  We study the directional excitation of optical surface waves controlled by
the magnetic field of light. We theoretically predict that a spinning magnetic
dipole develops a tunable unidirectional coupling of light to TE-polarized
Bloch surface waves (BSWs). Experimentally, we show that the helicity of light
projected onto a subwavelength groove milled in the top layer of a 1D photonic
crystal (PC) controls the power distribution between two TE-polarized BSWs
excited on both sides of the groove. Such a phenomenon is shown to be mediated
solely by the helicity of the magnetic field of light, thus revealing a
magnetic spin-orbit interaction. Remarkably, this magnetic optical effect is
clearly observed with a near-field coupler governed by an electric dipole
moment: it is of the same order of magnitude as the electric optical effects
involved in the coupling. The magnetic spin-orbit interaction opens new degrees
of freedom in the manipulation of light and offers appealing novel
opportunities in the development of integrated optical functionalities.
