Abstract
The spectral distribution of light emitted from a scanning tunnelling microscope junction not only bears its intrinsic plasmonic signature but is also imprinted with the characteristics of optical frequency fluc- tuations of the tunnel current. Experimental spectra from gold-gold tunnel junctions are presented that show a strong bias (Vb) dependence, curiously with emission at energies higher than the quantum cut-off (eVb); a component that decays monotonically with increasing bias. The spectral evolution is explained by developing a theoretical model for the power spectral density of tunnel current fluctuations, incorporating finite temperature contribution through consideration of the quantum transport in the system. Notably, the observed decay of the over cut-off emission is found to be critically associated with, and well explained in terms of the variation in junction conductance with Vb. The investigation highlights the scope of plasmon-mediated light emission as a unique probe of high frequency fluctuations in electronic systems that are fundamental to the electrical generation and control of plasmons.
Highlights
Plasmonics has long been dominated by the ability of passive structures to concentrate externally incident electromagnetic radiation into volumes of significantly sub-wavelength dimensions[1]
From previous experimental[15,16,17,18,19,20,21,22,23] and theoretical[24,25,26,27,28] studies into light emission from tunnel junctions, the scanning tunnelling microscope (STM), it is understood that optical frequency fluctuations in the tunnel current (IT) excite both localized surface plasmons (LSP)[29,30,31] as well as propagating surface plasmon polaritons[32,33,34]
The overall maximum energy of light emission from a Au-Au tip-sample junction (TSJ) is likely to be limited by the onset of inter-band transitions ~2.4 eV [Experimentally, the overall light emission detection window is decided by the photo-detector threshold and onset of the inter-band transitions in case of Au with Vb further limiting the emission to ~ħωco + ΔE]
Summary
− ω 2 that (Equation (10)), quantifies the strength of the IT fluctuations at energy ħω, which stimulates the junction LSPs and decides the emission intensity. It is dependent on the tunnel gap, Vb, electronic properties of TSJ material and T. The finite T effects get incorporated into the above expression through the functions f± (Ex) (Equations (3) and (4)) along with the T dependence built into Eq (10)
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