  We develop a theoretical framework for the description of light emission from
plasmonic contacts based on the nonequilibrium Green function formalism. Our
theory establishes a fundamental link between the finite-frequency quantum
noise and AC conductance of the contact and the light emission. Calculating the
quantum noise to higher orders in the electron-plasmon interaction, we identify
a plasmon-induced electron-electron interaction as the source of experimentally
observed above-threshold light emission from biased STM contacts. Our findings
provide important insight into the effect of interactions on the light emission
from atomic-scale contacts.
