Nanophotonic Force Nanoscopy for Ultrafast Dynamic Imaging of Nanostructures

Light and plasmonic nanoparticle interaction creates many physical effects, including the generation of non-equilibrium charge carriers, enhanced electromagnetic near-field, and photothermal effect. Particularly, the plasmonic photothermal effect, given by the optical absorption induced by surface plasmons, is well studied because of its broad applications. The plasmonic photothermal effect has been associated with fields as diverse as near-field optics, cell biology, drug delivery, cancer therapy, chemical reactions, nanofabrication, biomedical imaging, sensing, and so on. The photothermal effect of nanoparticles has been characterized by various tools and techniques. For example, photothermal dynamics of nanoparticles have been observed with optical microscopy using the pump and probe approaches but are diffraction limited. At the single particle level, electron microscopy can assess the nanoparticle’s morphology, surface potential, and phase transformations, and cathodoluminescence has been used for probing the thermal profiles of nanowires. However, these methods based on electron microscopy require a vacuum environment and a conductive substrate. The measured thermal profiles oftentimes do not represent the nanoparticles in an ambient condition because the thermal properties of nanoparticles are highly associated with the environment. Moreover, because of the slow scanning speed of the electron beam, the dynamical evolution of the thermal profile due to non-stationary heat transfer is unmeasurable. On the other hand, nanoscopic thermal profiles have been measured using micromechanical probes, where the near-field energy extinction and the thermal relaxation govern the thermal field and heat flow. The thermal expansion triggered by a modulated laser resonantly drives a micromechanical cantilever, through which one can assess the thermal profile of nanoparticles in situ. However, the dynamic information can be lost due to the relatively slow response of the cantilever. Although improved mechanical designs of the scanner have enabled a high-speed atomic force microscope (Hs-AFM) that can reach up to 1300 frames per second, a single image -frame still takes hundreds of microseconds, significantly longer than the thermal relaxation time of nanoparticles, which is in the nanosecond regime<i>.</i><br/><br/>In this talk, I will introduce an ultrafast visualization of dynamic heat transfer in the nanosecond temporal regime with a spatial resolution of around 10 nm. We show the heating and cooling stages of a single gold nanoparticle using nearfield optical force imaging, termed decoupled optical force nanoscopy (Dofn). Our technique, different from conventional near-field scanning optical microscopy, probes optical forces originating from the interactions between the illuminated nanoparticle and the probe.