Avoiding photothermal noise in laser assisted STM

S. M. LANDI; A. V. BRAGAS; COY, JULIO; O.E. MARTINEZ

ULTRAMICROSCOPY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 1999

0304-3991

Thermal expansion produced by laser irradiation of the tunneling junction is analyzed, as a necessary step towardsdetection and identi"cation of other laser induced currents in scanning tunneling microscopy (STM). Solving a tridimensionalheat di!usion model, the amplitude of thermal expansion as a function of the modulation frequency (u) of the lightpower, rolls o! as 1/u while the in-phase component rolls o! as 1/u2, both computed at the Gaussian beam center. Butshifted from the center a dephasing mechanism appears due to the lateral di!usion of the heat, and the in-phase thermalcontribution drops to zero. This behavior can be used to increase the signal to noise ratio without the need of driving theexperiment at high frequencies, frequently over the usual cuto! frequency of STM ampli"ers. Experiments were carriedon using a low power laser on highly oriented pyrolitic graphite (HOPG) and gold samples, showing a qualitativeagreement with the model.