Plasmonic dimer nanoantennas are characterized by a strong enhancement of the optical field, leading to large nonlinear effects. The third harmonic emission spectrum thus depends strongly on the antenna shape and size as well as on its gap size. Despite the complex shape of the nanostructure, we find that for a large range of different geometries the nonlinear spectral properties are fully determined by the linear response of the antenna. We find excellent agreement between the measured spectra and predictions from a simple nonlinear oscillator model. We extract the oscillator parameters from the linear spectrum and use the amplitude of the nonlinear perturbation only as scaling parameter of the third harmonic spectra. Deviations from the model only occur for gap sizes below 20 nm, indicating that only for these small distances the antenna hot spot contributes noticeable to the third harmonic generation. Because of its simplicity and intuitiveness, our model allows for the rational design of efficient plasmonic nonlinear light sources and is thus crucial for the design of future plasmonic devices that give substantial enhancement of nonlinear processes such as higher harmonics generation as well as difference frequency mixing for plasmonically enhanced terahertz generation.

Molecules are ubiquitous in natural phenomena and man-made products, but their use in quantum optical applications has been hampered by incoherent internal vibrations and other phononic interactions with their environment. We have now succeeded in turning an organic molecule into a coherent two-level quantum system by placing it in an optical microcavity. This allows several unprecedented observations such as 99% extinction of a laser beam by a single molecule, saturation with less than 0.5 photon, and nonclassical generation of few-photon super-bunched light. Furthermore, we demonstrate efficient interaction of the molecule-microcavity system with single photons generated by a second molecule in a distant laboratory. Our achievements pave the way for linear and nonlinear quantum photonic circuits based on organic platforms.

We investigate the role of electron-hole correlations in the absorption of freestanding monolayer and bilayer graphene using optical transmission spectroscopy from 1.5 to 5.5 eV. Line shape analysis demonstrates that the ultraviolet region is dominated by an asymmetric Fano resonance. We attribute this to an excitonic resonance that forms near the van-Hove singularity at the saddle point of the band structure and couples to the Dirac continuum. The Fano model quantitatively describes the experimental data all the way down to the infrared. In contrast, the common non-interacting particle picture cannot describe our data. These results suggest a profound connection between the absorption properties and the topology of the graphene band structure.The material properties and the atomic structure of graphene are intimately connected. Most electronic effects can be understood by the unique band structure deduced from a tight-binding model of uncorrelated electrons [1]. A prominent example is the constant optical absorption for photon energies in the infrared wavelength range. It is a consequence of the linear dispersion relation near the K points in the Brillouin zone, the so-called Dirac cones. The absorption is given by fundamental constants alone as the product of the fine structure constant in vacuum α ≈ 1/137 and π [2,3,4], and it is independent of the velocity of the Dirac fermions. Here, we demonstrate experimentally by line shape analysis that a single-particle model cannot describe the absorption spectrum of freestanding graphene in the visible and ultraviolet spectral region. The saddle point (M) in the band structure (see Fig. 1) causes a van-Hove singularity with a divergent density of states, allowing for a strong optical transition [5]. In this case, electron-hole correlations can lead to effects beyond the single-particle picture. An excitonic resonance at an energy slightly below the van-Hove singularity becomes possible. At a saddle point, the excitonic resonance takes a Fano shape as the discrete exciton state couples to the continuum formed by the band descending from the saddle point [5,6]. In the following we show that the Fano model of the excitonic resonance describes the complete optical spectrum of graphene from the ultraviolet all the way down to the infrared part of the electromagnetic spectrum.A Fano resonance occurs when a discrete state couples to a continuum of states [6]. The resulting spectrum has * These authors contributed equally.