Nanophotonics for biomedical superresolved imaging

The resolution of every imaging system is limited by the diffraction spot size that equals γ/(2 NA), where γ is the optical wavelength and NA is the numerical aperture of the lens. This limit is obtained due to the wave nature of light, and it is considered as an unsurpassable bound for most practical cases. Nevertheless, the resolution limit is obtained under certain assumptions, such as source monochromaticity, unpolarized light, short time imaging, and so on. In fact, for many practical situations, additional a priori information can assist in surpassing the classical resolution limit. This assertion is supported by the classical works of Toraldo di Francia and Luckosz [1-3], describing a system by the numbers of degrees of freedom it can transmit, instead of the space-bandwidth product. Later on, the signal-to-noise ratio (SNP) of the image was also included in the computation [4]. Thus, owing to the invariance of the information throughput of a system, the spatial resolution of a system can be enhanced, at the expense of other degrees of freedom [5]. As examples, the resolution in one axis can be enhanced by sacrificing the resolution on the orthogonal direction [2,6], or the two orthogonal polarizations can be used for doubling the resolution in the case of a polarization-independent object [7]. Methods based on extrapolation of the spectrum (e.g., [8]) try to squeeze the SNR degree of freedom [5].