The Optics Laboratory

NSOM EM Apparatus

The apparatus we use is an NSOM that has been modified so that an electrical connection is made to the metal that forms the aperture of the probe. A voltage source and current preamplifier allow us to use the NSOM probe as a mobile tunneling contact to electromigrate a spot that we have identified with shear-force topographic imaging. A schematic drawing shows the NSOM

The electronics for the electromigration (tunneling) are shown with a little more detail here:

A close up of the probe near the sample,

shows that one corner of the probe is usually the dominant location for both the force feedback and the tunneling. This may be slightly offset from the optical images. The cartoon here is much compressed in the vertical direction, as the tunnel spacing of ~0.5 nm ~10 times smaller than the aperture, which is in turn smaller than the metal cladding thickness (as shown).

The particle that is electromigrated in this system is the oxygen, so we need to be able to image its location. Fortunately, variations in oxygen content produce variations in the reflectivity:

From data in Anisotropy and oxygen-stoichiometry dependence of the dielectric tensor of YBa2Cu3O7-d (0²d²1) by J. Kircher, M.K. Kelly, S. Rashkeev, M. Alouani, D. Fuchs, and M. Cardona.

For far-field work, the curve for electric field parallel to a is appropriate, but boundary conditions for reflection NSOM,

combined with the stronger dependence for electric field parallel to c cause the latter to dominate in NSOM. NSOM has plenty of light polarized along c in the near-field, again due to the boundary conditions.

The idea of the experiment is to use different doses of electrons (with various energies) and examine the size of the effected region. Qualitatively, one expects the following (at some energy):

The electron energy dependence, the mechanism that limits the effected area size, and the rates can all give us important information to determine the mechanism of electromigration in this system.

The program: