Wave field restoration using focal-depth extension techniques under dynamic hollow-cone illumination

Wave field restoration has been achieved based on focal-depth extension under dynamic hollow-cone illumination using an optical microscope. Two kinds of focal-depth extension techniques, such as moving focus averaging and an objective annular aperture, were compared and their effectiveness for transmission electron microscopy applications is discussed. Both techniques enabled resolution enhancement at the same level and effectively obtained aberration-free phase and amplitude images separately by canceling out the rotationally invariant component of system wave aberrations, such as spherical aberration. The influence of nonlinear interference on image contrast was confirmed as significant by using a figure 8 shaped spatial frequency filter.