The mechanism of supperresolution found in dielectric microlenses has been a puzzle for a long time. Many proposals or models have been put forward but the underlying mechanism still remains unclear. Here we experimentally demonstrate that, when lifting a microlens above a sample, the defocused images taken by the microlens behave like those of conventional lenses, thus excluding evanescent waves as a likely mechanism for superresolution. We further employ a heating method to scan across optical resonant conditions of a microsphere while simultaneously monitoring the image resolution. The null result also excludes photonic nanojets to be the mechanism for superresolution. We also find the microlenses exhibit an interesting diameter-dependent resolving power, indicating effects beyond ray optics. Finally, we discover that the superresolution is mainly achieved by coherent imaging, assisted by surface plasmons of the underneath samples.
In the field of optical microscopy, it is well known that spatial resolution is limited by diffraction of light. There have been variously efforts to achieve resolution beyond diffraction limit. However, most previous methods rely on nonlinearities of fluorescence, and thus require high-intensity lasers or special labeling strategies. In 2011, a novel superresolution technique based on a dielectric microlens and simple bright field microscope was demonstrated. The scheme is unexpectedly simple, since neither labeling nor intense laser is necessary, while the resolution is significantly higher than diffraction limit. Nevertheless, the contrast of bright field microscope is poor. In this work, we combine a dielectric microlens along with confocal laser scanning microscopy to considerably enhance image contrast. By simply inserting a microsphere onto the sample, the resolution is undoubtedly better than diffraction limit of the objective. Meanwhile, the contrast exhibits almost one order of enhancement. Field of view and magnification of the microlens imaging system are also characterized. Comparing with bright field microscope, laser scanning confocal microscopy provides better contrast under this microslens assisted super-resolution scheme. Our finding will contribute to material science and biomedicine research.
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