Abstract
We propose a route to the spectral separation of optical spin angular momentum based on spin-dependent Fano resonances with antisymmetric spectral profiles. By developing a spin-form coupled mode theory for chiral materials, the origin of antisymmetric Fano spectra is clarified in terms of the opposite temporal phase shift for each spin, which is the result of counter-rotating spin eigenvectors. An analytical expression of a spin-density Fano parameter is derived to enable quantitative analysis of the Fano-induced spin separation in the spectral domain. As an application, we demonstrate optical spin switching utilizing the extreme spectral sensitivity of the spin-density reversal. Our result paves a path toward the conservative spectral separation of spins without any need of the magneto-optical effect or circular dichroism, achieving excellent purity in spin density superior to conventional approaches based on circular dichroism.
Highlights
By developing a temporal coupled mode theory (CMT)[32,33] for the Fano chiral resonator, we prove that the handedness of the spin eigenvector is projected onto the temporal domain as an opposite temporal shift, thereby leading to an antisymmetric Fano response in the spectral domain
We propose a new pathway for the nonmagnetic achievement of optical spin angular momentum based on the spin-dependent separation of Fano resonance spectra
By developing a spin-form temporal CMT for the chiral resonator, we unveil the origin of the spin-dependent antisymmetric Fano resonance in perfect agreement with the scattering matrix calculations
Summary
The temporal phase shift corresponding to Fano resonance[35], especially in the opposite direction for the ê+ and ê− spin modes, exposes only when both conditions of θ ≠ 0 (chiral medium) and τh ≠ τv (birefringent mirrors) are met at the same time. The spin-density spectra for different |qs| is plotted, showing a smaller bandwidth for larger |qs|, which is again associated with the decrease in the spectral separation between the ê+ and ê− modes (Fig. 4d). The effect of material loss for optical spin switching is presented in terms of (e) the spin density and (f) reflectance spectra: for different values of intrinsic quality factor of each resonant mode Qint = 2 × 102 to 103.
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