Efficient light generation from enhanced inelastic electron tunnelling

Abstract

Light emission from biased tunnel junctions has recently gained much attention owing to its unique potential to create ultracompact optical sources with terahertz modulation bandwidth 1-5. The emission originates from an inelastic electron tunnelling process in which electronic energy is transferred to surface plasmon polaritons and subsequently converted to radiation photons by an optical antenna. Because most of the electrons tunnel elastically, the emission efficiency is typically about 10 −5-10 −4. Here, we demonstrate efficient light generation from enhanced inelastic tunnelling using nanocrystals assembled into metal-insulator-metal junctions. The colour of the emitted light is determined by the optical antenna and thus can be tuned by the geometry of the junction structures. The efficiency of far-field free-space light generation reaches ~2%, showing an improvement of two orders of magnitude over previous work 3,4. This brings on-chip ultrafast and ultracompact light sources one step closer to reality. Electrons can tunnel through a metal-insulator-metal (MIM) junction either elastically or inelastically. For elastic tunnelling, electrons tunnel across the barrier layer without energy loss 6. However, the inelastic tunnelling process may create either phonons or photons as the electrons lose part of their energy in the gap and transition to a lower energy state in the metal counter-electrode. This process can be enhanced in the presence of surface plasmon polaritons around the MIM junction, as first discovered in 1976 1. Later theoretical 2 and experimental 7-10 studies increased the appeal of the MIM junction because of its ultra-small footprint and ultra-large modulation bandwidth. However, the main challenge for light generation from inelastic electron tunnelling is its low external quantum efficiency (EQE), a production of internal quantum efficiency (IQE) and radiation efficiency. Generally, the IQE describes the efficiency of the inelastic tunnelling event and can be increased by designing a plasmonic structure with a large local density of optical states (LDOS) 7,11,12 , and the radiation efficiency can be improved by introducing a high-quality optical antenna 13,14. Recently, light emission from electrically driven optical antennas made by amorphous (polycrystalline) plasmonic material has been demonstrated 3,4 with quantum efficiencies up to 10 −4. Compared with amorphous or polycrystalline plasmonic material, single-crystalline material has lower plasmonic loss 15 , which can further enhance the performance of the inelastic tunnel junction. Here, we use single-crystalline silver (Ag) nanocrystals to form tunnel junctions with gap distances of ~1.5 nm. Through geometrical engineering of the junctions to optimize the LDOS and radiation efficiency, we obtain a far-field light that the device could be integrated into photonics and/or plasmonic systems for on-chip applications 23-26. In principle, the emission frequency of the MIM junction device could cover a range from ultraviolet to mid-infrared, a...

Indirect Detection of the Light Emission in the Local Tunnel Junction

“…This gives rise to a so‐called Purcell effect, resulting in the occurrence of additional optical states and, consequently, leading to a significant enhancement of the photon generation (Figure ). Similar effect of the light emission enhancement up to few percent of quantum efficiency has been demonstrated in a recent study with two single‐crystal silver particles . Note that MDM contact under applied bias can also be considered as a compact photosensitive element due to its nanoantenna optical rectification properties .…”

“…Note, that according to the previous studies, the magnitude of bias related to the abovementioned features corresponds to the regime of efficient photon generation in a tunnel junction. The experimental data in previous studies demonstrate that the maximum light emission efficiency and feature of I ( V ) curve are observed at the same bias. Nevertheless, the authors left the discussion of this phenomenon beyond the scope of their works.…”

Electrically-driven Yagi-Uda antennas for light

“…1d). The efficiency of IET can be strongly enhanced by a high local density of optical states (LDOS) [20][21][22] . IET offers distinct advantages such as the absence of any active materials resulting in a large bandwidth while offering efficiencies of up to 2% 22 .…”

“…Even though quite some effort has already been devoted to optical Yagi-Uda antennas 4 -best possible design parameters have been found for both vertical 5 as well as in-plane emitting antennas 6 and different kinds of antennas have been realized [7][8][9][10][11][12] -the main drawback of the hitherto approaches is that the light is not generated locally but bulky lab-scale setups are needed as excitation sources. Generating light locally at the nanoscale is possible by different means, for example via scanning tunneling microscopes (STMs) 13,14 , carbon nanotubes [15][16][17][18] , quantum dots 19 and optical antennas [20][21][22] . However, obtaining directed electrically driven emission is only possible by utilizing STMs 23,24 , which again involves bulky lab-scale setups, or by twisting the arms of electrically driven dipole antennas in order to break the point symmetry 25 .…”

Efficient Surface Plasmon Polariton Excitation and Control over Outcoupling Mechanisms in Metal–Insulator–Metal Tunneling Junctions

“…Until recently, prohibitively low electron‐to‐photon conversion efficiency (10 4 –10 7 electrons result in a single photon14–16,21–23) due to the large momentum mismatch between the initially excited MIM‐TJ cavity mode (MIM‐SPP) and the radiating mode, coupled with an inelastic tunneling event probability of 10%,24 has prevented practical applications of MIM‐TJs. Recently, we have shown that the electron‐to‐SPP conversion efficiency can be experimentally as high as 1% in a planar Al‐AlO x ‐Cr‐Au MIM‐TJ,17 and Qian et al25 demonstrated 2% electron‐to‐photon emission efficiency in a resonant Ag–polymer–Ag nanocrystal tunnel junction, both within one order of magnitude agreement with theoretical inelastic tunneling models,24,26 demonstrating the potential of MIM‐TJs in nano‐scale optoelectronics.…”