Controlling electron transfer channels in a plasmonic scanning tunneling microscope junction under light excitation

Controlling electron transfer channels (ETCs) activated by n-photon absorption is essential for photo-assisted scanning tunneling microscopy. Here, we demonstrate systematic control of ETCs among n = 0, 1, and 2 by tuning the bias voltage and current and using field emission resonance (FER) and plasmon-assisted FER peaks as reliable indicators for photon order n. We highlight that the transition of electron tunneling from the n = 0 to n = 1 process together with the transition of FER to plasmon-assisted FER peaks can occur via decreasing the current under constant excitation laser power. In addition, we show that higher order photoemission channels could be activated by further reducing the current after the intensities of plasmon-assisted FER peaks reach the maximum. The emergence of a sloped background accompanied by the suppression of plasmon-assisted FER peaks is a characteristic signature of the photoemission process. These findings provide an efficient and practical approach for identifying and controlling ETCs in photoexcited scanning tunneling microscope junctions.