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

摘要

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.