Quenching the thermal contribution in laser assisted scanning tunneling microscopy

When irradiating the sample of a scanning tunneling microscope with a modulated light intensity, light absorption results in tip and sample heating and expansion at the modulation frequency, obscuring other possible laser induced mechanisms. This thermal noise limits the use of light modulation when very high spatial resolution is desired in fluorescence or nonlinear optics near field experiments, being an extreme case the measurement of the optical rectification with scanning tunneling microscopes. In this work we describe a method in which the thermal expansion at the modulation frequency can be reduced by orders of magnitude. The method is based on the irradiation of the sample with two interfering laser beams at different frequencies and incidence direction, giving light fringes traveling in the illuminated zone. Solving the heat diffusion equation we predict the thermal behavior of sample and find a good agreement with experimental data. (C) 2000 American Institute of Physics. [S0021-8979(00)02321-5].