Superradiantly Limited Linewidth in Complementary THz Metamaterials on Si-Membranes

Complementary double split ring THz resonators fabricated on a 10 mu m thin Si-membrane are studied. The linewidths of the fundamental LC mode and dipolar mode are drastically narrowing with increased resonator spacing. The extracted decay rate of the LC mode as a function of the resonator density shows a linear dependence, evidencing a collective superradiant effect of the resonator array. Furthermore, it is shown that a metamaterial can be designed for a low superradiant broadening of the resonance at high resonator densities, that is, in the metamaterial condition. The use of a thin membrane as a substrate is crucial, since it shifts the THz surface plasmon polaritons modes to much higher frequencies, preventing them to couple to the LC mode and unveiling the superradiant broadening mechanism for a large range of lattice spacings. At higher frequencies, not interfering with the high Q LC mode, other additional modes, which are ascribed to photonic crystal modes, form in the Si-membrane. The angle-dependent band structure is mapped and corresponding simulated electric field distributions are shown.