Our latest work is published in ACS Photonics [Link]. We demonstrated CW-driven non-linear electron transfer in a plasmonic STM junction.
Nonlinear optical processes require high light intensities. Consequently, they are typically induced by ultrashort pulsed-laser excitation, which utilizes the temporal confinement of the laser electric field. In the paper, we demonstrate that multiphoton photocurrents can be generated in a plasmonic scanning tunneling microscope (STM) junction using continuous-wave excitation. This unusual process is enabled by extreme field enhancement and confinement via the localized surface plasmon resonance in the STM junction. The photocurrent exhibits a power-law dependence on the incident intensity, with the exponent, representing the optical nonlinearity, varying between 3 and 1. This variation depends on both the incident photon energy and the bias voltage applied to the junction. The bias-voltage dependence of the photocurrent displays characteristic steps, which can be replicated by calculating the transmission of nonthermal carriers through the potential barrier of the tunnel junction. Our results provide unambiguous experimental evidence for the transfer of nonthermal photogenerated electrons under continuous-wave excitation in plasmonic nanojunctions.