So I meant to post this the other day, sorry!
The Orbital Angular Momentum of Light — Lecture II, Miles Padgett
Miles spoke today with admiration about “papers you wish you had written”. Every lecture of Miles’ can be considered as “one you wish you had given”, and certainly one that it is a pleasure to watch. He has a special talent for presenting things you already thought you knew in a way that you had never thought of before.
Miles spent the first 10 minutes discussing “creativity – the art of science”. In trying to develop new ideas we create more as a collective than as individuals, each new idea leading to new sub-ideas. However, an idea quickly deemed as “good” can stifle additional ideas. Creating an environment in which new ideas can flourish without bias is an important part of any research group.
Miles then went on to begin discussing the orbital angular momentum (OAM) of light. Beams carrying OAM possess helical wavefronts of constant phase. Think of a Fusilli pasta for example, which looks like the wavefront of a OAM = 3 beam. The sets of modes with well-defined OAM, such as e Laguerre-Gauss modes, form a basis for any beam. OAM can be generated and measured using spiral phase holograms or other devices, and has found interesting application in a number of fields of optics and physics in general, such as spiral interferometry, optical vortex cornographs, acoustic vortices, electron vortices. OAM has been used recently in free space optical and radio communications in which case OAM gives spatial mode division multiplexing, which could lead to increased bandwidth. Optical vortices appear frequently in nature. Whenever 3 ore more plane waves interfere, charge one optical vortices are formed. Vortices appear at points of phase singularities. Speckle patterns can be composed of tangled lines or loops of vortices.
There’s an app for that! “ihologram” can be used to create spiral phase holograms on your iphone, ipad.
The last third of the lecture was geared towards entanglement in photonic OAM. Correlations can be seen between different OAM modes and superpositions of OAM modes, or OAM and angle. Miles touched quickly on Bell inequality tests with OAM in a 2D subspace. OAM of different values allows one to produce sine curves of any frequency, and then one can engineer a square wave coincidence profile. However, to use this in a Bell inequality imposes a post-selection or fair sampling loophole.