  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.
