The detection of light created in the STM tunnel junction (see also: Luminescence from the STM tip) requires diverse modifications of the original STM set-up. The aim of our set-up is to obtain a robust and versatile instrument which allows regular topography scans and electron tunneling spectroscopy at liq. He temperature in UHV. Under these conditions it must, moreover, be able to collect a substantial part of the luminescence emitted from the tip-sample junction. As the tips in the STM are exchangable, the optical path must be readjustable in-situ.

The developed PSTM is shown on the image above on the left hand side inside its UHV chamber and cryostat. The doors of the cryostat are opened allowing a view inside. The set-up consists of 3 independent optical paths, each starting with a lens which is positioned a few millimeters away from the STM tip apex (right image above). The lenses are retractable and adjustable.

The light generated at the tip-sample junction propagates to the outside of the UHV chamber along free beam paths which conserve all important light properties, like angular distribution, polarization and temporal information. At the same time the heat input from the outside to the encapsulated STM-assembly is kept at a minimum as the light beams pass through the two copper shields of the He cryostat through very small holes. For all 3 paths the light direction can, of course, be reversed allowing optical excitation inside the tunnel junction.

<- The STM tip and the surface of the sample can be observed in-situ from outside the UHV chamber: The tip wire (diam. 0.3 mm) appears as the vertical rod with the tip apex slightly below the center of the picture. The mirror image of the tip reflected from the polished sample surface can be seen near the lower edge of the image. The tip will enter into tunneling contact exactly in the middle between the tip and its mirror image. The white ring surrounding approximately this point is the mirror image of the edge of the lens observing the tunnel junction from the opposite side.