Kai Braun

Three-dimensional optical antennas: Nanocones in an apertureless scanning near-field microscope

A sharp-tipped gold nanocone and the vertically aligned metallic tip of a near-field optical microscope together form a three-dimensional optical antenna with a highly controllable gap. Confocal measurements with different laser modes show the efficient axial excitation of the cones with a longitudinally polarized field. In the antenna configuration, extremely strong field enhancement up to a factor of 100 is obtained by tuning the gap between the two sharp tips down to few nanometers.

Tailoring gold nanostructures for near-field optical applications

A method of combined thin-film deposition, electron beam lithography, and ion milling is presented for the fabrication of gold and silver nanostructures. The flexibility of lithographical processes for the variation of geometric parameters is combined with three-dimensional control over the surface evolution. Depending on the etching angle, different shapes ranging from cones over rods to cups can be achieved. These size- and shape-tunable structures present a toolbox for nano-optical investigations. As an example, optical properties of systematically varying structures are examined in a parabolic mirror confocal microscope.

Au Nanotip as Luminescent Near-Field Probe

We introduce a new optical near-field mapping method, namely utilizing the plasmon-mediated luminescence from the apex of a sharp gold nanotip. The tip acts as a quasi-point light source which does not suffer from bleaching and gives a spatial resolution of ≤25 nm. We demonstrate our method by imaging the near field of azimuthally and radially polarized plasmonic modes of nonluminescent aluminum oligomers.

Enhancement of Radiative Plasmon Decay by Hot Electron Tunneling

Here we demonstrate that photon emission induced by inelastic tunneling through a nanometer single gap between a sharp Au tip and an Au substrate can be significantly enhanced by the illumination of the junction with 634 nm laser light with an electric field component oriented parallel to the tip-axis, i.e., perpendicular to the sample. Analyzing photoluminescence (PL) spectra recorded as a function of bias voltage allows us to distinguish between PL from (1) the decay of electron-hole pairs created by the laser excited sp/d interband transition with a characteristic band at 690 nm and (2) the red-shifted radiative decay of characteristic plasmon modes formed by the gap. Since the electroluminescence spectra (without laser) already show the plasmonic gap modes, we conclude that the enhanced intensity induced by laser illumination originates from the radiative decay of hot electrons closely above the Fermi level via inelastic tunneling and photon emission into the plasmon modes. Since these processes can be independently controlled by laser illumination and the amplitude of the bias voltage, it is of great interest for designing new switchable photon emission plasmonic devices.

Superluminescence from an optically pumped molecular tunneling junction by injection of plasmon induced hot electrons

Summary Here, we demonstrate a bias-driven superluminescent point light-source based on an optically pumped molecular junction (gold substrate/self-assembled molecular monolayer/gold tip) of a scanning tunneling microscope, operating at ambient conditions and providing almost three orders of magnitude higher electron-to-photon conversion efficiency than electroluminescence induced by inelastic tunneling without optical pumping. A positive, steadily increasing bias voltage induces a step-like rise of the Stokes shifted optical signal emitted from the junction. This emission is strongly attenuated by reversing the applied bias voltage. At high bias voltage, the emission intensity depends non-linearly on the optical pump power. The enhanced emission can be modelled by rate equations taking into account hole injection from the tip (anode) into the highest occupied orbital of the closest substrate-bound molecule (lower level) and radiative recombination with an electron from above the Fermi level (upper level), hence feeding photons back by stimulated emission resonant with the gap mode. The system reflects many essential features of a superluminescent light emitting diode.

Plasmonic oligomers in cylindrical vector light beams

Summary We investigate the excitation as well as propagation of magnetic modes in plasmonic nanostructures. Such structures are particularly suited for excitation with cylindrical vector beams. We study magneto-inductive coupling between adjacent nanostructures. We utilize high-resolution lithographic techniques for the preparation of complex nanostructures consisting of gold as well as aluminium. These structures are subsequently characterized by linear optical spectroscopy. The well characterized and designed structures are afterwards studied in depth by exciting them with radial and azimuthally polarized light and simultaneously measuring their plasmonic near-field behavior. Additionally, we attempt to model and simulate our results, a project which has, to the best of our knowledge, not been attempted so far.

Confocal and near-field spectroscopic investigation of P3HT:PCBM organic blend film upon thermal annealing

The poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) organic films are widely employed as electronic donor and acceptor in the field of organic film solar cell because of their high photovoltaic conversion efficiency. A home-built parabolic mirror assisted confocal and apertureless near-field optical microscope was used to investigate the degradation behavior of the film and to distinguish the donor and acceptor domains both topographically and optically. Under ambient condition, the degradation rates are decreased in the sequence of pristine P3HT, blend P3HT:PCBM film and pristine PCBM. N2 protection dramatically slows down the film degradation rate. Using confocal spectroscopic mapping, we are able to distinguish the local distributions of P3HT and PCBM. Micrometer PCBM aggregates were observed due to the thermal annealing process. Our experimental methods show the possibility to investigate morphology and the photochemistry properties of the organic solar cell films with high spatial resolution.

Extending the functions of scanning near-field optical microscopy

Advanced optical setups are continuously developed to gain deeper insight into microscopic matter. In this paper we report the expansion of a home-built parabolic mirror assisted scanning, near-field optical microscope (PMSNOM) by introducing four complementary functions. 1) We integrated a scanning tunneling feedback function in addition to an already existent shear-force feedback control mechanism. Hence a scanning tunneling microscope (STM)-SNOM is realized whose performance will be demonstrated by the tip-enhanced Raman peaks of graphene sheets on a copper substrate. 2) We integrated an ultrafast laser system into the microscope which allows us to combine nonlinear optical microscopy with hyperspectral SNOM imaging. This particular expansion was used to study influences of plasmonic resonances on nonlinear optical properties of metallic nanostructures. 3) We implemented a polarization angle resolved detection technique which enables us to analyze the local structural order of α-sexithiophene (α-6T). 4) We combined scanning photocurrent microscopy with the microscope. This allows us to study morphology related optical (Raman and photoluminescence) and electrical properties of optoelectronic systems. Our work demonstrates the great potential of turning a SNOM into an advanced multifunctional microscope.

Novel Parabolic Mirror Microscope Illuminated with Cylindrical Vector Beams for Confocal and Tip Enhanced Super Resolution Imaging

Despite the popular employment of objective lenses as focusing devices, parabolic mirror (PM) is intensively used in quite different scopes of science. For instances, mirrors with a small numerical aperture (NA) are normally used in astronomic telescopes and in telecommunications. Some groups also use high NA PMs in single molecule optics as efficient light collecting element [1-5]. In principle PMs are the perfect focusing elements: a parallel incoming beam along the optical axis of the PM can be focused to one point without any aberration. Furthermore by choosing proper deflecting coating material, PM is free from chromatic aberrations and can be used over wide optical frequencies. Nevertheless they are seldom used in imaging applications because of their bad off-axis properties. A small deviation from parallelism of the incoming beam or from the optical axis leads to tremendous aberrations and results in a small field of view. In confocal microscopy the sample is imaged point by point and thus is the perfect optical arrangement for a PM. Especially when combined with a scanning stage, so that the mirror stays perfect aligned with respect to the incident beam. Based on our experiences in the last years, we will demonstrate the integration of a PM in confocal and super resolution optical imaging systems. Applications in single molecule and particle imaging, as well as Raman scattering mapping of molecular monolayer will be shown.

Plasmon-enhanced light emission from an optically pumped bias-controlled tunneling junction (Conference Presentation)

Invited Ultra-small light sources which can be controlled electrically or optically are of great interest to nano-photonics. Electroluminescence induced by inelastic tunneling from an STM-junction is such an example, however hampered due to a small quantum efficiency. We observe enhanced emission of photons from such a tunneling junction by almost three orders of magnitudes by additionally exciting a gap-plasmon oscillation with laser irradiation. Either a pristine Au-substrate/Au-tip tunneling junction or a junction (Au-substrate/self-assembled molecular monolayer/Au-tip) with molecules chemically bound to the Au substrate is used. Analyzing the emission spectra from the junction recorded as a function of bias voltage for the Au-Au junction we conclude that the enhanced intensity is induced by laser illumination and originates from the radiative decay of hot electrons closely above the Fermi level via inelastic tunneling into the plasmon modes formed by the tip-substrate gap. In the presence of molecules in the gap, we observe a bias dependent spectral narrowing characteristic for superluminescence. The optically pumped molecular junction behaves as a bias-driven point source, operating at ambient conditions and providing almost three orders of magnitude higher electron-to-photon conversion efficiency than electroluminescence induced by inelastic tunneling without optical pumping. The enhanced emission can be modeled by rate equations taking into account the hole-injection from the tip (anode) into the highest occupied orbital of the closest substrate bound molecule (lower level) and radiative recombination with an electron from above the Fermi-level (upper level), hence feeding photons back by stimulated emission resonant with the gap mode.