Alfred J. Meixner

Double-pulse technique as an electrochemical tool for controlling the preparation of metallic nanoparticles

Various structures of silver nanoparticles were generated by means of the double-pulse technique. The interaction of the pulse parameters was modeled to demonstrate that the electrodeposition of nanoparticles can be variably controlled. The results derived showed how to create nanoparticle structures with respect to size, density and monodispersity according to the application goal.

A high numerical aperture parabolic mirror as imaging device for confocal microscopy

We explore the diffraction limited focusing and confocal imaging properties of a high NA parabolic mirror for confocal imaging and spectroscopy of nanoparticles and single molecules. Vector field calculations of the electric fields near focus for both linear and radially polarized illumination are discussed and show that the optical field can be similar tightly focused as in the case of a high NA objective lens. Furthermore they show that a high NA parabolic mirror allows an easy orientation of the polarization of the illuminating light in all spatial directions. The simulation of confocal imaging of single molecules is discussed and yields, that the use of radially polarized excitation light gives an easy access to their orientations.

Plasmonic Coupling of Bow Tie Antennas with Ag Nanowire

Ag nanowire with the receiving and transmitting Ag bow tie antenna pairs at its incident and emission ends was patterned on the SiO(2) substrate to realize an enhanced surface plasmon emission with a factor of 45 compared to the single Ag nanowire without antenna pairs. The receiving and transmitting bow tie antenna pairs enhanced the plasmon coupling and emission efficiencies of the Ag nanowire. And the maximum plasmon emission sensitively depended on the length of Ag nanowire, the arm length of bow tie antennas, and the incident angle of optical excitation. This enhanced plasmon emission was confirmed by finite-difference time-domain simulations and explored with analytical calculations using the impedance matching theory at optical frequency.

Temperature profile of fiber tips used in scanning near‐field optical microscopy

We have measured the temperature profile of aluminum coated fiber tips used for illumination‐mode scanning near‐field optical microscopy as a function of the optical input power with a micron sized thermocouple. The temperature coefficients vary from 20 K/mW for tips with a large cone angle to 60 K/mW for the narrow long ones. Temperatures of up to ≊470 °C have been measured close to the aperture with an optical input power of several mW before thermal damage of the coating occurred. The temperature profiles are analyzed theoretically taking into account the optical absorption, the thermal conductivity of the tip, as well as the heat loss to the environment.

The Histidine Kinase AHK5 Integrates Endogenous and Environmental Signals in Arabidopsis Guard Cells

Background Stomatal guard cells monitor and respond to environmental and endogenous signals such that the stomatal aperture is continually optimised for water use efficiency. A key signalling molecule produced in guard cells in response to plant hormones, light, carbon dioxide and pathogen-derived signals is hydrogen peroxide (H2O2). The mechanisms by which H2O2 integrates multiple signals via specific signalling pathways leading to stomatal closure is not known. Principal Findings Here, we identify a pathway by which H2O2, derived from endogenous and environmental stimuli, is sensed and transduced to effect stomatal closure. Histidine kinases (HK) are part of two-component signal transduction systems that act to integrate environmental stimuli into a cellular response via a phosphotransfer relay mechanism. There is little known about the function of the HK AHK5 in Arabidopsis thaliana. Here we report that in addition to the predicted cytoplasmic localisation of this protein, AHK5 also appears to co-localise to the plasma membrane. Although AHK5 is expressed at low levels in guard cells, we identify a unique role for AHK5 in stomatal signalling. Arabidopsis mutants lacking AHK5 show reduced stomatal closure in response to H2O2, which is reversed by complementation with the wild type gene. Over-expression of AHK5 results in constitutively less stomatal closure. Abiotic stimuli that generate endogenous H2O2, such as darkness, nitric oxide and the phytohormone ethylene, also show reduced stomatal closure in the ahk5 mutants. However, ABA caused closure, dark adaptation induced H2O2 production and H2O2 induced NO synthesis in mutants. Treatment with the bacterial pathogen associated molecular pattern (PAMP) flagellin, but not elf peptide, also exhibited reduced stomatal closure and H2O2 generation in ahk5 mutants. Significance Our findings identify an integral signalling function for AHK5 that acts to integrate multiple signals via H2O2 homeostasis and is independent of ABA signalling in guard cells.

Tighter focusing with a parabolic mirror

We demonstrate experimentally and theoretically that a parabolic mirror (PM) with a high numerical aperture (NA) of 1 focuses a radially polarized laser mode to the smallest diffraction-limited spot at a fixed NA and wavelength, having an area of 0.134 lambda(2). The measurements were performed with a confocal microscope, using the PM as a focusing and collecting element. The results stand in accordance with the theoretical calculations presented by Davidson and Bokor [Opt. Lett. 29, 1318 (2004)], who predicted a reduction in the total focal spot size of 43% as compared with an aplanatic lens.

Imaging Nanometre‐Sized Hot Spots on Smooth Au Films with High‐Resolution Tip‐Enhanced Luminescence and Raman Near‐Field Optical Microscopy

A novel near-field optical microscope based on a parabolic mirror is used for recording high-resolution tip-enhanced photoluminescence (PL) and Raman images with unprecedented sensitivity and contrast. The measurements reveal small islands on the Au surface with dimensions of only a few nanometres with locally enhanced Au PL. These islands appear as nanometre-sized hot spots in tip-enhanced Raman microscopy when benzotriazole molecules adsorbed on the Au surface serve as local sensors for the optical field. The spectra show that localized plasmons are the cause of both the locally enhanced Au PL and enhanced Raman scattering. This finding suggests that the dispersive background in the surface-enhanced Raman spectra can be explained simply by the enhanced Au PL in the gap. Furthermore, our results show that the surface flatness must be better than 1 nm, to provide an optically homogeneous substrate for near-field enhanced PL and Raman spectroscopy.

Holographic detection of photochemical holes

Abstract The application of the holographic detection method to optical hole-burning spectroscopy is investigated. Crossed laser beams were used to induce spatially periodic changes in the spectral hole depth for chlorin (2,3-dihydroporhyrin) molecules embedded in a polyvinylbutyral (PVB) matrix at 4.2 K. It is shown that the holographic technique is a sensitive zero-background method for detecting weak signals associated with photochemical holes and is well suited to determine homogeneous linewidths. The results obtained demonstrate that the holographic technique represents an efficient tool for high-resolution studies in inhomogeneously broadened bands of complex organic molocules.

Direct measurement of standing evanescent waves with a photon-scanning tunneling microscope

We present a detailed analysis of a standing evanescent wave that is caused by total internal reflection of an Ar-ion laser beam on a glass prism and investigate the coupling to a subwavelength dielectric tip of a photon-scanning tunneling microscope that is raster scanned at a close distance over the prism surface. The intensity of the evanescent field is spatially modulated with a period of 239.2 nm. It decays exponentially with a constant of 103.9 nm with increasing distance from the prism surface. Precise measurements of the standing evanescent wave can be used to calibrate the scanner and permit one to determine the spatial resolution and the coupling efficiency of the tip.

Direct imaging single molecule diffusion in a solid polymer host

Abstract The time evolution of the microscopic lateral diffusion and dynamic reorientation of individual dye molecules dispersed in a thin polymer film is probed by means of scanning near-field optical microscopy at room temperature and ambient conditions. On a sub-micrometer scale we identify regions where the diffusion is drastically non-random, while the statistical average of the trajectories of 97 molecules in a 4 μm × 4 μm section displays random-walk behavior.