Philip H. Jones, Onofrio M. Maragò & Giovanni Volpe
Fig. 3.10 — Spin-curl force
The spin–curl force (black arrows) emerges in the presence of a gradient of polarisation and is non-conservative, as can be seen by calculating the work done by this force along the dashed line. The polarisation state of the light is indicated by the ellipses on the background: the x– and y-components of the electric field oscillate in phase quadrature and their amplitudes (indicated by the length of the axes of the ellipses) are such that the intensity of the field is constant (so that there is no gradient force).