Experimental analysis of sub-micrometer opticalintensity distributions after an opaque disk

Generation of subwavelength beam sizes is a fascinating challenge with several implications. The observation of a 120 nm laser spot in the visible part of the spectrum is reported here. It has a size variation of less than 10% in a distance of $ 50\;\unicode{x00B5}{\rm m} $50µm along the axis of propagation. This so-called Arago spot results from the diffraction of the light from a laser diode by the edges of an absorbing disk. Applications are discussed and hollow beams carrying orbital angular momentum with a 400 nm diameter dark spot in the center are evidenced. This paves the way toward atom lithography via atom guiding or new spectroscopy on forbidden transitions.