Incident-angle-modulated molecular plasmonic switches: a case of weak exciton-plasmon coupling

We have designed an angularly tunable plasmonic system that consists of Au nanodisks in combination with molecules of photoswitchable resonance, spiropyran, to gain new insights into weak exciton-plasmon couplings. In the weak exciton-plasmon coupling regime, switching molecular resonance can induce localized surface plasmon resonance (LSPR) peak shifts due to the change in the refractive index of the molecular materials. On the basis of the angle-resolved spectroscopic study of the nanodisk-spiropyran system both with and without UV irradiation, we reveal an unusual "zigzag" curve for the LSPR peak shifts (due to the photoswitching of the molecular resonance) as a function of the original LSPR peak wavelength. A further theoretical analysis attributes the "zigzag" curve to two significant competing effects that depend on the incident angle of the probe light: plasmon-enhanced molecular resonance absorption and LSPR sensitivity to the surroundings' refractive index.