Microlens Super-resolution Imaging Based On Photon Nanojet Effect

The objects with features of around half of illumination wavelength cannot be resolved by a conventional optical microscope due to the classical diffraction limit.Recent studies demonstrate that microslens assisted by a conventional microscope have a capability to resolve features beyond the diffraction limit. This method receives extensive attentions from the scientific community. In this context, we focus on the imaging properties and its mechanism of the microlens based on the properties of photonic nanojet generated by the microlens, the main research works are listed below:1. Two types of object samples containing point object and line object are used in the experiment. The imaging properties of high index immersed BTG microsphere lenses in the diameter range of 5-300 um are experimentally studied, especially the transition of the imaging properties to geometrical optics when the diameter of the microsphere lens increase. Our experimental results demonstrate that shifting the focal plane of the objective lens can result in both the super-resolved images and the interference ring images. Both the imaging properties are depended on the diameter of the microsphere. When the microsphere diameter increases, interference rings becomes more distinct, but the image contrast of the super-resolved image decrease until disappears. Theoretical simulation shows a super-strength focus generated at the shadow of the microsphere named "photonic nanojet", and it performs an important role in the image properties of the microsphere lens. In our test range, the diameter of the microsphere increases, the super-resolved capability of the microsphere decreases, this relates to the full width of half maximum (FWHM) and the position of the maximum.2. The far-field imaging properties of high index microsphere lens spatially separated from the object are experimentally studied, and a comparison has been made with the properties of the microsphere lens with low index. Our experimental results demonstrate that both the microsphere lens still have the super-resolved capability when they space from the surface of the object sample. By different immersed mode, both the microsphere lens can be formed by the photonic nanojet.which supports the super-resolved capability of the microsphere. Along the incident direction, photonic nanojet of the low index microsphere can perform a long persistence length, which results in a long image space of the super-resolved image of the low index microsphere lens. The high index microsphere lens performs larger magnification and field of view due to its high refractive index and larger diameter.3. A dielectric planar cavity coupled microsphere lens is fabricated and its effects on the imaging of sub-surface nanostructures have been studied. The dielectric planar cavity structure is composed of a low refractive index silica layer and a high refractive index SU-8 layer,our experimental results illustrate that the nanostructures of data-recorded Blu-ray disc can be clearly resolved. Optical images of the object with higher contrast can be obtained , which illustrates that the dielectric planner cavity structure in our experiments can improve the transmission of specific spatial frequencies into the near-field imaging system.The stationary waves generated by the incident light and the reflected light from the substrate material can result in resonantly enhanced effect. so the planer cavity structures can preserve the beam energy along z-axis with a very low loss factor, effectively supports the super-resolved capability of the microsphere lens.4. Based on the photonic nanojet theory and soft-lithography method. a removable elastomer nicrolens arrays films has been designed and fabricated. The microlens array as a thin film can be removed and attached to the tested samples with sub-wavelength sizes, and synchronously forms magnified virtual images, which can effectively improve the observed field of view. Theoretical analysis reveals that the microlens array can form photonic nanojet, which endows the microlens array with unusual abilities to illuminate object surfaces and excite more near-filed high frequency information. The properties of the photonic nanojet included by its position,intensity and FWHM are determined by the wavelength of the incident light, the diameter and arrangement of the microlens array and the refractive index of the surrounding medium.