Light often has spin angular momentum, more commonly referred to as polarization. Only in the past 30 years have researchers been able to impart orbital angular momentum, or OAM (see the article by Miles Padgett, Johannes Courtial, and Les Allen, Physics Today, May 2004, page 35). Instead of the usual flat wavefront, light with nonzero OAM has a helical wavefront and corkscrews in the direction of propagation. The OAM quantum number m defines how tight the corkscrew motion is. For example, m = 1 means a full rotation in a period, and m = 2 means two full rotations in a period.
With OAM, additional information can be encoded per photon. That advantage could improve modern optical networks, which use only intensity modulations to send information. What’s more, unlike polarization, which has two states and combinations thereof, m can take any integer value. Unfortunately, OAM is hard to measure—it typically requires a room’s worth of bulky optics. Ritesh Agarwal at the University of Pennsylvania and his colleagues have found a compact way to detect light’s OAM through the induced photocurrent in a device that’s only tens of micrometers across. Their newly published technique could reduce the space required to read out information encoded in the OAM.