A Fine Point on Light’s Angular Momentum

Padgett and Allen reply: In our experiment, the particle was Teflon and had an absorption of a few percent, which meant it could be trapped within optical tweezers. In the review, we called it transparent to contrast it with the earlier work of H. He and coworkers. ¹ In hind-sight, as Anthony Siegman suggests, we should have described it as “slightly, or partially, absorbing.”

Both the spin and orbital angular momentum of a beam can always be calculated from the transverse components of linear momentum. This transverse linear momentum arises both from the azimuthal phase gradient (orbital AM) and a combination of the beam’s intensity gradient and polarization (spin AM). For circularly polarized annular beams, as Siegman correctly describes, the azimuthal linear momentum from the spin contribution has equal and opposite senses on the inner and outer edges of the ring. As a consequence the particle spins, as shown in figure 4 of our article. A detailed consideration can be found in reference ² .