Anke Horneber

Nonlinear optical point light sources through field enhancement at metallic nanocones

A stable nonlinear optical point light source is investigated, based on field enhancement at individual, pointed gold nanocones with sub-wavelength dimensions. Exciting these cones with near-infrared, focused radially polarized femtosecond beams allows for tip-emission at the second harmonic wavelength (second harmonic generation, SHG) in the visible range. In fact, gold nanocones with ultra-sharp tips possess interesting nonlinear optical (NLO) properties for SHG and two-photon photoluminescence (TPPL) emission, due to the enhanced electric field confinement at the tip apex combined with centrosymmetry breaking. Using two complementary optical setups for bottom or top illumination a sharp tip SHG emission is discriminated from the broad TPPL background continuum. Moreover, comparing the experiments with theoretical calculations manifests that these NLO signatures originate either from the tip apex or the base edge of the nanocones, clearly depending on the cone size, the surrounding medium, and illumination conditions. Finally, it is demonstrated that the tip-emitted signal vanishes when switching from radial to azimuthal polarization.

Polarization‐Dependent SERS at Differently Oriented Single Gold Nanorods

We investigated the influence of the orientation of individual gold nanorods on the polarization-dependent single-particle surface-enhanced Raman spectroscopy (SERS) of adenine. Higher-order laser beams (radial and azimuthal polarizations) have been used in combination with a parabolic mirror-assisted confocal optical microscope. Based on the photoluminescence (PL) patterns of the single gold nanorods and the simulated electric-field distribution in the focus, we distinguished between isolated gold rods and clusters as well as single nanorods with different orientations. We found that for single gold nanorods lying flat on the substrate, the longitudinal particle plasmon resonance (PPR) mode can be excited more efficiently with the in-plane field component in the focus of an azimuthally polarized laser beam, which enables the observation of stronger enhanced adenine Raman spectra from the single gold nanorods compared to the case of a radially polarized beam.

Local Observation of Phase Segregation in Mixed-Halide Perovskite

Mixed-halide perovskites have emerged as promising materials for optoelectronics due to their tunable band gap in the entire visible region. A challenge remains, however, in the photoinduced phase segregation, narrowing the band gap of mixed-halide perovskites under illumination thus restricting applications. Here, we use a combination of spatially resolved and bulk measurements to give an in-depth insight into this important yet unclear phenomenon. We demonstrate that photoinduced phase segregation in mixed-halide perovskites selectively occurs at the grain boundaries rather than within the grain centers by using shear-force scanning probe microscopy in combination with confocal optical spectroscopy. Such difference is further evidenced by light-biased bulk Fourier-transform photocurrent spectroscopy, which shows the iodine-rich domain as a minority phase coexisting with the homogeneously mixed phase during illumination. By mapping the surface potential of mixed-halide perovskites, we evidence the higher concentration of positive space charge near the grain boundary possibly provides the initial driving force for phase segregation, while entropic mixing dominates the reverse process. Our work offers detailed insight into the microscopic processes occurring at the boundary of crystalline perovskite grains and will support the development of better passivation strategies, ultimately allowing the processing of more environmentally stable perovskite films.

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.

Plasmonon-enhanced Raman spectroscopy: From plasmonic nanogap to SERS and TERS

Summary form only given. Tip-enhanced Raman spectroscopy (TERS) witnesses its active development since more than 15 years. Based on the lightning-rod and plasmonic resonance effects, the generation of strongly confined electromagnetic field within a nanogap between a sharp tip apex and a sample surface under laser illumination even makes the visualization of single molecules possible. Our group has actively contributed to the development of TERS in a variety of aspects, such as low temperature TERS of dye molecules at 10K, and TERS combined with fs- or ps-laser excitation. We have investigated a variety of radiation scenarios from plasmonic nanogaps. To realize a precise control over the nanogap distance down to 1 nm, we combined shear-force and tunneling current feedbacks with confocal optical microscopes. Both electrical and optical excitation can be applied to the plasmonic nanogap. This allows one to study either separately or simultaneously the plasmonic resonance effect and electric DC field on the optical radiation. In this talk, we will report our latest progresses of using TERS 1) to determine the nanometer-sized morphology in organic semiconductor crystalline film and 2) to identify molecular orientation changes within monolayer adsorbates. With this work, we content that TERS has past the prove-of-the-concept phase and has grown into an effective technique finding wide applications.