Fig. 1: BIC-engineered Au–TiO₂ metasurface for high-density singlet-oxygen generation.

a Conceptual illustration of photoexcitation in aqueous media: a q-BIC metasurface concentrates green light to convert triplet oxygen (³O₂) into ¹O₂, enabling localized cytotoxicity. b Device architecture and unit cell. A square array of TiO₂ elliptical nanopillars (height h) on SiO₂ is capped with an ultrathin Au layer. An in-plane scaling factor S multiplies all lateral dimensions---major/minor diameters S × 2A, S × 2B and lattice periods S × P_x, S × P_y---while h remains fixed. Structural asymmetry is controlled by the relative axis rotation θ between neighboring ellipses (asymmetry parameter \(\alpha =\sin \theta\)). The factor S tunes the resonance wavelength without altering α. c Scaling of the bare TiO₂ q-BIC quality factor with asymmetry: Q = kα^m (fit m ≈ −2.109), validating the characteristic Q ∝ α⁻² behavior near the symmetry-protected BIC. d Angle-resolved reflectance map of the bare TiO₂ metasurface, showing a narrowband q-BIC dispersion that peaks at the Γ point, with its narrowest linewidth at 538 nm (green). e Radiative and absorptive quality factors as a function of the structural asymmetry \(\alpha =\sin (\theta )\). The radiative quality factor Q_rad (blue) is obtained by fitting the simulated Q-α dependence with Q_rad = kα^m, whereas the absorptive quality factor Q_abs (red dashed line) is calculated from the integrated multilayer system. The intersection Q_rad = Q_abs indicates the critical-coupling condition, yielding θ ≈ 0.26 rad. f Simulated absorptivity of the Au–TiO₂ metasurface at λ = 532 nm as a function of the relative rotation angle θ. The absorptivity reaches its maximum at θ ≈ 0.26 rad (red dashed line), consistent with the predicted loss-matching (critical-coupling) condition in e