Mid-infrared quasi-BIC resonances with sub-wavelength slot mode profiles in germanium-based coupled guided-mode resonance structures

We experimentally demonstrate a novel quasi-bound state in the continuum(BIC) resonance in the mid-infrared wavelength region with the resonant electric field confined as a slot mode within a low-refractive-index medium sandwiched between high-index layers. The structures studied here comprise coupled amorphous germanium guided-mode resonance(GMR) structures with a top one-dimensional grating layer and bottom uniform layer separated by a low-index silicon nitride layer. The slot-mode profile within the silicon nitride layer with mode field confinement >30% is achieved as a solution to the electromagnetic wave propagation through the coupled GMR structure with the dominant field component being perpendicular to the layers. The quasi-BIC resonance in symmetric 1D grating structures can be observed even at normal incidence when considering a realistic excitation beam with finite angular spread. The measured transmission peak is found to redshift(remain almost unchanged)under classical(full-conical) mounting conditions. The highest quality factor of ～400 is experimentally extracted at normal incidence under a classical mounting condition with a resonance peak at 3.41 μm wavelength. Such slot-mode GMR structures with appropriately chosen low-index intermediate layers can find applications in resonantly enhanced sensing and active photonic devices.