Optical Near-Field Microscopy

Getting really close

Abbe’s famous formula regarding the achievable spatial resolution of optical microscopy is based on the fact that the field components in the vicinity of sub-wavelength objects decay exponentially with distance to the object. As a rule of thumb, the decay length is roughly comparable to the extent of the object. Thus, to detect the field components of a 30-nm diameter object, one need to get really close to the object, i.e., one needs to get into the optical near-field. Tiny apertures in a metal film (on a fiber tip or on the edge of a glass corner) brought into proximity to the object under investigation can serve as a converter of optical near-fields to propagating optical waves. Scanning such a tip over a surface enables acquiring two-dimensional images with sub-wavelength spatial resolution. Our group has applied this technology to a variety of systems including metamaterials and biological systems such as, e.g., pores in cell membranes.

Noise is the signal

The tiny volume of large light intensity formed by the evanescent field of such an aperture can also be used to follow dynamics with high temporal resolution. The basis is a technique called fluorescence correlation spectroscopy (FCS). In FCS, the laser-induced fluorescence of dye molecules is recorded by a sensitive and fast detector. In many situations, the signal levels are low and the signal looks much like noise at first sight. However, upon calculating the autocorrelation function of such noisy signals, the dynamics of the molecules can be revealed over many orders of magnitude in time. For example, diffusion or transport of molecules can be investigated in spite of the fact that only a single molecule may be present in the excitation volume at any instant in time. The tiny excitation volume results in a large sensitivity with respect to fastdiffusion/transport.

A complete list of publications can be found here.