Tobias Utikal

Quantitative modeling of the third harmonic emission spectrum of plasmonic nanoantennas.

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.

Excitonic Fano Resonance in Free-Standing Graphene

We investigate the role of electron-hole correlations in the absorption of free-standing 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 noninteracting particle picture cannot describe our data. These results suggest a profound connection between the absorption properties and the topology of the graphene band structure.

Spectroscopic detection and state preparation of a single praseodymium ion in a crystal.

The narrow optical transitions and long spin coherence times of rare earth ions in crystals make them desirable for a number of applications ranging from solid-state spectroscopy and laser physics to quantum information processing. However, investigations of these features have not been possible at the single-ion level. Here we show that the combination of cryogenic high-resolution laser spectroscopy with optical microscopy allows one to spectrally select individual praseodymium ions in yttrium orthosilicate. Furthermore, this spectral selectivity makes it possible to resolve neighbouring ions with a spatial precision of the order of 10 nm. In addition to elaborating on the essential experimental steps for achieving this long-sought goal, we demonstrate state preparation and read out of the three ground-state hyperfine levels, which are known to have lifetimes of the order of hundred seconds.

Few-photon coherent nonlinear optics with a single molecule

Photons are efficiently funnelled into a single molecule if they are nearly resonant with the sharp molecular transition. In this condition, the coherent nonlinear optical effect can be induced with only a few photons without high-finesse cavities.

Coherent Coupling of a Single Molecule to a Scanning Fabry-Perot Microcavity

Organic dye molecules have been used in great many scientific and technological applications. Among these, the polycyclic aromatic hydrocarbons (PAH) behave very favourably as quantum emitters when embedded in organic crystals at temperatures below 4K. PAHs can serve as solid-state emitters with remarkable optical properties such as near-unity quantum efficiency and high photostability. However, transitions to short-lived molecular vibrational levels reduce the scattering cross section by a branching ratio of about 30–50%. In order to overcome this shortcoming and render the molecule as a two-level system, one can enhance its zero-phonon emission by using the Purcell effect, e.g., via coupling to an optical cavity, to obtain a near-unity branching ratio.

Spectroscopic detection of single Pr3+ ions on the 3H4−1D2 transition

Rare earth ions in crystals exhibit narrow spectral features and hyperfine-split ground states with exceptionally long coherence times. These features make them ideal platforms for quantum information processing in the solid state. Recently, we reported on the first high-resolution spectroscopy of single ions in yttrium orthosilicate nanocrystals via the transition at a wavelength of 488 nm. Here we show that individual praseodymium ions can also be detected on the more commonly studied transition at 606 nm. In addition, we present the first measurements of the second-order autocorrelation function, fluorescence lifetime, and emission spectra of single ions in this system as well as their polarization dependencies on both transitions. Furthermore, we demonstrate that by a proper choice of the crystallite, one can obtain narrower spectral lines and, thus, resolve the hyperfine levels of the excited state. We expect our results to make single-ion spectroscopy accessible to a larger scientific community.Rare earth ions in crystals exhibit narrow spectral features and hyperfine-split ground states with exceptionally long coherence times. These features make them ideal platforms for quantum information processing in the solid state. Recently, we reported on the first high-resolution spectroscopy of single Pr

Chip-Based All-Optical Control of Single Molecules Coherently Coupled to a Nanoguide

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Coherent coupling of a single molecule to a scanning Fabry-Pérot microcavity

ions in yttrium orthosilicate (YSO) nanocrystals. While in that work we examined the less explored

Nonlinear optics with single molecules

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Nonlinear Plasmon Optics

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