Photosensitizing Metasurface Empowered by Enhanced Magnetic Field of Toroidal Dipole Resonance.

Photochemical reaction exploiting an excited triplet state (T 1 ) of a molecule requires two steps for the excitation, i.e., electronic transition from the ground (S 0 ) to singlet excited (S 1 ) states and intersystem crossing to the T 1  state. A dielectric metasurface coupled with photosensitizer that enables energy efficient photochemical reaction via the enhanced S 0 →T 1 magnetic dipole transition is developed. In the direct S 0 →T 1 transition, the photon energy of several hundreds of meV is saved compared to the conventional S 0 → S 1 →T 1 transition. To maximize the magnetic field intensity on the surface, a silicon (Si) nanodisk array metasurface with toroidal dipole resonances is designed. The surface of the metasurface is functionalized with ruthenium (Ru(II)) complexes that work as a photosensitizer for singlet oxygen generation. In the coupled system, the rate of the direct S 0 →T 1 transition of Ru(II) complexes is 41-fold enhanced at the toroidal dipole resonance of a Si nanodisk array. The enhancement of a singlet oxygen generation rate is observed when the toroidal dipole resonance of a Si nanodisk array is matched with the direct S 0 →T 1 transition wavelength of Ru(II) complexes.