Avoiding photothermal noise in laser assisted scanning tunneling microscopy

Thermal expansion produced by laser irradiation of the tunneling junction is analyzed, as a necessary step towards detection and identification of other 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 (ω) of the light power, rolls off as 1/ω while the in-phase component rolls off as 1/ω2, both computed 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-phase thermal contribution drops to zero. This behavior can be used to increase the signal to noise ratio without the need of driving the experiment at high frequencies, frequently over the usual cutoff frequency of STM amplifiers. 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.