Laser-induced scanning tunneling microscopy: Linear excitation of the junction plasmon.

We introduce the cross-polarized double-beat method for localized excitation of the junction plasmon of a scanning tunneling microscope with femtosecond laser pulses. We use two pulse trains derived from a Ti:sapphire laser operating at a repetition frequency of f(s)=76 MHz, with a relative shift between their carrier frequencies ω(a)/2π=f(s)+f(b) controlled with an acousto-optic modulator. The trains are cross-polarized and collinearly focused on the junction, ensuring constant radiation flux. The anisotropic susceptibility of the junction plasmon mixes the fields, which modulate the tunneling current at f(b) (the difference between carrier beat and repetition frequency) at base-band frequencies that can be used for direct detection of the tunneling current. The interferometric cross-correlation of the pulses and the polarization dependence of the mixing identify the coupling to the radiation to be through the coherent z-displacement of the tip plasmon. Single Ag atoms are used to demonstrate microscopy under irradiation. In the linear coupling regime, the laser-induced displacement of the plasmon is operationally indistinguishable from the mechanical displacement of the junction gap.