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...

We present optical trapping with a 10 nm gap resonant coaxial nanoaperture in a gold film. Large arrays of 600 resonant plasmonic coaxial nanoaperture traps are produced on a single chip via atomic layer lithography with each aperture tuned to match a 785 nm laser source. We show that these single coaxial apertures can act as efficient nanotweezers with a sharp potential well, capable of trapping 30 nm polystyrene nanoparticles and streptavidin molecules with a laser power as low as 4.7 mW. Furthermore, the resonant coaxial nanoaperture enables real-time label-free detection of the trapping events via simple transmission measurements. Our fabrication technique is scalable and reproducible, since the critical nanogap dimension is defined by atomic layer deposition. Thus our platform shows significant potential to push the limit of optical trapping technologies.

Harnessing the optical properties of noble metals down to the nanometer-scale is a key step towards fast and low-dissipative information processing. At the 10-nm length scale, metal crystallinity and patterning as well as probing of surface plasmon (SP) properties must be controlled with a challenging high level of precision. Here, we demonstrate that ultimate lateral confinement and delocalization of SP modes are simultaneously achieved in extended self-assembled networks comprising linear chains of partially fused gold nanoparticles. The spectral and spatial distributions of the SP modes associated with the colloidal superstructures are evidenced by performing monochromated electron energy loss spectroscopy with a nanometer-sized electron probe. We prepare the metallic bead strings by electron beam-induced interparticle fusion of nanoparticle networks. The fused superstructures retain the native morphology and crystallinity but develop very low energy SP modes that are capable of supporting long range and spectrally tunable propagation in nanoscale waveguides.

Proteins are the most specific yet versatile biological self-assembling agents with a rich chemistry. Nevertheless, the design of new proteins with recognition capacities is still in its infancy and has seldom been exploited for the self-assembly of functional inorganic nanoparticles. Here, we report on the protein-directed assembly of gold nanoparticles using purpose-designed artificial repeat proteins having a rigid but modular 3D architecture. αRep protein pairs are selected for their high mutual affinity from a library of 10(9) variants. Their conjugation onto gold nanoparticles drives the massive colloidal assembly of free-standing, one-particle thick films. When the average number of proteins per nanoparticle is lowered, the extent of self-assembly is limited to oligomeric particle clusters. Finally, we demonstrate that the aggregates are reversibly disassembled by an excess of one free protein. Our approach could be optimized for applications in biosensing, cell targeting, or functional nanomaterials engineering.