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In classical microscopy, the diffraction of light limits the resolving power of the system, which restricts the detailed observation of nanoscale elements. From the cut-off frequency of the optical transfer function, the lateral resolution of an optical microscope can thus be quantified as 0.5 Lambda/NA, where Lambda and NA are the wavelength of the light source and the numerical aperture of the microscope objective, respectively. A white-light microscope thus allows the visualisation of objects having a minimum size that is just greater than half of the wavelength of the illumination in air. Recently, several far-field methods have been developed in order to overcome this limitation. Microsphere-assisted microscopy is one such recent technique which allows the diffraction limit to be broken. In 2011, Wang & al. developed two-dimensional super-resolution imaging through glass microspheres. They showed that microsphere-assisted microscopy distinguishes itself from others by being able to perform label-free and full-field acquisitions. In addition, with only slight modifications of classical white-light microscopy, microsphere-assisted microscopy makes it possible to reach a lateral resolution of a few hundred nanometres (~λ/7 in immersion). Placing a microsphere on a sample, directly or at a few hundred nanometres distance allows the generation of a super-resolved virtual image of the object, which is then collected by a microscope objective. Currently several studies have been aimed at providing a better understanding of the super-resolution phenomenon in microsphere imaging. Now we know that the performance in microsphere-assisted microscopy depends on the optical and geometrical parameters. According to many studies on the PJ prediction, the imaging process in microsphere-assisted microscopy can be addressed by considering the sphere as a photonic jet lens. However, recently, we demonstrated that the photonic jet (PJ) generation by a microsphere, and considered here as the point spread function, is not small enough to justify this resolution improvement in the imaging process. Although, the size of the focus spot overcomes the diffraction limit, the full width at half maximum of the PJ waist is around a third of the wavelength, which is lower than the super-resolving power which is around λ/7 in immersion. However, the PJ phenomenon can explain the imaging through the microsphere, the nature of the image, the position of the image plane and the lateral magnification dependence provided by the microsphere. Moreover, we have investigated the contrast in the virtual image according to the relative phase difference between close point sources. When the point sources are initially in-phase, the two virtual images cannot be distinguished. However, when the point sources are initially out-of-phase, the two virtual images can be clearly distinguished. Our work considers a third hypothesis by contributing to the investigation of the role of whispering gallery modes (a radially evanescent wave) in the microsphere assisted microscopy mechanism through numerical simulations using a finite element method.
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