Toward the development of a three-dimensional mid-field microscope

Recently, an extraordinary transmission of light through small holes (<200 nm) in a thin metallic film has been described. This phenomenon has been shown to be the result of the photon-plasmon interaction in thin films where a periodic structure (such as a set of holes) is embedded in the film. One of the extraordinary results is that the beam that passes through a hole has a very small diffraction in extreme contrast to the wide angle predicted by diffraction theory. Based on this effect, we propose here a new type of microscopy that we term mid-field microscopy. It combines an illumination of the sample through a metallic hole-array with far-field collection optics, a scanning mechanism and a CCD. When compared to other high resolution methods, what we suggest here is relatively simple because it is based on a thin metallic film with an array of nano-sized holes. Such a method can be widely used in high-resolution microscopy and provide a novel simple-to-use tool in many life-sciences laboratories. When compared to near-field scanning optical rnicroscopy (NSOM), the suggested mid-field method provides a significant improvement. This is chiefly for three reasons: 1. The penetration depth of the microscope increases from a few nanometers to a few micrometers, hence the name mid-field microscope. 2. It allows one to measure an image faster because the image is measured through many holes in parallel rather then through a single fiber tip used in conventional near-field microscopy, and 3. It enables one to perform three-dimensional reconstruction of images due to a semi-confocal effect. We describe the physical basics of the photon-plasmon interaction that allows the coupling of light to the surface plasmons and determines the main spectral characteristics of the device. This mechanism can be ascribed due to the super-periodicity of the electron oscillations on the metallic surface engendered by the grating-like structure of the hole-array.