Light's twist

Abstract

That light travels in straight lines is a statement of the obvious. However, the energy and momentum flow within light beams can twist to form vortices such as eddies in a stream. These twists carry angular momentum, which can make microscopic objects spin, be used to encode extra information in communication systems, enable the design of novel imaging systems and allow new tests of quantum mechanics.

Keywords: optical vortex; orbital angular momentum; structured light.

Figure 1.

When laser beams have helical phasefronts, their energy and momentum twist around the beam axis, and the beam carries an orbital angular momentum of $\ell \hslash$ per photon.

Figure 2.

When a standard laser beam illuminates a forked diffraction grating, the first-order diffracted beam has helical phase fronts and consequentially carries orbital angular momentum.

Figure 3.

When a helically phased laser beam (ℓ=3) is incident on microscopic particles, they are set into rotation around the beam axis.

Figure 4.

Nonlinear crystals can double the frequency of an incident laser beam, conserving the energy in the optical fields. In addition to the conservation of energy, the conservation of momentum leads to a doubling of the OAM per photon.

Figure 5.

The linear Doppler shift can be derived from the work done against the force exerted by an incident light beam (a). Similarly, a rotational Doppler shift can be derived from the work done against a torque (b).