Stefan Linden

Spatio-spectral characterization of photonic meta-atoms with electron energy-loss spectroscopy

Scanning transmission electron microscopy in combination with electron energy-loss spectroscopy is a powerful tool for the spatial and spectral characterization of the plasmonic modes of lithographically defined photonic meta-atoms. As an example, we present a size dependence study of the resonance energies of the plasmonic modes of a series of isolated split-ring resonators. Furthermore, we show that the comparison of the plasmonic maps of a split-ring resonator and the corresponding complementary split-ring resonator allows a direct visualization of Babinet’s principle. Our experiments are in good agreement with numerical calculations based on a discontinuous Galerkin time-domain approach.

Manipulation of Airy surface plasmon beams.

We demonstrate experimentally the manipulation of Airy surface plasmon beams in a linear potential. For this purpose, we fabricate dielectric-loaded plasmonic structures with a graded refractive index by negative-tone gray-scale electron beam lithography. Using such carefully engineered potentials, we show that the bending of an Airy surface plasmon beam can be fully reversed by the potential.

Distance-dependence of the coupling between split-ring resonators and single-quantum-well gain

We present low-temperature femtosecond pump-probe experiments on arrays of silver split-ring resonators coupled to single quantum wells, in which we vary the geometrical separation between the two components to study the variation of coupling with distance. Its strength is found to decrease exponentially with increasing separation with a 1/e length on the order of 8 nm. We further link our experimental results to numerical calculations of the near fields which show the same distance-dependence as the coupling strength in the experiment. Together, this confirms the assumption of a near-field-assisted coupling mechanism.

Second harmonic generation spectroscopy on second harmonic resonant plasmonic metamaterials

The field enhancement resulting from the resonant excitation of plasmonic modes in metallic nanostructures allows to amplify nonlinear processes such as second harmonic generation in plasmonic metamaterials. Thus far, nonlinear experiments with metallic nanostructures relied on the resonant enhancement of the pump light by a plasmonic mode. Here, we report on second harmonic generation spectroscopy on plasmonic metamaterials that only exhibit a plasmonic resonance at twice the pump frequency. We also observe in this case a strong enhancement of the second harmonic generation signal, even though the driving pump field is not resonantly enhanced. The experimental data can be explained in terms of the anharmonic oscillator model and classical symmetry selection rules applied to nonlinear metamaterials. Our findings pave the way for further optimizations of the nonlinear response of plasmonic metamaterials.

Optimal Second-Harmonic Generation in Split-Ring Resonator Arrays

Previous experimental measurements and numerical simulations give evidence of strong electric and magnetic field interaction between split-ring resonators in dense arrays. One can expect that such interactions have an influence on the second harmonic generation. We apply the Discontinuous Galerkin Time Domain method and the hydrodynamic Maxwell-Vlasov model to simulate the linear and nonlinear optical response from SRR arrays. The simulations show that dense placement of the constituent building blocks appears not always optimal and collective effects can lead to a significant suppression of the near fields at the fundamental frequency and, consequently, to the decrease of the SHG intensity. We demonstrate also the great role of the symmetry degree of the array layout which results in the variation of the SHG efficiency in range of two orders of magnitude.

From isolated metaatoms to photonic metamaterials: Mapping of collective near-field phenomena with EELS

We investigate the evolution of plasmonic modes during the transition from metaatoms to photonic metamaterials by electron energy-loss spectroscopy. Interactions between metaatoms have a strong effect on the near-field distribution of metamaterials.

Reflective Metasurfaces for Incoherent Light To Bring Computer Graphics Tricks to Optical Systems

The normal mapping technique is widely used in computer graphics to visualize three-dimensional (3D) objects displayed on a flat screen. Taking advantage of optical properties of metasurfaces, which provide a highly efficient approach for manipulation of incident light wavefront, we have designed a metasurface to implement diffuse reflection and used the concept of normal mapping to control its scattering properties. As a proof of principle, we have fabricated and characterized a flat diffuse metasurface imitating lighting and shading effects of a 3D cube. The 3D image is displayed directly on the illuminated metasurface and it is brighter than a standard white paper by up to 2.4 times. The designed structure performs equally well under coherent and incoherent illumination. The normal mapping approach based on metasurfaces can complement traditional optical engineering methods of surface profiling and gradient refractive index engineering in the design of 3D security features, high-performance planar optical diffusers, novel optical elements, and displays.

Transverse Anderson localization of surface plasmon polaritons

We investigate the effect of disorder on the propagation of surface plasmon polaritons in arrays of evanescently coupled dielectric loaded surface plasmon polariton waveguides. Diagonal disorder is implemented by randomly varying the heights of the waveguides. Real-space as well as momentum-space images of the surface plasmon polariton intensity distribution in the waveguide arrays are recorded by leakage radiation microscopy. In real space, increasing disorder results in a transverse localization of surface plasmon polaritons. In the momentum distribution, we observe for the first time a disorder-induced transition from a continuous band to a set of discrete modes.

Fluorescence enhancement by a dark plasmon mode

We investigate the fluorescence properties of colloidal quantum dots coupled to gold nanowire antennas. By varying the wire length, the plasmon modes of the nanoantennas are successively tuned through the emission band of the quantum dots. We observe a pronounced fluorescence enhancement both for short and long nanoantennas. These findings can be attributed to the coupling of the quantum dots to the bright dipole plasmon mode and the dark quadrupol plasmon mode, respectively. This interpretation is supported by numerical calculations of the far-field scattering spectra and the radiation rates.

Invited Article: Direct phase mapping of broadband Laguerre-Gaussian metasurfaces

We report on the fabrication of metasurface phase plates consisting of gold nanoantenna arrays that generate Laguerre-Gaussian modes from a circularly polarized Gaussian input beam. The corresponding helical phase profiles with radial discontinuities are encoded in the metasurfaces by the orientation of the nanoantennas. A common-path interferometer is used to determine the orbital angular momentum of the generated beams. Additionally, we employ digital holography to record detailed phase profiles of the Laguerre-Gaussian modes. Experiments with different laser sources demonstrate the broadband operation of the metasurfaces.