Simons Program Abstract

Creating an Optical Spanner To Demonstrate
the Spin and Orbital Angular Momentum of Light

Yiwei Li, Phillips Academy, Andover, MA; Hamsa Sridhar, Harvard
University; John Noé, Laser Teaching Center, Stony Brook University

Optical tweezers are an exciting application of physics that use the momentum of photons in a focused laser beam to exert a force on and ultimately trap microscopic particles. Such tweezer devices are used extensively in biological fields to non-invasively manipulate microscopic matter such as DNA. It is possible for laser beams to carry angular momentum, and such beams can exert a torque on trapped particles, causing them to rotate [1]. This additional degree of motional control makes possible many more tweezer applications, such as microscopic motors and fluid pumps.

Light can have two distinct types of angular momentum: spin angular momentum (SAM) and orbital angular momentum (OAM). SAM is carried by circularly polarized light, light in which the electric field vector traces out a circle. Such was proven in 1936 by Richard Beth, when he demonstrated the effects of spin angular momentum on a birefringent wave-plate [2]. On the other hand, OAM arises from laser beams with helical wavefronts, commonly known as optical vortices. Normally the momentum vector in a beam of light is parallel to the direction of its propagation; in an optical vortex beam the momentum vector includes a small azimuthal component that creates angular momentum.

Our overall goal is to create a circularly polarized optical vortex tweezer that can transfer both spin and orbital angular momentum, similar to the "optical spanner" described by Simpson, Dholakia, Allen, and Padgett [1]. In their device the two types of angular momentum could be combined to vary the speed of rotation of trapped Teflon particles.

We initially attempted to achieve the π/2 phase shift necessary for creating circularly polarized light by tilting and/or combining several unmarked 1/4 wave-plates found in the lab. Recently we have tried an alternative and more promising method that involves total internal reflection within two right triangular prisms [3]. Currently we are also reworking the existing tweezers setup to include an optical vortex beam created with a spiral Fresnel zone plate [4].

We would like to thank the Simons Foundation for funding this research, and Prof. Harold Metcalf for establishing and supporting the Laser Teaching Center.

[1] N. B. Simpson, K. Dholakia, L. Allen, and M. J. Padgett, "Mechanical equivalence of spin and orbital angular momentum of light: an optical spanner," Optics Letters 22 52 (1997).

[2] Richard Beth, "Mechanical Detection and Measurement of the Angular Momentum of Light," Physical Review 50 115 (1936).

[3] Kristine Horvat, Laser Teaching Center project, Spring 2007,

[4] Nityan Nair, "Diffraction With a Twist," Simons Program abstract, August 2008.

Yiwei Li
August 2008
Laser Teaching Center