Principles of vortex light generation from electronically excited nanoscale arrays

It has recently been shown possible to directly generate an optical vortex (a beam of light endowed with orbital angular momentum) by spontaneous emission from a molecular exciton array. This contrasts with most established methods, which typically rely on the modification of a conventional beam by an appropriate optical element (for example, a q-plate) to impose the requisite helical twist of a vortex. The new procedure is achieved by nanofabricating a chiral arrangement of chromophores into a ring of specifically configured symmetry, supporting a doubly degenerate (conjugated) exciton with the appropriate azimuthal phase progression. It emerges that the symmetry elements present in the phase structure of the optical field, produced by emission from these degenerate excitons on a array, exhibits precisely the sought character of an optical vortex. The highest order of exciton symmetry, including the corresponding splitting of the electronic states, dictates the maximum magnitude of the topological charge. Work is now progressing on computer simulations aiming to reveal the detailed pattern of polarization behaviour in the emitted light, in which the vector character of the beam progresses around the phase singularity along the beam propagation axis. Significantly, this analysis points to the emission of radiation with polarization varying over the beam profile.