Abrupt lateral beam shifts from terahertz quasi-bound states in the continuum

Bound states in the continuum (BICs) are notable in photonics for their infinite Q factors. Perturbed BICs, or quasi-BICs (QBICs), have finite but ultra-high Q factors, enabling external coupling. So far, most studies have focused on the momentum-space properties of BICs and QBICs, with few discussions on their properties in real space. Here, we experimentally demonstrate that QBICs can induce abrupt lateral beam shifts. By applying Brillouin zone folding to a compound grating waveguide, we form a QBIC band where all states become QBICs. When excited at specific incident angles, these QBICs produce sudden lateral beam shifts, rapidly disappearing as frequencies deviate from the QBIC band. Using terahertz imaging, we capture these beam shifts at different incident angles, characterizing the QBIC band. This work offers alternative insights into QBIC behaviors and supports the development of advanced sensors and wavelength division (de) multiplexers.