Scanning electron microscope image of a TriboleverTM. The four faces of the open pyramid in the middle of the sensor act as highly reflective mirrors. Together with the polished end faces of four optical fibers, they constitute four separate optical interferometers. A sharp, electrochemically etched tungsten tip of 100 µm is inserted through the pyramid and glued in place, the sharp end of the tip thus protruding from the rear side of the pyramid. [2,3]. We use four optical fibers, etched and polished at the end, each pointing towards one of the four pyramid faces, to set up four simultaneously operating, optical interferometers. The laser beam coming through each of these optical fibers generates two reflections: one from the pyramid face to which it is pointing and the other from the polished end face of the fiber itself. These two reflected beams interfere with each other and the resulting intensity is measured by a photodiode, which is connected to a home-built amplifier. Each fiber is glued in a nanomoter that is used to bring the end face to within a few wavelengths (780 nm) from the pyramid face at such a distance that the derivative of the interference signal with respect to changes in the distance is at its maximum; this procedure maximizes the sensitivity. The optical path difference between the two reflections changes when the pyramid is displaced, either in the normal direction or in one of the lateral directions. In this way, each interferometer measures a projection of the three- dimensional displacement vector of the central pyramid. The combination of the four interferometer signals is used to accurately determine the components of this displacement along the X-, Y- and Z-directions. Standard RHK electronics is used to drive the scanning motion and control the data acquisition. It is also used for part of the data analysis. Home-built drift-compensation electronics allows as to counteract slow thermal drifts in the instrument with slowly varying offset voltages on the scan piezos. This allows us to perform scans at extremely low velocities, as low as 0.01 nm/s, without significant contributions to the tip speed due to thermal drift. A detailed description of the entire instrument is given in Ref. [1].
