?Does high harmonic generation conserve angular momentum

High harmonic generation is an extreme nonlinear process in which infrared or visible radiation is frequency up-converted into the extreme ultraviolet and x-ray spectral regions. As a parametric process, high harmonic generation should conserve the radiation energy, momentum and angular momentum. Indeed, conservation of energy, momentum and orbital angular momentum have been demonstrated. On the other hand, conservation of spin (polarization) angular momentum has thus far never been studied, neither experimentally nor theoretically.
 
I will present the first study on the role of spin angular momentum in extreme nonlinear optics. In our experiment, we generate high harmonics of bi-chromatic elliptically-polarized pump beams that interact with isotropic media. While observing that the selection rules qualitatively correspond to spin conservation, we unequivocally find that the process of converting pump photons into a single high-energy photon does not conserve angular momentum. In one regime, we find that this major discrepancy can be explained if the harmonic photons are emitted in pairs.