Abstract

A distinct advantage of far-field light microscopy is the non-destructive imaging of the interior of biological specimens. An obvious disadvantage is the limited spatial resolution, which for more than a century has been paradigmatic. The concept of stimulated emission depletion (STED-) microscopy¹ and the related concept of Ground-State-Depletion (GSD) Microscopy² allow for a fundamental breaking of the diffraction barrier. The physics of breaking the diffraction barrier is not just the ‘depletion’ of the excited or the ground state, rather it is the saturation^1,2 of the depletion. Saturation disentangles the final size of the fluorescent spot from the diffraction limited spot size of the focal light beams involved. The passband of the optical transfer function of the system is automatically enlarged. Saturation signifies a nonlinear intensity relationship, but importantly, in both cases the non-linearity is based on processes that are per se linear: 1-photon absorption and stimulated emission.

© 2003 Optical Society of America