Nonlinear quantum tunneling effects in nanoplasmonic environments: two-photon absorption and harmonic generation

We use a quantum mechanical approach to derive a set of linear and nonlinear quantum conductivity coefficients for metal-insulator-metal structures with nanometer sized gaps. The immediate proximity of metallic objects generates a tunneling AC current density that endows the gap region with additional linear and nonlinear coefficients that in turn trigger linear and nonlinear absorption, and second-and third-harmonic generation. For example, a vacuum gap approximately 0.8 nm thick displays an effective vertical bar chi((2))vertical bar similar to 0.1 pm/V for adjacent objects composed of dissimilar metals and an effective vertical bar chi((3))vertical bar similar to 10(-20) m(2)/V-2 for either similar or dissimilar metals, increasing exponentially for smaller gaps. Field localization inside the gap ensures that harmonic generation arising from the gap region overwhelms intrinsic metal second-and third-order nonlinearities. (C) 2014 Optical Society of America