Fano-resonant excitations of generalized optical spin waves

While chiral materials possess spin-form wave functions as their eigenmodes, optical spin excitations cannot be obtained solely in chiral materials due to a lack of spin impedances. To date, spin excitation has generally been induced through non-conservative methods, such as circular dichroism or magneto-optical effects. In this chapter, we describe a conservative approach to optical spin excitation and control based on the spin-dependent appearance of Fano resonance. Starting from the development of the spin-form temporal coupled mode theory for 2D and 3D chiral resonances, the origin of the spin-Fano interactions is demonstrated in terms of the link between the spin eigenmodes in the polarization domain and anti-symmetric Fano resonances in the spectral domain. By comparing this spin-dependent Fano-resonant system with other optical spin materials, such as chiral, circular dichroic, and birefringent media, we discuss the impact of our results toward the realization of optical spintronics, such as applications of highly selective spin switching and unpolarized spinning operations.