The conventional optical coherence tomography (OCT) images based on enhanced scattering and the photothermal (PT) images based on enhanced absorption of the localized surface plasmon (LSP) resonance of Au nanorings (NRIs) in a bio-tissue sample are demonstrated with the scans of an OCT system (1310-nm system), in which the spectral range covers the LSP resonance peak wavelength, and another OCT system (1060-nm system), in which the spectral range is away from the LSP resonance peak wavelength. A PT image is formed by evaluating the modulation frequency (400 Hz) response of an excitation laser with its wavelength (1308 nm) close to the LSP resonance peak at 1305 nm of the Au NRI solution. With the scan of the 1310-nm OCT system, the Au NRI distribution in the bio-tissue sample can be observed in both conventional OCT and PT images. However, with the scan of the 1060-nm OCT system, the Au NRI distribution can be clearly observed only in the PT image. The diffusion process of Au NRIs in the bio-tissue sample can be traced with the scan of either OCT system. Based on phantom experiments, it is shown that the PT image can help in resolving the ambiguity of a conventional OCT image between the enhanced scattering of Au NRIs and the strong scattering of a tissue structure in the 1310-nm OCT scanning. Also, under the condition of weak intrinsic sample scattering, particularly in the scan of the 1060-nm system, the PT signal can be lower than a saturating level, which is determined by the excitation power. By increasing OCT system signal-to-noise ratio or M-mode scan time, the PT signal level can be enhanced.

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.