This picture is of a ‘photonic molecule’, the latest development in our increasingly sophisticated control of photons. Bayer and colleagues (Phys. Rev. Lett. 81, 2582-2585; 1998) have built a range of these structures to test how pure modes of trapped light behave in them.

Until a few years ago, our manipulation of light was crude: creating photons of visible light by heating matter until its electrons vibrate violently enough to radiate; filtering them; bouncing them off mirrors. The light is generally incoherent, and has a wide range of colours. Lasers are a much more refined tool, producing a narrow beam of nearly monochromatic, coherent (in-phase) light.

Even cleverer are photonic-bandgap materials, where scattering off periodic arrays of a dielectric prevents certain wavelengths of light from propagating. Such materials can be used to bend light around corners, and trap it in cavities.

Light can also been trapped in ‘photonic atoms’. A semiconductor layer is sandwiched between two mirrors, and an island is etched out of the overall structure. The refractive-index barrier between the semiconductor and the surrounding air allows photons to leak only slowly out of the sides; this confinement means that only resonant photon modes are excited. This technology may be used to make efficient semiconductor lasers with a single excited mode.

To take the idea a stage further, Bayer et al.linked pairs of gallium-arsenide photonic atoms with narrow bridges. As the bridge length was reduced, new photonic states emerged that are similar to the bonding and antibonding orbitals of the hydrogen molecule.

The aim now is to build more complicated photonic molecules to study this photon-matter interaction, and perhaps to learn something, by analogy, about electronic states in real molecules.