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Based on the scanning of photothermal optical coherence tomography (PT-OCT), the photothermal behavior of a flowing medium generated by the enhanced absorption of localized surface plasmon resonance of incorporated Au nanorings (NRIs) is observed. In particular, the effects of air bubble generation and thermally induced bubble size oscillation in a flowing medium through the incorporation of Au NRIs and modulated laser illumination are demonstrated. The size oscillation of the air bubble produces the vibration of the flowing medium, which is synchronized with the laser modulation, for generating PT-OCT signal. At the resonance frequency of flowing-medium vibration, the PT-OCT signal reaches the maximum level. The resonance frequency is related to the mass density and viscosity of the flowing medium and is independent of the flow speed of the medium in a vessel. Such a relation can be used for in situ monitoring the mass density and viscosity of a flowing medium.

The reduction of average extinction cross section of a localized surface plasmon (LSP) resonance mode under the random orientation condition of Au nanoring (NRI) distribution is first numerically demonstrated. The reduction range depends on the geometry symmetry property of the electron oscillation axis in the LSP resonance mode. Then, by increasing the ring height, an optimized Au NRI geometry is designed to make the resonance wavelengths of its cross-ring dipole mode and axial dipole mode the same. In such an Au NRI, a few higher-order axial LSP modes are discovered. Also, under the condition of random orientation distribution, the ranges of extinction, scattering, and absorption cross section reductions from the corresponding maximum levels of optimized excitations are significantly decreased, when compared with the counterparts of the Au NRIs of a smaller ring height.