setup that allows undergraduate and graduate students to observe and better understand fundamental physical processes.

A remote laboratory for optical levitation of charged droplets

“…A detailed description of the experimental set-up can be found in [9], and only a brief description is given here. A diode pumped solid state laser, with a maximum power of 2 W is vertically aligned and focused into a glass cell.…”

“…The lower electrode is either positive or negative, and the upper electrode is grounded. A detailed discussion on the behaviour of a charged trapped droplet in an electric field can be found in the recent work by Isaksson et al [9]. (iv) It is also possible to use a feedback loop where the position of the droplet is controlled by changing the power of the laser.…”

“…where d is the diameter of the droplet and λ is the wavelength of the laser (λ = 532 nm). The size of the trapped droplets can also be measured by the phase difference between the position of the droplet and the driving AC fields, as described in the paper by Isaksson et al [9].…”