Parametric downconversion and the orbital angular momentum of light

The orbital angular momentum of light is distinct from the intrinsic angular momentum associated with the polarization state of the light. Thus, linearly polarized laser modes can carry angular momentum. Such orbital angular momentum is associated with light beams with helical wave fronts, one class of which are the Laguerre-Gaussian laser modes. The azimuthal phase factor exp(ilφ) in their field distribution gives rise to an orbital angular momentum equivalent to lh per photon. Degenerate parametric downconversion can be considered as the inverse process to second-harmonic generation. An incoming pump wave generates two fields, signal and idler, with half the pump frequency. Accordingly, a Laguerre-Gaussian pump beam produces signal and idler beams.