Safe Experimentation in Optical Levitation of Charged Droplets Using Remote Labs

Summary

Optical levitation is a method for levitating micrometer-sized dielectric objects using laser light. Utilizing computers and automation systems, an experiment on optical levitation can be controlled remotely. Here, we present a remotely controlled optical levitation system that is used both for educational and research purposes.

Abstract

The work presents an experiment that allows the study of many fundamental physical processes, such as photon pressure, diffraction of light or the motion of charged particles in electrical fields. In this experiment, a focused laser beam pointing upwards levitate liquid droplets. The droplets are levitated by the photon pressure of the focused laser beam which balances the gravitational force. The diffraction pattern created when illuminated with laser light can help measure the size of a trapped droplet. The charge of the trapped droplet can be determined by studying its motion when a vertically directed electrical field is applied. There are several reasons motivating this experiment to be remotely controlled. The investments required for the setup exceeds the amount normally available in undergraduate teaching laboratories. The experiment requires a laser of Class 4, which is harmful to both skin and eyes and the experiment uses voltages that are harmful.

Introduction

The fact that light carries momentum was first suggested by Kepler when he explained why the tail of a comet always points away from the sun. The use of a laser to move and trap macroscopic objects was first reported by A. Ashkin and J. M. Dziedzic in 1971 when they demonstrated that it is possible to levitate micrometer sized dielectric objects¹. The trapped object was exposed to an upward directed laser beam. Part of the laser beam was reflected on the object which imposed a radiation pressure on it that was sufficient to counterbalance gravity. Most of the light, however, was refracted through the dielectric object. The change of the directi....

Protocol

NOTE: The laser used in this experiment is a class IV laser delivering up to 1 W of visible laser radiation. All personnel present in the laser laboratory must have conducted adequate laser safety training.

1. Hands-On Experimental Protocol

1. Safety
   1. Make sure everyone in the lab is aware that a laser will be turned on.
   2. Turn on the laser warning lamp in the lab.
   3. Check that no watch or metal rings are worn and put on the laser goggles.
   4. Check that the four light absorbing boards, closest to the experiment, are in place.
   5. Check the space between the laser and the absorbing board f....

Results

When the laser beam is well aligned, and the bottom plate is clean, the drops are almost immediately trapped. When a droplet is trapped it can stay in the trap for several hours, giving plenty of time for investigations. The radius r of the droplets is in the range of 25 ≤ r ≤ 35 µm and the charge has been measured between 1.1x10^-17 ±1.1x10^-18 C and 5.5x10^-16 ±5.5x10^-17 C. The size of the droplets stays, according to our measurements, c.......

Discussion

This work presents a setup for carrying out a modern physics experiment in which droplets are optically levitated. The experiment can be performed either in a traditional hands-on way or remotely. With the remote system establishment, students and researchers all over the world can get access to the experimental set-up. This also guarantees the users’ safety, since they do not need to be in presence of the high-power laser and electric fields required for the experiment. In addition, the users can interact with the.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
GEM 532	Laser Quantum		Green laser with adjustable power between 50 mW and 2 W
Lateral Effect Position Sensor	THOR Lab	PDP90A	PSD to sensor the position of the droplet in the pipette
Advanced Educational Spectrometer Kit, Metric	THOR Lab	EDU-SPEB1/M	Mirrors and other elements to control the laser beam
Pipette	Self made		The chamber were the droplet is trapped was specially made for this setup
AC/DC Power supply	Keithley Instruments, Inc.	2380-500-30	A power supply to generate the electric field (0V - 500V DC)
Power Distribution Unit	APC	AP7900	A PDU to remotelly connect the lab instrumentation