Optical vortices on a chip

An international research group led by scientists from the University of Bristol and the Universities of Glasgow (UK) and Sun Yat-sen and Fudan in China, have demonstrated integrated arrays of emitters of so call 'optical vortex beams' onto a silicon chip. The work is featured on the cover of the 19 October issue of Science magazine (Integrated compact optical vortex beam emitters, Xinlun Cai, Jianwei Wang, Michael J. Strain, Benjamin Johnson-Morris, Jiangbo Zhu, Marc Sorel, Jeremy L. O'Brien, Mark G. Thompson, Siyuan Yu, Science, 19 October 2012).

Contradicting traditional conception, light in such beams does not propagate in straight rays. Instead, its energy travels in a spiral fashion in a hollow conical beam shape. The beams therefore look very much like a vortex or cyclone, with its light rays 'twisted' either left-handed or right-handed.  In theory, there is no limit to how twisted the light rays can be.

In quantum mechanics, this feature is associated with the 'orbital angular momentum' (OAM) of photons – photons in such beams can be thought to orbit around the beam axis, somewhat similar to the movement of planets around the Sun or electrons around a nucleus.

When such light interacts with matter, it asserts a rotational force (a torque) on the matter; therefore it can be used as so called 'optical spanners' in addition to 'optical tweezers', which can rotate as well as trap microscopic particles or droplets. Different degree of twist can also be used to transmit information – allowing more information to be carried by a single optical signal, and increasing the capacity of optical communications links.

Light beams at the same frequency but with different OAM values can be used to transmit different streams information.

Single particles of light (photons) can use these different degrees of twist to represent quantum information, where a single photon can be twisting both clockwise and anti-clockwise at the same time.