Coupled orbital angular momentum conversions in a quasi-periodically poled LiTaO3 crystal
The orbital angular momentum (OAM) of light is a tool that has numerous promising applications, ranging from quantum sensing, quantum communication and realizations of quantum information protocols. In order to produce OAM modes, one needs to act upon the phase front of a light pulse and essentially make it look like a corkscrew, by delaying some parts more than others. This is all fine unless the wavelength of the light pulse is far away from the operating range of the tools at one's disposal. For example, one of the most flexible instruments that one can use is a spatial light modulator (SLM), an array of microscopic cells which are individually addressable (a bit like the pixels of a screen) and which can delay the phase by a programmable amount. SLMs are great, but accept a limited range of frequencies. In particular, producing OAM in the UV range is very hard and manufacturers are doing their best to extend the working range of SLMs.
In this Optics Letters article, Fang and collaborators thought outside the box: make OAM at lower frequencies and then combine the pulses in a non-linear medium to produce OAM at higher frequencies. This is a non-linear process that allows one to "fuse" single photons together, so that a number of low-frequency photons may end up in a single photon with higher frequency. This idea has been around for a while, but the difficult bit was to maintain the spatial properties of the initial photons, which Fang and colleagues managed to do by employing an engineered crystal. This allowed them to overcome many of the issues encountered in previous attempts and to generate high-frequency OAM modes with remarkable efficiency.