Light emission from Na/Cu(111) induced by a scanning tunneling microscope

Abstract

Measurements of light emission from a scanning tunneling microscope probing a Na overlayer on the ͑111͒ surface of Cu are reported along with the results of a model calculation that essentially agree with the experimental ones. The observed light-emission spectra show two characteristic features depending on the bias voltage. When the bias voltage is smaller than the energy of the second quantum well state formed outside the Na overlayer the light emission is due to a plasmon-mediated process, while for larger biases light emission is mainly caused by quantum well transitions between the two levels.

Two-Electron Photon Emission from Metallic Quantum Wells

“…3) give rise to plasmon mediated emission by inelastic tunneling from the tip Fermi level only to the lower QWS. The emission occurs predominantly at hν < eU − E 1 as expected [3,19].…”

“…At 0.6 ML, these states are located at E 1 0:4 eV and E 2 2:1 eV. Previously, photon emission due to two processes has been reported from these layers [3,19]. At low U, i.e., eU < E 2 , electrons tunnel inelastically from the tip Fermi level to the lower QWS and emit photons.…”

“…This influx was then used as input in a calculation of the light emission intensity along the line of Ref. [19]. Figure 3 displays results from our model calculations obtained for U 2 V and I 10, 100, and 300 nA, respectively.…”

“…The Keldysh Green's function (GF) formalism provides a suitable theoretical framework for calculating the intensity of the emitted light from a system out of equilibrium such as an STM under finite bias. The intensity can be written [19]…”

“…The integrations run over a volume between the tip and sample where the electrons and photons interact efficiently. G is a factor describing the enhancement of the electromagnetic vacuum fluctuations in the tip-sample cavity, and Π < is the Fourier transform of the currentcurrent GF, −i j z ( r ′ , 0)j z ( r, t) , which in the case of allowed light emission can be expressed in terms of a current matrix element between the initial and final electron states [19]. The detailed calculations of electron states and matrix elements employs a one-dimensional model for the system.…”