|
In optical microscopy, the diffraction of the light as well as the coherence limits the resolving power of the system. Observation of nanoscale elements through an optical microscope appears often to be restricted. Assuming an incoherent light source and a circular pupil, the lateral resolution of an optical microscope can thus be quantified by the cut-off frequency of the optical transfer function, given by 0.5 λ/NA, where λ 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, in ideal cases, such as features of MOEMS-based components and bacteria. In reality, imperfections or misalignment of optical components makes the resolution limit worse. Recently, several far-field methods have been developed in order to overcome this limitation, such as stimulated-emission-depletion microscopy and the negative-refractive-index superlens. In 2004, a new potential far-field microscopy technique based on the scanning photonic jet beam was proposed, leading to a lateral resolution of ~λ/3. In 2011, Wang et al. experimentally introduced the phenomenon of two-dimensional super-resolution imaging through a glass microsphere. 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) which is adequate, for example, for the visualization of adenoviruses using the fluorescence effect. Placing a microsphere on (or above) a sample allows the generation of a super-resolved virtual image of the object which is then collected by a microscope objective. Although the super-resolution phenomenon is still not well understood, we now know that the performance of microsphere-aided microscopy depends on the optical and geometrical parameters. Recently, we successfully demonstrated the label-free combination of microsphere-assisted microscopy with dark-field illumination in order to image translucent samples. Random glass nanofeatures, as well as brain cell morphology, have been observed. Future work is now in progress towards the extension of 3D object inspection using interferometry.
|
