Orbital angular momentum of twisted light: Chirality and optical activity

Optical activity is conventionally understood as a natural difference in the optical responses of chiral materials with opposite handedness. It stems from the quantised spin angular momentum ±h per photon, with the ± representing either left- or right-handed circular polarisations. Less well known, until recently, was the possibility that matter might also respond in a similar, discriminatory way to the handedness of twisted light, or 'optical vortices', whose orbital angular momentum (OAM) is quantised as ℓh per photon, where ℓ is the topological charge whose sign determines a wavefront twist to the left or right. Initial studies focusing on whether, in spectroscopic applications, chiral matter might respond differently to the vortex handedness of +ℓ and -ℓ beams, failed to identify any viable mechanism. However, in the last few years, theory and experiment have both supplied ample evidence that, under certain conditions, such forms of interaction do exist-and as a result, the field of chirality and optical OAM is beginning to flourish at a pace. This topical review presents a survey of this new field, working up from a description of those initial studies to the cutting-edge experiments now taking place. Analysing the fundamental mechanisms provides for a revision of previous precepts, broadening their scope in the light of recent advances in understanding, and highlighting a vibrant synergy between the fields of optical activity and twisted light.