Orbital angular momentum lasers

Light can be tailored to carry angular momentum well beyond the restriction of its two spin states, left- and right-circularly polarized light, by imbuing it with orbital angular momentum (OAM). OAM is controlled by imparting finer and finer azimuthal phase gradients, twisting the wavefront ever tighter in one of two helicities, clockwise or anticlockwise. This can be done directly within a laser — OAM lasers — by imprinting an intracavity twist on the circulating light, but it requires judicious laser cavity design to break nature’s angular momentum degeneracy. Without this, the laser produces equal measures of both helicities, for no net OAM. We review the physics of OAM lasers, covering diverse symmetry-breaking approaches such as gain or loss control, asymmetric cavity geometries and geometric phase control. Structured matter allows this symmetry breaking to be done at the microscale and nanoscale, for OAM lasers based on topological matter, photonic crystals and optical breaking of chiral symmetry in microring cavities, as well as leveraging non-Hermitian photonic design at exceptional points. The exciting prospect of using structured matter to engineer twisted light is discussed along with the opportunities and challenges ahead. This Review covers the intriguing physics behind orbital angular momentum lasers, summarizing the exciting prospects at the interface between structured light and structured matter.