Physicists in Spain and the US have shown that the orbital angular momentum, or twistedness, of light can be made to vary rapidly in time. The researchers demonstrated what they call “self-torque” by generating high-frequency harmonics from two intense infrared laser pulses offset very slightly in time. They argue that light with this property could in future provide an ideal tool for manipulating molecules and other nanometre-sized structures.

Spin angular momentum is a familiar property of light, being manifest in the polarization of the light. But since 1992 it has been known that light exhibits a distinct property known as orbital angular momentum (OAM). This results in a twisting of a beam’s wavefront around its propagation axis so that the light takes on a spiral shape with zero intensity at its core. A beam can in principle have any amount of twistedness, with greater twist meaning that the wavefront rotates more quickly.

Physicists are using OAM to develop a range of new technologies. The number of data channels in a fibre-optic cable can be boosted by sending down light with multiple values of OAM, just as it can be enhanced via greater frequency bandwidth. Beams with OAM can also increase the resolution of microscopy and be used to manipulate microscopic objects such as nanoparticles, quantum dots and even living cells.

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