A far-field optical microscope (OM) is a powerful noninvasive, nondestructive tool to study sub-micrometer structures and organisms, which has been used for decades to study the interactions between light and matter in the spatial domain. We report here a sophisticated label-free OM method with superspatial resolution to visualize ZnO nanoparticles. Of three femtosecond pulses, two served as pumps at 1000 nm and the other one served as a probe at 774 nm. The two pumps (one of Gaussian shape and the other of toroidal shape) were generated with a phase difference of 180°. When the conventional pump–probe approach was used in the absence of a second toroidal pump, a ZnO nanoparticle was observed to show a particle size of 445 nm because of the limit of diffraction. In contrast, when the second toroidal pump was applied out of phase, the obtained OM image showed a ZnO nanoparticle down to 96 nm. We demonstrated for the first time that the reported phase-modulated pump–probe approach has an ability for spatial resolution beyond its optical diffraction limit and a potential for label-free imaging applications in nanomaterials and life sciences.

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基于具有亚衍射分辨率的调相飞秒泵浦-探针方法的无标记光学显微镜

远场光学显微镜（OM）是研究亚微米结构和生物的强大无创，无损工具，数十年来一直用于研究空间域中光与物质之间的相互作用。我们在这里报告了一种具有超空间分辨率的先进的无标签OM方法，以可视化ZnO纳米颗粒。在三个飞秒脉冲中，两个在1000 nm处用作泵浦，另一个在774 nm处用作探针。产生两个泵（一个为高斯形状，另一个为环形），相位差为180°。当在没有第二台环形泵的情况下使用传统的泵浦探针方法时，由于衍射极限，观察到ZnO纳米颗粒的粒径为445 nm。相反，当第二台环形泵异相施加时，得到的OM图像显示出低至96nm的ZnO纳米颗粒。我们首次证明了所报道的调相泵浦探针方法具有超出其光学衍射极限的空间分辨率能力，并且具有在纳米材料和生命科学中无标签成像应用的潜力。