Relation of the angular momentum of surface modes to the position of their power-flow center.

We show that the value of the total angular momentum (AM) carried by a surface mode can be interpreted as representing the transverse position of the center or balance point of the power flow through the mode. Especially in the lossless cases, the value of the Abraham AM per unit power (multiplied by the square of the speed of light in vacuum) is exactly the same as the transverse position of this power-flow center. However, the Minkowski counterpart becomes proportional to that position with a coefficient in the form of 1 + η, where η is determined mainly by the constitutive parameters of media.