Observation of bound photonic drumhead surface state in the radiation continuum

Nodal chain (NC) semi-metal forms a degeneracy ring in momentum space. With the projection of NC in the boundary, there is a special surface dispersion appearing at the crystal surface and termed drumhead surface state (DSS). Previously, experimental investigation on photonic NC and DSS has been made with metallic photonic crystals at microwave frequencies. However, the far-field detection of DSS and its coupling with the radiative mode have not addressed. Here, we studied the DSS through far-field angle-resolved reflection measurement and simulation, and found the bound property in the radiation continuum of the DSS center at Γ point in surface Brilliouin zone. Based on the temporal coupled-mode theory, we numerically obtain the radiative quality factor (Q_r) of the DSS from reflection spectrum. Q_r in Γ point tends to infinity for the DSS, leading to a bound state in the continuum (BIC). Our work establishes a connection between BIC and photonic topological surface state.