Structured Gaussians From Geometric Quantisation

We develop a geometric description of structured Gaussian beams, a form a structured light, by applying geometric quantisation and symplectic reduction to the 2D harmonic oscillator. Our results show that the geometric quantisation of the oscillator's reduced phase space coincides with the modal Poincar\'e sphere in optics. We explicitly consider the case of the Generalised Hermite-Laguerre-Gauss modes, identifying their interbasis expansions with rotations of the reduced phase space and the geometric data accompanying the quantisation. This description simplifies the presentation of $SU(2)$ symmetries in structured light beams and is extensible to other symmetry groups.