Orbital Angular Momentum of Light

Orbital angular momentum (OAM) of light is the portion of a beam's angular momentum that depends on the spatial distribution of the electromagnetic field rather than polarization. It includes an origin-dependent external component (r × P) and an origin-independent internal component associated with helical wavefronts and optical vortices. Helical modes are labeled by an integer topological charge m, which sets the phase winding e^{imφ} around the beam axis; common examples are Laguerre-Gaussian modes with nonzero azimuthal index. Individual photons in such modes carry quantized OAM along the propagation axis Lz = mħ. Classical and quantum formulations express OAM through spatial derivatives of the field, and measurable features include a central intensity null and characteristic spiral phase structure.

Decomposition of total angular momentum into spin and orbital parts for massless photons is theoretically problematic, prompting alternative uses of helicity. Practical work focuses on generating, detecting, and exploiting OAM: spatial light modulators, spiral phase plates, diffractive fork gratings and q‑plates create specific m states, while measurements use spiral-fringe counting, holographic filters and other interferometric or diffractive techniques. Applications span increased-channel telecommunications, quantum information encoding, radio astronomy and integrated sensing. Recent advances include geometric‑phase (Pancharatnam-Berry) metasurfaces for OAM generation and proposals for direct excitonic emission of structured light.