Stefan Grafström

Scanning force microscopy investigation of the Pb(Zr0.25Ti0.75)O3/Pt interface

We report on a novel approach for the investigation of the Pb(ZrxTi1−x)O3/Pt interface applying scanning force microscopy techniques. Ferroelectric samples (PZT film /Pt/SiO2/Si) were polished at a shallow angle (∼6.1°) thereby enlarging the film cross section from a 430 nm film thickness to a width of more than 4 μm. Piezoresponse force microscopy and Kelvin probe force microscopy were applied in order to deduce the dielectric polarization P and local potential distribution over the full cross section. We clearly observe a transition layer with a thickness of ∼240 nm which manifests itself both in a gradual decrease of the piezoresponse signal as a function of film thickness and in a corresponding variation of the surface potential. Furthermore, after polarization reversal due to a dc voltage applied to the tip, a different retention behavior was observed within the transition layer. The results are tentatively attributed to negatively charged defects accumulated at the PZT/Pt interface.

Surface plasmon transmission across narrow grooves in thin silver films

We report on the direct measurement of surface plasmon transmissivity of narrow grooves in thin silver films using near-field optical microscopy in an attenuated-total-reflection setup. For different groove widths, we observe characteristic changes in transmissivity that are attributed to resonant gap modes. The results are in good agreement with existing theoretical predictions.

Polarization conversion through collective surface plasmons in metallic nanorod arrays

For two-dimensional (2D) arrays of metallic nanorods arranged perpendicular to a substrate several methods have been proposed to determine the electromagnetic near-field distribution and the surface plasmon resonances, but an analytical approach to explain all optical features on the nanometer length scale has been missing to date. To fill this gap, we demonstrate here that the field distribution in such arrays can be understood on the basis of surface plasmon polaritons (SPPs) that propagate along the nanorods and form standing waves. Notably, SPPs couple laterally through their optical near fields, giving rise to collective surface plasmon (CSP) effects. Using the dispersion relation of such CSPs, we deduce the condition of standing-wave formation, which enables us to successfully predict several features, such as eigenmodes and resonances. As one such property and potential application, we show both theoretically and in an experiment that CSP propagation allows for polarization conversion and optical filtering in 2D nanorod arrays. Hence, these arrays are promising candidates for manipulating the light polarization on the nanometer length scale.

FM demodulated Kelvin probe force microscopy for surface photovoltage tracking

The surface photovoltage (SPV) of a structured semiconductor surface is deduced via detection of the contact potential difference measured with Kelvin probe force microscopy (KPFM). Our setup is based on a quantitative KPFM method complemented with modulated laser illumination in order to measure SPV. A high lateral resolution and quantitative values for the SPV are obtained by operating the KPFM in the so-called frequency modulation (FM) mode which is advantageous compared to the amplitude sensitive (AM) mode as proven by our simulations. In contrast to similar studies based on scanning tunnelling microscopy, KPFM offers the clear advantage that there is virtually no electric DC field between tip and sample and, therefore, the SPV is not affected by the presence of the tip.

Probing the nanoscale electro-optical properties in ferroelectrics

We present an approach to inspecting the electro-optical properties of a ferroelectric crystal on the nanometer scale by applying a confined electric field E between a pointed optical fiber and the sample under investigation. Monitoring the optical transmission of barium titanate (BaTiO3) provides a complete image of the ferroelectric domain distribution in a single scan, including also antiparallel domains. The spatial resolution of ∼250 nm in this experiment is determined by the confinement of the electric field.

Three-Dimensional Electric Field Probing of Ferroelectrics on the Nanometer Scale Using Scanning Force Microscopy

Nanoscale investigations of ferroic systems are currently of clue interest in device fabrication and analysis. We show that scanning force microscopy (SFM) is of valuable help in addressing questions of both dynamic and static stability of domains and domain walls. In this contribution polarization sensitive modes of SFM, i.e. piezoresponse force microscopy (PFM) and Kelvin force probe microscopy (KPFM) are contrasted for the internal and external electric field measurements. These techniques provide unique resolution of the sample topography including chemical heterogeneity and the domain wall width, recording of hysteresis loops on the nanometer scale, as well as the transient response when inducing ferroelectric domain switching. Emphasis is laid onto the future possibilities in measuring device limiting physical properties which are related to interface problems. Here SPM tools applied to the local inspection provide unique insight to this problematic.

Impact of optical in-plane anisotropy on near-field phonon polariton spectroscopy

The authors report a spectroscopic near-field investigation using a tunable free-electron laser in combination with a scattering near-field optical microscope. They excite optically uniaxial LiNbO3 close to a phonon resonance in the infrared regime, thereby exciting a phonon polariton resonance in the coupled tip-sample system. They find that the resonance shows a clear dependence on the orientation of the optical axis of the birefringent crystal within the surface plane. This provides evidence that in addition to the dominant contribution of the dipole moment parallel to the tip axis, also the component along the surface is sensed in such a scattering experiment.

Metallic nanorod arrays: negative refraction and optical properties explained by retarded dipolar interactions

We show that the optical properties of arrays of parallel-aligned metallic nanorods can be understood by means of a retarded dipolar interaction model. Exemplarily, arrays of gold nanorods having various lengths and diameters are investigated experimentally. A strong diameter dependence of the long-axis surface plasmon resonance (LSPR) as well as a lower energy limit of this mode for varying length was found. The model also shows that, for small nanorod distances (d<λ/2), the optical properties are independent of the azimuthal angle of the incoming plane wave and of the detailed arrangement of the nanorods. Furthermore, the model was used to explain the dependence of the LSPR on the angle of incidence and to find the conditions for which negative and extraordinary positive refractions occur in these structures.

Near-field spectroscopy with white-light illumination

We report on near-field optical spectroscopy based on the illumination of the sample with white light from a Xe arc lamp through a tapered optical fiber. The light transmitted through the sample is analyzed with a grating spectrometer in the spectral range between 400 and 750 nm. Our setup provides a unique possibility for recording detailed spectroscopic information within a short acquisition time. Near-field spectra acquired on gold clusters measuring 100 nm in diameter and 20 nm in height reveal a wavelength-dependent transmittivity with both reduced and enhanced light intensities probably stemming from surface plasmon excitation.

Lamellar ferroelectric domains in PbTiO3 grains imaged and manipulated by AFM

Abstract Atomic force microscopy in combination with piezoresponse force microscopy are applied to inspect and manipulate the lamellar ferroelectric domains of a non-continuous polycrystalline PbTiO 3 film. A former study showed such films to exhibit a net integral polarization direction with every grain being randomly oriented. However, the results presented here demonstrate a lamellar domain structure inside most of these single crystalline grains which is attributable to 90° domain walls. This lamellar domain distribution might be a result of mechanical strain at the surface and the interface to the substrate as predicted from theoretical calculations for epitaxially grown PbTiO 3 films. In a switching experiment, the domains of a single grain were manipulated, showing that the lamellar structure recovers. This indicates that the lamellar domain arrangement is energetically favored in these samples.