Optical measurement of torque exerted on an elongated object by a noncircular laser beam

We have developed a scheme to measure the optical torque exerted by a laser beam on a phase object by measuring the orbital angular momentum of the transmitted beam. The experiment is a macroscopic simulation of a situation in optical tweezers, as orbital angular momentum has been widely used to apply torque to microscopic objects. A hologram designed to generate ${\mathrm{LG}}_{02}$ modes and a CCD camera are used to detect the orbital component of the beam. Experimental results agree with theoretical numerical calculations, and the strength of the orbital component suggest its usefulness in optical tweezers for micromanipulation.

Figure 1

Experimental setup for measurement of torque on the phase plate (rod).

Figure 2

Gray scale profile and image of the circular rod.

Figure 3

First order modes from a ${\mathrm{LG}}_{02}$ hologram with a Gaussian input. The graph shows a line scan through the image array.

Figure 4

First order modes from a ${\mathrm{LG}}_{02}$ hologram with a ${\mathrm{LG}}_{02}$ input.

Figure 5

Signal difference from two center pixels, for the rotation of a rod (with a circular profile) through $360°$.

Figure 6

Interference patterns showing the phase shift of circular profile rods, with varying contact print exposure times.

Figure 7

Signal strength (difference in intensity of the two center pixels) relative to phase thickness of different rods (circular profile rods).