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Optical physics

Optical physics

Chip puts a twist on light

19 Oct 2012
Tiny rings put a twist on light

Physicists in the UK and China have produced silicon devices measuring just a few thousandths of a millimetre across that can endow light beams with a twistedness associated with orbital angular momentum. The researchers say that by varying this property over a range of values, such devices could increase the amount of bandwidth available for telecommunications and underpin extremely powerful quantum computers.

A light beam’s “spin angular momentum” is a familiar property associated with its polarization, the direction in which its electric field vibrates. But light can also possess orbital angular momentum (OAM), which causes a beam’s wavefront to change direction in time. Whereas an ordinary collimated beam has a wavefront that remains fixed at right angles to its direction of propagation, a beam with OAM will see its wavefront rotate around the propagation axis, creating a spiral or vortex. The greater the orbital angular momentum, the tighter the spiral.

Generating OAM involves varying a beam’s phase across a plane at right angles to its path. In contrast, a collimated beam has a uniform phase across this plane. Physicists have come up with a number of ways of doing this, such as placing asymmetric lenses or holograms in the path of a laser beam. This latter approach, pioneered by Miles Padgett at Glasgow University in the UK, involves using a computer to create a grating with many columns of pixels that split into a pitchfork shape at the centre of the grating.

Useful but bulky

These techniques have led to a number of specialized applications, such as using laser beams to rotate particles in devices known as optical spanners. But the components involved – such as lenses or hologram plates – are bulky. Greater exploitation of OAM will probably require smaller devices that can be integrated into chips, since many proposed applications require the generation of large numbers of closely packed vortices. Last year Christopher Doerr and Lawrence Buhl at Bell Laboratories in the US reported making a silicon-chip-based system containing a spoke-like arrangement of waveguides, with the phase between neighbouring waveguides offset slightly in order to emit light with OAM. But measuring 1.0 × 1.4 mm, the device was large by the standards of modern integrated circuits.

In the latest work, Siyuan Yu of Bristol University and colleagues have made silicon devices consisting of straight waveguides connected to modified micro-ring resonators. These resonators usually trap light in the same way that the whispering gallery in St Paul’s Cathedral in London confines sound waves. But Yu’s group carved out a series of tiny bumps on the inside surface of the rings, so creating circular diffraction gratings that allow light to escape the rings. Crucially, the researchers realized that by adjusting the distance between bumps, they could give light a twist. With that distance equal to the light’s wavelength, all the light rays should be emitted at right angles to the plane of the rings, so creating a planar wavefront. But if there is a mismatch, they reasoned, the emission angle should vary along the rings’ circumference, which creates the unevenness of phase needed for OAM.

To show they could in fact generate light with OAM, the researchers merged the light from a 15-μm diameter ring with a circularly polarized reference beam. The resulting interference pattern showed the hoped-for signature – a spiral pattern with the right number of arms, given the amount of OAM added to the light. “These spirals are exactly what theory predicts should be seen, so there is no ambiguity at all in our result,” says Yu.

The team then hooked up three such rings to a single waveguide and found that each ring produced spiral emissions with the same number of arms. This simultaneous emission is important, says Yu, because it shows that the rings are reproducible and can therefore potentially be made in large quantities.

“A very clever idea”

Padgett is enthusiastic about the work. He says that it “opens the way for OAM to be an integral part of integrated optics”. Doerr, who is now at the US company Acacia Communications, is also complimentary about the new device, describing its underlying principle as “a very clever idea”. He believes that the device’s compactness might make it suitable for 2D imaging of objects, such as biological cells, which can alter the OAM value of light passing through them. But he argues that, unlike his group’s chip, the Bristol design will not lead to a significant increase in optical-communication bandwidth because each OAM state would require a different wavelength.

Yu is addressing this shortcoming and says that he and his colleagues are working on changing the OAM values associated with a particular wavelength by varying the refractive index of the rings electrically. Indeed, he says that they aim to produce devices that can emit different OAM values at the same time. This, he claims, could enhance telecommunication bandwidth, by increasing the number of channels available, and boost the power of quantum computers – devices, still under development, that promise much faster data crunching by processing multiple quantum states simultaneously. “Currently, quantum computers rely on electron spin or photon spin, which only have two states, whereas OAM has many states,” he explains.

The new device is described in Science.

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