Theory of tip-surface gap dependence in laser-assisted scanning tunneling microscopy

Nonlinear characteristics of laser-assisted tunneling in the context of scanning tunneling microscopy (STM) are investigated by combining Bardeen's formalism and the WKB approximation. Atomic-scale images of the second-order nonlinear response calculated for graphite are dependent on the tip-sample distance. At small distance, this response is proportional to the square of the derivative of the surface local density of states (LDOS), giving trigonal images peaking atop A sites. However, at large distance, the images are proportional to the square of the LDOS, giving trigonal images of B sites. The present formalism can also predict atomic-scale STM images induced by laser irradiating other surfaces such as silicon.