Quasi-bound states in the continuum with high Q-factors in metasurfaces of lower-index dielectrics supported by metallic substrates

For observing high-Q quasi-bound states in the continuum (BIC), the metasurfaces should be made of high-index materials, restricting their applications due to the limited material functionalities. In this work, we demonstrate that high-Q quasi-BIC can also be obtained by using lower-index dielectrics, provided that the metasurfaces are supported by metallic substrates. Strong field confinement assisted by plasmon excitation on the surface of metallic substrate enables realizing quasi-BIC even when using the lower-index materials such as zinc oxide. The numerical results show that high Q-factors in the order of several hundreds can be obtained with such metasurfaces of lower-index materials. They do not exhibit, however, quasi-BIC when supported by dielectric substrates due to the strong mode leakage originating from the low index contrast. Quite interestingly, metasurfaces made of high-index dielectrics supported by metallic substrates exhibit lower Q-factors compared with the metasurfaces of lower-index dielectrics due to the stronger penetration of mode field into the metallic substrate. The presented results can find important applications for photonic purposes, including efficient UV generation and low-threshold lasing from the lower-index dielectric metasurfaces.