Many biological molecules come in two mirror-image forms, each of which can have very different properties such as toxicity or reactivity. Simple and compact devices for sensing the two ‘chiral’ forms, referred to as being either left- or right-handed, would therefore be useful in a variety of applications. Detecting a molecule’s chirality is not easy however, and requires a special form of light known as circularly polarized light. Makoto Kuwata-Gonokami and co-workers from the University of Tokyo in Japan1 have now developed a micro-scale semiconductor device that produces this type of light.

Circularly polarized light displays the same chiral property as the molecules it is used to detect — its polarization rotates as the light beam propagates with either a right-handed or left-handed spiral. This property makes circularly polarized light useful for investigating chiral molecules because it interacts with each chiral type differently.

Kuwata-Gonokami and his co-workers fabricated their compact light source by embedding tiny crystalline ‘quantum dots’ in a semiconducting material. “We have shown that it is possible to take advantage of light–matter interaction using an artificial periodic chiral structure to induce an imbalance in the amplitude of right- and left-handed circular components,” explains Kuwata-Gonokami.

Fig. 1: Schematic illustration of a surface patterned with a repeating array of chiral structures that influence the degree of circular polarization of the emission from a quantum dot

The team patterned the surface of two semiconductor devices with an array of structures known as gammadions (see image) — one device with a left twist and the other with a right twist. They showed that the polarization orientation of the quantum dot emission depended on the twist direction of the gammadions. The structures generated circularly polarized light with a degree of polarization as high as 26%. However, calculations indicate that this could be further increased to 91% if the position of the quantum dot relative to the surface pattern could be better controlled.

Whereas sources that can generate either left- or right-circularly polarized light have been created before using liquid crystal materials, the advantage of the semiconductor approach is that the devices can be much thinner and, therefore, find a wider range of applications. “Detecting the chirality of biomolecules is important for medical sensing,” says Kuwata-Gonokami. “Our compact solid-based circularly polarized emitter could lead to such a device.”