We report on the direct observation of highly confined transition molecular switch on azobenzene based monolayer. The transition was induced by UV light through the optical field enhancement occurring in the proximity of a sharp STM probe. A 0.5 nm reduction of the thickness of the azobenzene monolayer is detected in real time on a minute nearly circular region of about 50 nm of diameter. We demonstrated that the thickness change is compatible with rigorous molecular modelling of the polymer used. The area of collective molecular switching, is actually a small bit of information, functional to high-density optical storage investigations. (C) 2004 Elsevier B.V. All rights reserved.