MODES IN EPSILON NEAR-ZERO STRUCTURES – FROM BULK LEAKY PLASMONS TO EMBEDDED EIGENSTATES

Epsilon near zero (ENZ) materials have been capturing the attention of different research communities in the last decade. Strong light-matter interaction enabled by ENZ regime has been exploited for realization of Purcell-effect enhancement, perfect absorbers, enhanced nonlinear effects and active opto-electronic devices. ENZ regime was also shown to enable peculiar phenomena due to inherent “wavelength stretching” – tailoring the radiation pattern and directivity, as well as the effect of supercoupling [1]. However, both natural and artificial ENZ materials suffer from high losses which has been a major obstacle for their application. While the intrinsic material loss is unavoidable, radiation loss of the modes supported by structures with ENZ can be minimized through the concept of embedded eigenstates (EE) or bound states in the continuum (BIC). Namely, it has recently been shown that ENZ materials are instrumental for engineering EEs, which offer theoretically unlimited Q-factors and field enhancements [2].
Here we explore structures with ENZ materials supporting EEs and quasi-EEs with the goal of minimizing radiation loss and thereby improving the Q-factors and field enhancements of the supported leaky bulk plasmon modes. Furthermore, we propose a multilayer non-Hermitian PT-symmetric structure, shown at the inset of figure 1(a), in order to compensate for both material and radiation loss. In this way we obtain quasi-EEs and demonstrate a Q-factor of ~108, along with extreme field enhancement of ~104, figure 1(a) and 1(b). Lastly, we demonstrate that these values can be made arbitrarily high with increasing the number of layers. We believe that these results are promising for light trapping, non-linear optical effects and sensing.