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.
In chip-scale sensors, light is usually detected through interactions with matter. The bulk of a material’s response follows the dipole approximation, and unlike the vanishingly small quadrupole and subsequent terms in the expansion, it lacks phase information about the light. A clockwise helical wavefront and a counterclockwise helical wavefront have the same donut-shaped intensity and thus the same electrical response in that approximation. Measuring the phase is essential for characterizing light that has OAM.
To perform the measurements, Agarwal and his colleagues found a material whose symmetry forbids the dipole term under normal incidence of light. The higher-order terms, which are sensitive to the phase, aren’t drowned out. The group’s chosen material was tungsten ditelluride, a Weyl semimetal (see Physics Today, December 2019, page 24). Besides the right symmetry, it also has a strong nonlinear optical response.
In the group’s experiment, specially designed U-shaped electrodes, shown in the figure, probed the photocurrent in WTe2 that results from an incident laser’s phase gradient. The electrode shape gave the researchers a method to distinguish between current from the phase gradient and that from the intensity gradient and other effects. The resulting photocurrent took on discrete values based on the magnitude of m and the chirality.
The team’s next step is to test light composed of a combination of OAM modes. The goal is to break down the resulting electrical signal by the quantum numbers and the percentage they represent of the overall incident beam. With the math already worked out, the group members are currently improving the design of the electrodes. (Z. Ji et al., Science 368, 763, 2020.)