Talbot Focussing - a new effect of periodic structures and its utilization

The Talbot effect is a well know phenomenon, in which periodic structures reproduce themselfes at multiples of the Talbot distance. Also it was discovered by Lohmann, that for some fractional Talbot distances, self similarities with a conversion from phase to amplitude appear. This effect was called the fractional Talbot effect. Here we discuss a new phenomenon of periodic gratings which we call Talbot focussing. In this case, the observation is not a reproduction of the initial distribution but instead a very sharp focus in the center of the cell. The theory of this effect and applications to metrology are discussed.