Nature Nanotech. http://doi.org/z89 (2015)

Many organic dipolar molecules display nonlinear optical behaviour: the square of an applied electric field — associated with light, for example — contributes to the magnitude of the molecule's induced polarization. Second-order nonlinear response has many applications, including laser frequency doubling.

In three or two dimensions, however, a set of identical dipoles will arrange in a pairwise manner, with neighbouring dipole moments being antiparallel to each other. As a result, any nonlinear optical response of the whole array cancels out. In contrast, the one-dimensional equilibrium arrangement has dipoles lined up head-to-tail, leading to an enhanced nonlinear optical response. But how do you coax the molecules onto a line?

Sofie Cambré and colleagues have found the trick: carbon nanotubes can provide just the right confinement. The authors were able to encapsulate certain asymmetric dye molecules in nanotubes, and observed a nonlinear optical response enhancement of a factor of more than fifty. Tube diameter and synthesis temperature act as experimental knobs for adjusting the degree of alignment. The filled nanotubes are chemically inert, making them ideal building blocks for novel nonlinear optical materials.