Slow Light Effect by Photonic Angular Momentum States

We show that the photonic angular momentum states (AMSs) in metamaterials can be utilized to achieve slow light. The photonic AMS possesses a helical phase, the orbital-angular-momentum degree of freedom in photons. Unlike the traditional slow-light schemes in metamaterials by using orthogonal polarizations, the photonic AMSs are dipole forbidden for all polarizations of free-space radiation due to the existence of helical phase. Numerical simulation for a metamaterial design proves the feasibility of this scheme, in which the indirectly excitation of dark photonic AMSs burns some transmission dips in the otherwise high transmission window. With the unique merits of dark photonic AMSs, this slow light scheme can realize a high group index $n_g$ greater than $\sim500$.