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Metal-dielectric superlens with ultra-flat phase of the modulation transfer function

We optimize the silver-dielectric layered flat lens in terms of three criteria:the variance of modulation transfer function (MTF) phase, the absolute values of the MTF phase, and the variance of this part of the phase spectrum which corresponds to the propagating waves only. All three characterise limited dependence of MTF on the range of spatial frequencies. The structure supports the resonant tunnelling of propagating and evanescent waves at distances of several wavelengths with a high transmission efficiency. Simulations are performed using the transfer matrix method and verified with the FDTD one. The resolution expressed with FWHM of the point spread function normalized with respect to wavelength reaches the value as small as 0.12, for the amplitude transmission of 40% and the thickness of the multilayer equals to 2.4 micrometers. For structures considered, we identify and explain a phenomenon that resembles focusing at a finite distance from the back plane.