Electrically Driven Hot-Carrier Generation and Above-Threshold Light Emission in Plasmonic Tunnel Junctions

Abstract

Localized surface plasmons (LSPs) in metal nanostructures have attracted much attentionfor their role in generating non-equilibrium hot carriers (HCs) for photochemistry 1-3 , photodetection 4,5 and photoluminescence 6 . In addition to optical excitation, LSPs and HC dynamics can be driven electrically via inelastic tunneling. LSP-mediated light emission in tunnel junctions 7-13 commonly features photon energies below the threshold set by the applied voltage bias. Recent work [14][15][16][17][18] has reported photon energies significantly above that threshold, while the underlying physical origin remains elusive. Proposed mechanisms include higher-order electron-plasmon and electron-electron interactions 15,17-20 , and blackbody radiation of hot electrons 16,21 . We report measurements of light emission in tunnel junctions of different plasmonic materials and reveal that HCs generated by nonradiative decay of electrically excited plasmons play a key role in above-threshold light emission. We observed the crossover from above-to below-threshold light emission regime by controlling the tunneling current. There is a giant material dependence of the photon yield, as much as four orders of magnitude, much greater than the plasmon-enhanced radiative efficiency of the tunneling gap. The spectral features of light emission are consistent with a proposed mechanism that incorporates the plasmonic field enhancement and a non-equilibrium HC distribution parametrized by a bias-dependent Boltzmann factor. The effective temperatures of HCs are found to correlate with the bias voltage, rather than the dissipated electrical power. Electrically driven HC generation (above 2000 K under modest voltage) and plasmon-enhanced light emission could open new strategies for chemistry, optoelectronics and quantum optics.Electrically driven light emission from tunnel junctions is of great interest for a variety of technologies requiring efficient optoelectronic integration and conversion at the nanoscale, such

Thousand-fold Increase in Plasmonic Light Emission via Combined Electronic and Optical Excitations

“…Figure 1C shows the light emission spectra (ℏw > ℏwexc ≈ 1.58 eV) measured from a Au junction. In the EL case, above-threshold emission can be generated via multi-electron interactions in the low current limit (~100 nA) 19 , and hot carrier recombination in the high current limit (~100 $%) 24 . For PL at zero bias, in addition to the hot carrier 26 or intraband transition 36 induced photoluminescence, tunneling electrons can also undergo a Raman process (electronic Raman scattering, ERS) before emitting a photon with a different energy 30,31 .…”

“…2D) incident power at 0.46 mW. Our past work 24 on light emission from electrically driven junctions shows that, using a normalization method, one can separate the voltage-independent LSP spectrum, 5()). Here we perform a similar analysis (see Supporting Information Sec.…”

“…In our previous work 24 , we used a voltage dependent effective temperature 6 &,, in a Boltzmann factor, ! !ℏ#/% !…”

“…Recent studies [18][19][20][21][22][23] of plasmonic light emission in tunnel junctions have reported a strong upconversion effect, with generated photon energy (ℏw) significantly above the energy threshold of the incident electrons (eV with V the applied bias). While multiple processes can produce such above-threshold photons, recent work 24,25 has revealed the dominant role of LSP-induced hot carriers. Similarly, upconversion photoluminescence ( ℏw > ℏw exc with ℏw exc the excitation photon energy) from plasmonic nanoparticles [26][27][28][29] has also been observed, and this phenomenon can be explained by mechanisms such as hot carrier luminescence 26 , anti-Stokes electron Raman scattering 27,30,31 , and other surface-enhanced anti-Stokes processes [32][33][34] .…”

Highly-efficient electrically-driven localized surface plasmon source enabled by resonant inelastic electron tunneling

“…Black vertical arrows indicate the respective cutoff frequency with the quantum relation . Note that the small portion of emission with could be attributed to multielectron processes or hot-electron light emission 50 , 51 . b Calculated vacuum source efficiency (color lines) and SP radiation enhancement (black line) at various voltages.…”