Super-resolution imaging of low-contrast periodic nanoparticle arrays by microsphere-assisted microscopy* *Project supported by the National Natural Science Foundation of China (Grant No. 61673287).

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We use the label-free microsphere-assisted microscopy to image low-contrast hexagonally close-packed polystyrene nanoparticle arrays with diameters of 300 and 250 nm. When a nanoparticle array is directly placed on a glass slide, it cannot be distinguished. If a 30-nm-thick Ag film is deposited on the surface of a nanoparticle array, the nanoparticle array with nanoparticle diameters of 300 and 250 nm can be distinguished. In addition, the Talbot effect of the 300-nm-diameter nanoparticle array is also observed. If a nanoparticle sample is assembled on a glass slide deposited with a 30-nm-thick Ag film, an array of 300-nm-diameter nanoparticles can be discerned. We propose that in microsphere-assisted microscopy imaging, the resolution can be improved by the excitation of surface plasmon polaritons (SPPs) on the sample surface or at the sample/substrate interface, and a higher near-field intensity due to the excited SPPs would benefit the resolution improvement. Our study of label-free super-resolution imaging of low-contrast objects will promote the applications of microsphere-assisted microscopy in life sciences.