Finite-difference time-domain analysis of scanning tunneling microscope light emission spectra

The finite-difference time-domain (FDTD) method has been applied to the spectral analysis of scanning tunneling microscope (STM) light emission. We found that FDTD analysis provides predictions that are consistent with those of the dielectric theory of STM light emission if the radius of curvature of the tip front is less than 50nm. In the radius range above 50nm, discrepancies between these theories were considerbale in predictions of the efficiency of STM light emission but moderate in those of its spectral shapes. STM light emission spectra of rods and disks embedded in substrates, to which the dielectric theory of STM light emission is not applicable, were calculated by the FDTD method, and we deduced that the spectral features specific to the rod material become distinct when the lateral size of the rod is comparable to or larger than that of localized surface plasmons determined by the dielectric theory of STM light emission; a thickness of 3-5 nm is additionally required for the disk.