Resonant scattering enhanced photothermal microscopy
Photothermal (PT) microscopy is currently the most efficient approach for the detection and spectroscopy of individual non-fluorescent nano-objects solely based on their absorption. The nano-objects in the current PT microscopy are usually non-resonant with the probe laser light and the PT signal is mainly generated from the interaction of incident probe light and heating light induced thermal lens around the imaged object. Inspired by the high sensitivity of the scattering field from the nano-objects near optical resonance to the variation of local refractive index, here in this paper, we developed a novel strategy of resonant scattering enhanced PT microscopy where the imaged nano-objects are near resonant with the probe laser light, and we demonstrated it by using gold nanorods (NRs) with tunable longitudinal surface plasmon resonances. It was found that the PT signal of gold NR near resonant with the probe light shows dramatic variation in narrow resonance wavelength range as small as 15 nm and the maximal amplitude of PT signal in this range can be enhanced as high as 43 times compared with the weak PT signal of gold NR non-resonant with the probe light. Theoretical analysis indicates that the obtained strong PT signal is mainly caused by the heat induced variation of the polarizability of gold NR. Our novel work demonstrates the first resonant scattering enhanced PT imaging of plasmonic nanoparticles, paving the way for the development of PT microscopy with ultra-high sensitivity toward sensing, imaging, and spectroscopy of nanoscopic objects in complex environment.