Ultra-narrowband terahertz circular dichroism driven by planar metasurface supporting chiral quasi bound states in continuum

TL;DR

This paper demonstrates ultra-narrowband terahertz circular dichroism using a planar silicon metasurface supporting high-Q quasi-bound states in continuum, enabling precise wave control at very narrow linewidths.

Contribution

It introduces a novel planar silicon metasurface design supporting symmetry-protected and parameter-tuned quasi-BICs for ultra-narrowband THz chirality.

Findings

Achieved linewidth below 0.06 GHz with a quality factor of 10^4.

Demonstrated controllable THz circular dichroism at ultra-narrow bandwidths.

Validated the effectiveness of quasi-BIC in enhancing THz wave manipulation.

Abstract

Terahertz (THz) chirality pursues customizable manipulation from narrowband to broadband. While conventional THz chirality is restricted by non-negligible linewidth and unable to handle narrowband well. Recently, the concept "quasi bound states in continuum" (quasi-BIC) is introduced to optics resonance system whose the quality factor can be extremely high with the ultra-low radiative loss, thus providing a conceptual feasibility for wave control with ultra-narrow linewidth. Herein, we construct quasi-BIC in a planar all-silicon THz metasurface with in-plane C2 and mirror symmetries breaking. Such system not only exposes the symmetry-protected BIC, but also exposes the parameter-tuned BIC assigned to single resonance type. An extremely narrow linewidth (below 0.06 GHz) with high quality factor (104 level) is obtained at quasi-BIC frequency, which achieves the ultra-narrowband THz chirality.