Photosensitizing Metasurface Empowered by Enhanced Magnetic Field of Toroidal Dipole Resonance

Photochemical reaction exploiting an excited triplet state (T₁ ) of a molecule requires two steps for the excitation, i.e., electronic transition from the ground (S₀ ) to singlet excited (S₁ ) states and intersystem crossing to the T₁  state. A dielectric metasurface coupled with photosensitizer that enables energy efficient photochemical reaction via the enhanced S₀ →T₁ magnetic dipole transition is developed. In the direct S₀ →T₁ transition, the photon energy of several hundreds of meV is saved compared to the conventional S₀ → S₁ →T₁ 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 S₀ →T₁ 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 S₀ →T₁ transition wavelength of Ru(II) complexes.