New aspects of light emission from STM

The recently discovered new mechanism of light emission from a scanning tunneling microscope (STM) compared with well-known conventional mechanisms is reviewed. The new mechanism was found for nanoscale structures which are composed of silicon ( Si) dangling bonds on Si( 001) surfaces. When an STM tip made of tungsten is located above Si dangling bond sites, visible light is emitted at a quantum efficiency of the order of 10(-6) photons/electron. Light is generated by radiative dipole transitions between the surface state of the tip and that of the sample. In the visible-light emission, optical selection rules still apply. The linear polarization of isochromat light emitted from the tunneling gap between an STM tip and a Si( 001) sample strongly depends on the bias voltage between the tip and the sample. The results show that the pi* and sigma* surface states of the sample contribute to the emission of p-polarized and s-polarized light, respectively, in accordance with optical selection rules.