Direct Excitation of Triplet State of Molecule by Enhanced Magnetic Field of Dielectric Metasurfaces

Abstract

Efficient excitation of a triplet (T1) state of a molecule has far‐reaching effects on photochemical reaction and energy conversion systems. Because the optical transition from a ground singlet (S0) to a T1 state is spin‐forbidden, a T1 state is generated via intersystem crossing (ISC) from an excited singlet (S1) state. Although the excitation efficiency of a T1 state can be increased by enhancing ISC utilizing a heavy atom effect, energy loss during S1→T1 relaxation is inevitable. Here, a general approach to directly excite a T1 state from a ground S0 state via magnetic dipole transition, which is boosted by enhanced magnetic field induced by a dielectric metasurface, is proposed. As a dielectric metasurface, a hexagonal array of silicon (Si) nanodisks is employed; the nanodisk array induces a strongly enhanced magnetic field on the surface due to the toroidal dipole (TD) resonance. A proof‐of‐concept experiment is performed using ruthenium (Ru) complexes placed on a metasurface and demonstrates that the phosphorescence is 35‐fold enhanced on a metasurface when the TD resonance is tuned to the wavelength of the direct S0→T1 transition. These results indicate that photon energy necessary to excite the T1 state can be reduced by more than 400 meV compared to the process involving the ISC. By combining optical measurements with numerical simulations, the mechanism of the phosphorescence enhancement is quantitatively discussed.

Photosensitizing Metasurface Empowered by Enhanced Magnetic Field of Toroidal Dipole Resonance

“…Fabrication of a Silicon Nanodisk Hexagonal Array: Details of the fabrication procedure are described in our previous papers. [31,40] A monolayer of a hexagonal array of polystyrene beads (PSBs) (Polysciences Inc.) with the diameter of 500 nm was formed on an amorphous Si film sputterdeposited on a SiO 2 substrate (SPF-210H, ANELVA). The size of PSBs were then reduced to 450 nm by isotropic etching with oxygen plasma (L-201D, ANELVA).…”

“…We demonstrated that enhanced magnetic field of the TD metasurface can strongly enhance spin‐flip direct S 0 →T 1 transition of Ru(II) complexes by photoluminescence excitation (PLE) spectroscopy. [ 31 ] This result indicates that the metasurface can be a platform to promote photochemical reactions utilizing triplet excited states of a molecule.…”

“…In our previous work, we developed a dielectric metasurface composed of a hexagonal array of thin Si nanodisks [31] which possesses the toroidal dipole (TD) modes. [32][33][34][35] TD is a current loop along a meridian of a torus in a dielectric resonator.…”

Magnetic and electric Purcell enhancement in a hybrid metal-dielectric nanostructure

“…The structure provides an option of having photon sources with selective excitations of magnetic and electric emission [38] . Such selective excitation can enhance and even monitor certain photochemical reactions, which only happens when magnetic dipole transitions are excited [39,40] . For example, singlet oxygen-mediated photoadditions can be improved through this method.…”

Generalized Transition Moment and Oscillator Strength for Optimal Control of Excited States Using Near-Field Light

“…At the same time, we aim to extend this work to a more general framework. Furthermore, the inclusion of magnetic interactions − or vibronic couplings is an important direction for further manipulation of the molecular excited states. Self-consistent interactions between molecules and light sources may change the excited states. , A stronger interaction between light and molecules to realize hybridized polaritonic states will also be an interesting new avenue of research.…”