Avoiding photothermal noise in laser assisted scanning tunneling microscopy

Thermal expansion produced by laser irradiation of the tunneling junction i s analyzed, as a necessary step towards detection and identification of oth er laser induced currents in scanning tunneling microscopy (STM). Solving a tridimensional heat diffusion model, the amplitude of thermal expansion as a function of the modulation frequency (omega) of the light power, rolls o ff as 1/omega while the in-phase component rolls off as 1/omega(2), both co mputed at the Gaussian beam center. But shifted from the center a dephasing mechanism appears due to the lateral diffusion of the heat, and the in-pha se thermal contribution drops to zero. This behavior can be used to increas e the signal to noise ratio without the need of driving the experiment at h igh frequencies, frequently over the usual cutoff frequency of STM amplifie rs. Experiments were carried on using a low power laser on highly oriented pyrolitic graphite (HOPG) and gold samples, showing a qualitative agreement with the model. (C) 1999 Published by Elsevier Science B.V. All rights res erved.