Magnetically induced linear,nonreciprocal, and tunable transparency

Abstract

Electromagnetically induced transparency (EIT) is defined as coherent optical nonlinearity that makes a medium transparent in a narrow spectral range around the absorption line, which was first observed in a three-level atomic system and had been caused by destructive interference between two pathways excited by two laser beams [1]. Due to the experimental possibilities, such as the extremely low temperature and the high intensity of lasers, the research and potential application of electromagnetically induced transparency in atomic systems are seriously limited. Later, this concept was extended to the classical optical systems, using gas-phase atomic [2,3], metamaterial/metasurface [4-7], plasmonic [8-9], optical [10-17], optomechanical [18-20] and superconducting [21,22] systems, which allow experimental implementation with incoherent light and operation at room temperature [23,24]. Among them there are all-optical analogues of EIT realized in optical resonant systems, such as metamaterial, plasmonic, photonic crystal and whispering-gallery-mode resonators [17]. Thus, in recent years the meta-atom (meta-system) based EIT analogs using coupled bright and dark resonators have been developed to mimic quantum destructive interference between excitation states in three-level atomic systems, resulting in a sharp transmission window within a broad absorption spectrum [8,9]. Such structures have ignited intensive research interests, as EIT produces enhanced transmission with an extremely strong dispersion, which shows great potentials for applications in slow light devices [25,26], optical buffers [27,28], ultrasensitive biosensing [29], and enhanced nonlinear effects [30]. In this letter we report, for the first time to our knowledge, about a completely new mechanism that opens a transparency window in an opaque medium using an external magnetic field, i.e. magnetically induced transparency (MIT). We consider a photonic crystal layer based on a metamaterial with a helical periodical structure (cholesteric liquid crystal (CLC)-like structure) in an external magnetic field, which exhibits magneto-optical activity. The dielectric permittivity and magnetic permeability tensors have the forms: