Brillouin zone folding THz metamaterial for nonreciprocal transmission

Nonreciprocal transmission, a critical functionality for terahertz (THz) isolators, remains challenging to achieve in the compact and bias-free systems. This work proposes a THz metamaterial leveraging Brillouin zone folding (BZF)-induced bound states in the continuum (BZF-BICs) to realize robust nonreciprocal transmission. At an incident intensity of 60 MW/cm 2 , the transmission spectra for opposite propagation directions exhibit stark asymmetry at 0.854 THz, yielding 44.87% and 0.18% transmission, where the isolation is 23.97 dB. In contrast, quasi-bound states in the continuum (Q-BICs) under identical conditions show negligible nonreciprocal transmission, where the transmission approaches zero in both directions at 0.856 THz. The enhanced performance of BZF-BICs arises from their high-quality (high-Q) resonance, enabled by reduced radiation losses through Brillouin zone folding. Further analysis reveals that the nonreciprocal intensity range (NRIR) depends on out-of-plane structural asymmetry, with optimized geometries achieving NRIR values up to 2.06. This work establishes BZF-BICs as a promising platform for integrated THz isolators.