Effect of silver film roughness on imaging property of BaTiO3 microsphere

Abstract

Due to the Abbe diffraction limit, the resolution of a traditional optical microscopy is limited to about half of the illumination wavelength. Recent studies show that super-resolution imaging through dielectric microsphere has emerged as a simple imaging technique to overcome the diffraction limit under the illumination of white light. However, for imaging through microsphere, sometimes it is needed to enhance the reflection of a sample by depositing a metallic thin film on the top of the sample. Metallic thin films with different surface roughness have different optical properties. However, the effect caused by the surface roughness of a metallic film on microsphere imaging is rarely studied. In this paper, we study the effects of silver films with different surface roughness deposited on the surfaces of samples on the imaging properties of BaTiO3 (BTG) microspheres. Silver thin films are deposited respectively at evaporation rates of 1.5-3 Å/s and 5-10 Å/s, and the surface roughness values (root mean square (RMS) values) of the obtained silver films are about 3.23 nm and 6.80 nm, respectively. Using a BTG microsphere to observe a sample with a silver film deposited on its surface, we find that the surface roughness of the silver film will affect the imaging resolution and the range of focal image position (RFIP) of the BTG microsphere. When we use a 15-μm-diameter BTG microsphere to observe a 250-nm-diameter microsphere array and 580-nm-diameter microsphere array, the RFIP of the BTG microsphere increases with the RMS of the silver film increasing from 3.23 to 6.80 nm. Moreover, a 200-nm-diameter microsphere array can also be clearly discerned. The simulation results obtained by the commercial software COMSOL show that when the surface of a microsphere array sample is deposited with a rough silver film, the electric field intensity is enhanced not only in the gaps between adjacent microspheres, but also on the silver particles due to the excitation of localized surface plasmons. We propose that the scattering effect and the local electric field intensity enhancement caused by the rough silver film allow more high-frequency information of the sample to be coupled into the BTG microsphere, and thus improving the resolution and RFIP of the microsphere. As the imaging law of microsphere imaging still needs to be explored, our research work will be helpful in further revealing the mechanism in microsphere imaging.