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Abstract

Nonlinear optical processes require high light intensities and, consequently, are typically induced by ultrashort pulsed-laser excitation utilizing the temporal confinement of the laser electric field. Here we demonstrate that multiphoton photocurrents can be generated in a plasmonic scanning tunneling microscope (STM) junction by continuous-wave excitation at sub-MW cm^–2 incident intensities. This 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. The exponent, representing the optical nonlinearity, varies between 3 and 1, depending on both the incident photon energy and the bias voltage applied to the junction. The bias-voltage dependence of the photocurrent shows characteristic steps that can be reproduced by calculating the transmission of nonthermal carriers through the potential barrier of the tunnel junction. Our results provide unambiguous experimental evidence for transfer of nonthermal photogenerated electrons under continuous-wave excitation in plasmonic nanojunctions.