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Optics

Droplets set light in a spin

Fusilli pasta is made by extruding dough through an appropriately shaped hole. A new method for making similar shapes in the optical field of light involves passing laser beams through droplets of liquid crystals.

The fact that certain forms of polarized light can carry a spin angular momentum has been known since the early twentieth century1 — today we associate this quantity with individual photons. But light can also carry an orbital angular momentum arising not from its polarization, but from its phase profile. Such beams have many uses in optical manipulation, imaging and even information processing, but generating or measuring these beams requires specialist lenses or holograms. Brasselet et al.2 now report in Physical Review Letters that such beams can be made simply by focusing a laser through a microscopic liquid-crystal droplet.

The spin angular momentum of light is associated with circular polarization, in which the optical field of a light beam rotates around the beam's axis. Circularly polarized light is described as being right-handed or left-handed, depending on the direction of rotation. At every point in the cross-section of a normal laser beam that has circular polarization, the waves of the optical field are in step and rotate together. This produces a 'plane wave', in which the wavefronts are parallel planes, each separated from the next by a distance of one optical wavelength (Fig. 1a). But in a beam carrying orbital angular momentum, the wavefronts instead form one or more continuous helices (Fig. 1b). If there is a single helix, the wavefront looks like a screw thread; if there are two helices, the wavefront looks like DNA; and for three helices, the wavefronts have the shape of fusilli pasta. In fact, beams can be made with any number of helices — the more helices there are, the larger the orbital angular momentum.

Figure 1: Light with a twist.
figure1

a, In plane waves of light, the wavefronts form planes that are separated from each other by a distance of one optical wavelength. b, Brasselet et al.2 pass plane waves of circularly polarized light (in which the optical field of the light rotates around the axis of the beam) through specially prepared droplets of liquid crystals to make helical waves. Here, the wavefront forms a helix, which confers orbital angular momentum on the light. (Figure adapted from ref. 2.)

The first laser beams that carried orbital angular momentum were made in 1992, by passing a normal laser beam through a system of lenses3. This opened the door to further studies, and led to the discovery a few years later that light beams carrying orbital angular momentum can act as 'optical spanners' that rotate microscopic objects4. Orbital angular momentum is now known to underpin many phenomena, including the Doppler shifts of spinning bodies, certain forms of Heisenberg's uncertainty principle and manifestations of quantum entanglement5.

Orbital angular momentum is still almost always introduced into normal laser beams using converter devices — usually, large optical components such as lens systems, holograms or precisely machined spiral wedges of plastic or glass. On a smaller scale, micromachining techniques have been used to fabricate miniature converters, primarily for use in optical tweezers6 (laser beams that can trap and move microscopic objects). The beauty of Brasselet and colleagues' system2 is that the high purity of the beam it produces is a natural consequence of the internal orientation of the molecules in the liquid-crystal droplet. This orientation arises from the conditions and reagents used to prepare the droplets, and so no complicated set-ups or machining techniques are required.

So how exactly does it work? Brasselet and colleagues' liquid-crystal material is birefringent, which means that horizontally polarized light travels through it at a different speed from vertically polarized light. This effect is used widely in optics to make waveplates that transform the polarization state of light. A half-waveplate reflects the electromagnetic field of a beam about the optic axis of the crystal, transforming right-handed circularly polarized light into left-handed, and vice versa. Brasselet and colleagues' droplets act as half-waveplates, but with an added twist. Not only do they interconvert left- and right-handed circularly polarized light, but the transmitted light also undergoes a geometric Pancharatnam–Berry phase delay — a change in phase that depends on the orientation of the optic axis of the liquid crystal. Because of the way in which the optic axes are orientated within the droplets, laser beams emerge with a helical wavefront (Fig. 1b), and hence with an orbital angular momentum. Pancharatnam–Berry phase delays have previously been used in macroscopic light-mode converters based on liquid crystals7, but never before has the effect been a natural consequence of microscopic droplet structure.

A surprising feature of Brasselet and colleagues' microscopic converter is that it works over a wide range of optical wavelengths — a feat previously made possible only using combinations of optical components8. In their present form, however, the inherent structure of the droplets2 means that the resulting beam contains only two intertwined wavefronts, whereas traditional approaches can generate any number of them. The challenge now will be to extend the droplet approach to yield larger numbers of intertwined wavefronts, and to construct a robust, miniature converter that can be used in practical applications. Given the apparent purity of the beams produced using Brasselet and colleagues' strategy, this is a challenge well worth pursuing.

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Padgett, M. Droplets set light in a spin. Nature 461, 600–601 (2009). https://doi.org/10.1038/461600a

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