ABSTRACT:Boosting nonlinear frequency conversion in extremely confined volumes remains a key challenge in nano-optics, nanomedicine, photocatalysis, and background-free biosensing. To this aim, field enhancements in plasmonic nanostructures are often exploited to effectively compensate for the lack of phase-matching at the nanoscale. Second harmonic generation (SHG) is, however, strongly quenched by the high degree of symmetry in plasmonic materials at the atomic scale and in nanoantenna designs.Here, we devise a plasmonic nanoantenna lacking axial symmetry, which exhibits spatial and frequency mode overlap at both the excitation and the SHG wavelengths. The effective combination of these features in a single device allows obtaining unprecedented SHG conversion efficiency. Our results shed new light on the optimization of SHG at the nanoscale, paving the way to new classes of nanoscale coherent light sources and molecular sensing devices based on nonlinear plasmonic platforms.

2The interaction of light and matter, i.e. absorption and emission of photons, can be considerably enhanced in the presence of strongly localized and therefore highly intense optical near fields 1 .Plasmonic antennas consisting of pairs of closely spaced metal nano particles have gained much attention in this context since they provide the possibility to strongly concentrate optical fields into the gap between the two metal particles [2][3][4] . Pairs of closely spaced metal nanoparticles supporting plasmonic gap resonances consequently find broad applications, e.g. in single-emitter surfaceenhanced spectroscopy 5,6 , quantum optics 7 , extreme nonlinear optics 8-10 , optical trapping 11 , metamaterials 12, 13 and molecular opto-electronics 14 .The success of metal-insulator-metal structures is based on two fundamental properties of their anti-symmetric electromagnetic gap modes. (i) As a direct consequence of the boundary conditions, the dominating field components normal to the metal-dielectric interfaces are sizable only inside the dielectric gap. This means that for anti-symmetric gap modes the achievable field confinement is not limited by the skin depth of the metal, but is solely determined by the actual size of the gap. (ii) Since the free electrons of the metal respond resonantly to an external optical frequency field, enormous surface charge accumulations, accompanied by ultra-intense optical near fields, will occur. In addition, with decreasing gap width, stronger attractive coulomb forces 4 across the gap lead to further surface-charge accumulation and a concomitantly increased near-field intensity enhancement. We therefore conclude that an experimental realization of atomic-scale concentration of electromagnetic fields at visible frequencies is possible but it requires atomic-scale shape control of the field-confining structure, i.e. the gap, as well as a careful assignment and selection of suitable optical modes.Here we achieve atomic-scale confinement of electromagnetic fields at visible frequencies by combining for the first time both atomic-scale shape control of the field confining structure 15 as well as a careful selection and assignment of suitable optical modes [16][17][18] . We study single-crystalline nanorods which self-assemble into side-by-side aligned dimers with gap widths below 0.5 nm. Sideby-side aligned nanorod dimers possess various distinguishable symmetric and anti-symmetric 3 modes in the visible range 19 . In contrast to previous work 20-22 we demonstrate full control over symmetric and anti-symmetric optical modes by means of white-light scattering experiments. We experimentally demonstrate the presence of atomic-scale light confinement in these structures by observing an extreme > 800 meV hybridization splitting of corresponding symmetric and antisymmetric dimer modes. Our results open new perspectives for atomically-resolved spectroscopic imaging, deeply nonlinear optics and attosecond physics, cavity optomechanics and ultra-sensing as well as quantum optics.To obtain nano...

Electrically connected resonant optical antennas hold promise for the realization of highly efficient nanoscale electro-plasmonic devices that rely on a combination of electric fields and local near-field intensity enhancement. Here we demonstrate the feasibility of such a concept by attaching leads to the arms of a two-wire antenna at positions of minimal near-field intensity with negligible influence on the antenna resonance. White-light scattering experiments in accordance with simulations show that the optical tunability of connected antennas is fully retained. Analysis of the electric properties demonstrates that in the antenna gaps direct current (DC) electric fields of 10(8) V/m can consistently be achieved and maintained over extended periods of time without noticeable damage.

The optimization of nonlinear optical processes at the nanoscale is a crucial step for the integration of complex functionalities into compact photonic devices and metasurfaces. In such systems, photon upconversion can be achieved with high efficiencies via third-order processes, such as third harmonic generation (THG), thanks to the resonantly enhanced volume currents. Conversely, second-order processes, such as second harmonic generation (SHG), are often inhibited by the symmetry of metal lattices and of common nanoantenna geometries. SHG and THG processes in plasmonic nanostructures are generally treated independently, since they typically represent small perturbations in the light-matter interaction mechanisms. In this work, we demonstrate that this paradigm does not hold for plasmon-enhanced nonlinear optics, by providing evidence of a sum frequency generation process seeded by SHG, which sizably contributes to the overall THG yield.We address this mechanism by unveiling a characteristic fingerprint in the polarization state of the THG emission from non-centrosymmetric gold nanoantennas, which directly reflects the asymmetric distribution of second harmonic fields within the structure and does not depend on the model one employ to describe photon upconversion. We suggest that such cascaded processes may also appear for structures that exhibit only moderate SHG yields. The presence of this peculiar mechanism in THG from plasmonic nanoantennas at telecommunication wavelengths allows gaining further insight on the physics of plasmon-enhanced nonlinear optical processes. This could be crucial in the realization of nanoscale elements for photon conversion and manipulation operating at room-temperature.

Gold nanostructures have important applications in nanoelectronics, nano-optics as well as in precision metrology due to their intriguing opto-electronic properties. These properties are governed by the bulk band structure but to some extend are tunable via geometrical resonances. Here we show that the band structure of gold itself exhibits significant size-dependent changes already for mesoscopic critical dimensions below 30 nm. To suppress the effects of geometrical resonances and grain boundaries, we prepared atomically flat ultrathin films of various thicknesses by utilizing large chemically grown single-crystalline gold platelets. We experimentally probe thickness-dependent changes of the band structure by means of two-photon photoluminescence and observe a surprising 100-fold increase of the nonlinear signal when the gold film thickness is reduced below 30 nm allowing us to optically resolve single-unit-cell steps. The effect is well explained by density functional calculations of the thickness-dependent 2D band structure of gold.