Ulrich Fischer
University of Münster, Physics, Alumnus
We demonstrate that scanning near-field optical microscopy based on a probe with a triangular aperture is capable of imaging single fluorescent molecules with an optical resolution of 30 nm. Numerical simulations agree well with experiment.
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A dynamic force distance control for scanning near-field optical microscopy (SNOM) based on a quartz tuning fork as piezoelectric force sensor is introduced. In contrast to a similar design for shear-force feedback, the tuning fork is... more
A dynamic force distance control for scanning near-field optical microscopy (SNOM) based on a quartz tuning fork as piezoelectric force sensor is introduced. In contrast to a similar design for shear-force feedback, the tuning fork is aligned in such a way that forces perpendicular to the surface are detected. Various near-field probes can be attached to the end of the tuning fork and serve as force sensing tip. The high force sensitivity is demonstrated for two different near-field probes by imaging the topography of organic samples. The tetrahedral tip, an apertureless high-resolution near-field probe used up to now mainly with tunnel current distance control, is for the first time successfully combined with a force distance control for SNOM. A similar distance control was used in conjunction with an only few millimeters short aluminum-coated tapered fiber tip as near-field probe. The suitability of this design for near-field optical fluorescence imaging is demonstrated. Furthermore, the dynamic force d...
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Scanning Near-field Optical Microscopy (SNOM) allows the investigation of optical properties on subwavelength scales. During the past few years, more and more attention has been given to this technique that shows enormous potential for... more
Scanning Near-field Optical Microscopy (SNOM) allows the investigation of optical properties on subwavelength scales. During the past few years, more and more attention has been given to this technique that shows enormous potential for imaging, sensing and modification at near-molecular resolution. This article describes the technique and reviews recent progress in the field.
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This work is devoted to the analysis of the shape of transmission spectra of composite systems including a thin adsorbed layer of absorbing dye molecules. It is demonstrated, that different from the often observed absorption... more
This work is devoted to the analysis of the shape of transmission spectra of composite systems including a thin adsorbed layer of absorbing dye molecules. It is demonstrated, that different from the often observed absorption characteristics in the transmission spectra as determined by the imaginary part of the layer’s dielectric function, the transmission spectra are in general determined by coherent light scattering of the molecules in the layer and an interference of this scattered light with the incident light. As a result, the shape of the dispersion of both, imaginary and real parts of the layer’s dielectric function, determines the transmission spectrum. Such spectra cannot be constructed from the transmission spectra of the initial system and the deposited film alone. It is shown that the shape of the transmission spectra of the composite films is described in general by an asymmetrical line like a Fano line and that the origin of such a shape can be associated with the physics underlying a Fano re...
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The Raman signal can be strongly increased for molecules deposited on a thin metal film in a configuration where the emission of the Raman signal into a glass support of the film is mediated by leaky surface plasmons (SPs). Here we show... more
The Raman signal can be strongly increased for molecules deposited on a thin metal film in a configuration where the emission of the Raman signal into a glass support of the film is mediated by leaky surface plasmons (SPs). Here we show that SP provides an efficient pathway to detect tip enhanced Raman scattering (TERS). Based on this result, we
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Increased transmission of a thin gold film by adsorbed layers of dye molecules. [Applied Physics Letters 80, 3715 (2002)]. Ulrich C. Fischer, Eugene Bortchagovsky, Jörg Heimel, René T. Hanke. Abstract. Thin multilayers of a negatively ...
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ABSTRACT Localized surface plasmons can be used to control near-field optical phenomena in the subwavelength range. Specifically, this chaper reviews recent results which show that localized surface plasmons can confine the optical... more
ABSTRACT Localized surface plasmons can be used to control near-field optical phenomena in the subwavelength range. Specifically, this chaper reviews recent results which show that localized surface plasmons can confine the optical intensity down to nanoscopic dimensions. The discussion first considers how a collection-mode near-field optical microscope can observe the squeezing of the plasmon field of metallic nanostructures deposited on a flat surface. Numerical simulations then provide illustrations of the confined fields associated with nanostructures which are feasible using current microfabrication techniques. Finally, we present arguments which explain how localized surface plasmons can deliver a significant amount of power to the very end of a tetrahedral tip used as the light source of an illumination-mode near-field optical microscope.
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Research Interests: Materials Engineering, Condensed Matter Physics, Materials Science, Optics, Microscopy, and 15 moreScanning Probe Microscopy, Photonics, Nanotechnology, Medicine, Scanning tunneling microscopy, Gold, Light, Surface plasmon resonance, Nanostructure, Wavelength, Photons, Optical Microscope, Biochemistry and cell biology, Plasmon, and Excitation
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By near field optics the diffraction limit of light microscopy can be avoided. Contact imaging by energy transfer is a simple scheme to achieve this goal. Scanning Near Field Optical Microscopy (SNOM) using a tapered metal coated fibre... more
By near field optics the diffraction limit of light microscopy can be avoided. Contact imaging by energy transfer is a simple scheme to achieve this goal. Scanning Near Field Optical Microscopy (SNOM) using a tapered metal coated fibre with an aperture at the tip seems to reach a resolution limit at 30 nm. Alternative probes offer the opportunity to extend the resolution to the nm range. The concept of the tetrahedral tip as an efficient SNOM probe and its performance in a hybrid SNOM/STM (Scanning Tunneling Microscope) mode is described, where optical images of 6 nm resolution can be obtained.
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We realized a combination of a scanning near field optical (SNOM) and a scanning tunneling microscope (STM) using the tetrahedral tip as a probe. The SNOM and the STM signal are acquired simultaneously during the scan. In the STM mode... more
We realized a combination of a scanning near field optical (SNOM) and a scanning tunneling microscope (STM) using the tetrahedral tip as a probe. The SNOM and the STM signal are acquired simultaneously during the scan. In the STM mode atomic resolution on pyrolytic graphite is routinely obtained. Simultaneous SNOM/STM investigations of thin silver films evaporated on glass show a lateral resolution of 6 nm in the near field optical signal. Absorption contrast in the optical image is obtained in images of evaporated silver films as well as of patches of purple membrane deposited on indium tin oxide (ITO) as a substrate.
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Functionalization, bringing new functions to standard approaches, is used in different fields of science. Functionalized probes have found wide application in atomic force microscopy, especially in force spectroscopy and for the specific... more
Functionalization, bringing new functions to standard approaches, is used in different fields of science. Functionalized probes have found wide application in atomic force microscopy, especially in force spectroscopy and for the specific recognition of single molecules. At the same time they have found only minor application in scanning near-field optical microscopy in spite of the possibility to obtain new information. This article considers examples of the use of functionalized tips in near-field optics and emphasizes the new approach based on tip-enhanced Raman scattering (TERS) with functionalized tips. The concept of Raman probe is described considering some possible perspectives as well as the demonstration of an internal standard in TERS based on a functionalized tip.
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We use a tetrahedral glass fragment which is coated with a thin film of gold (T-tip) to form a local tip source by illuminating the tip from within its transparent glass body. We demonstrate the potential of this apertureless tip source... more
We use a tetrahedral glass fragment which is coated with a thin film of gold (T-tip) to form a local tip source by illuminating the tip from within its transparent glass body. We demonstrate the potential of this apertureless tip source for tip-enhanced Raman scattering (TERS) as an alternative to the standard scheme with an external illumination. We found that T-tips with different optical properties result from the process of tip fabrication. For some tips, the Raman signal persists for a small number of molecules picked up by the retraction of the tip from a tunnel contact. In these cases, the tip itself gives rise to the TERS signal without a gold substrate close to the tip. Other tips only exhibit a TERS signal when there is a contact to a gold substrate. Tips covered by Raman active nonresonant molecules serve as a new kind of near-field Raman probe for local properties of a sample. As the Raman spectrum of the coated tip is sensitive to the local light intensity, it may be used to monitor the field-enhancement properties of metal films and metal nanostructures. A near-field Raman probe avoids problems due to bleaching of a fluorescent near-field probe and, furthermore, such a near-field Raman probe may be highly selective for specific surface properties. Copyright © 2009 John Wiley & Sons, Ltd.
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A triangular nanoaperture in an aluminium film was used previously as a probe in a scanning near-field optical microscope to image single fluorescent molecules with an optical resolution down to 30 nm. The high-resolution capability of... more
A triangular nanoaperture in an aluminium film was used previously as a probe in a scanning near-field optical microscope to image single fluorescent molecules with an optical resolution down to 30 nm. The high-resolution capability of the triangular aperture probe is because of a highly confined spot of the electric near field which emerges at an edge of the aperture, when the incident light is polarized perpendicular to this edge. Previous numerical calculations of the near-field distribution of a triangular aperture in a planar metal film using the field susceptibility technique yielded a nearly quantitative agreement with the experimental results. Using the same numerical technique we now explored the possibility for a further confinement of the electric near field and an increase in its intensity by modifications of the form of a triangular aperture. By introducing a kink on an edge pointing into the aperture, an arrow-shaped aperture is formed with one convex and three concave metal corners. It turns out that this form leads to a substantial further confinement of the near-field intensity at the convex corner. By extending the wings of this arrow-shaped aperture a further 5-fold increase of the intensity can be obtained without a deterioration of the confined spot.
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The need for system miniaturization and fast data processing has recently motivated the rapid expansion of nanophotonics and research in the manipulation of surface plasmons. These electromagnetic waves are intrinsically bound to... more
The need for system miniaturization and fast data processing has recently motivated the rapid expansion of nanophotonics and research in the manipulation of surface plasmons. These electromagnetic waves are intrinsically bound to metal-dielectric interfaces and can be confined and guided at deep subwavelength scale by metal nanostructures, such as plasmonic waveguides and nano-antennas. However, downscaling optical functionalities far beyond the diffraction limit poses challenges in dynamically controlling plasmon localization and directionality. Here, we present the tunable directional coupling of surface plasmons by transferring the spin momentum of light to the extrinsic angular momentum of the guided surface waves within sharply bent trajectories. On the basis of such a spin-to-orbit conversion, we demonstrated experimentally the selective non-radiative addressing of two nano-antennas connected at the ends of a curved nanoscale metallic waveguide, with a surface plasmon directio...
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In scanning near field optical microscopy (SNOM), a tip whose dimensions are small compared with the wavelength serves as a light-emitting antenna. Light is transmitted from a source via a waveguide structure to this antenna. An important... more
In scanning near field optical microscopy (SNOM), a tip whose dimensions are small compared with the wavelength serves as a light-emitting antenna. Light is transmitted from a source via a waveguide structure to this antenna. An important part of an SNOM probe — apart from the detailed structure of the tip — is the link between the macroscopic waveguide and the antenna of subwavelength dimensions. A partially metal-coated tetrahedral tip is considered to be a structure fulfilling these functions. Motivations for the choice of this structure are models of metal waveguide structures without cutoff and the fact that surface plasmons can be excited on planar metal films and along edges as well as on tips. Aspects of probe fabrication and first results of tests of their optical properties are reported.
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This work is devoted to the analysis of the shape of transmission spectra of stratified systems with a deposited thin adsorbed layer. It is demonstrated, that the transmission spectra are in general determined by coherent light scattering... more
This work is devoted to the analysis of the shape of transmission spectra of stratified systems with a deposited thin adsorbed layer. It is demonstrated, that the transmission spectra are in general determined by coherent light scattering of the molecules in the deposited layer and interference of this scattered light with the incident one. As a result, the shape of the dispersion of both imaginary and real parts of the layer's dielectric function determines the transmission spectrum instead of only the imaginary part as in the case of transmission spectra of adsorbed layers on a transparent bulk substrate. The fact that such spectra can not be constructed from the absorption spectra of the stratified system and the deposited film alone was often the reason for seeking of new interactions in such systems. We demonstrate that this non additive property of the spectra has to be expected for such systems in general also in the absence of such interactions and that the position of the minimal transmission in a normalized spectrum does not correspond to the position of an absorption line. It is shown that the shape of such spectra is described by an asymmetrical line like a Fano line and that the origin of such a shape can be associated with the physics underlying a Fano resonance. A new approach for the determination of the dielectric function of deposited films is proposed on the basis of the presented analysis. It is demonstrated experimentally and theoretically that the deposition of a thin absorbing layer can give rise to an increase in transmission of an initial system. Conditions of such an increase are presented.
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A new geometry and functional principle for submicroscopic sensors and light sources as potential probes for near field microscopy is presented. The basic idea is to use cleaved semiconductor crystals as probe material. Two advantages are... more
A new geometry and functional principle for submicroscopic sensors and light sources as potential probes for near field microscopy is presented. The basic idea is to use cleaved semiconductor crystals as probe material. Two advantages are envisaged: first, the fact that by cleaving crystalline material very sharp and well defined edges and perhaps corners can be fabricated, and secondly, the optoelectronic conversion can be integrated into the probe itself. Micrographs of a light-emitting structure and the related current-voltage characteristics are shown. The structure is formed by a Schottky contact consisting of GaAs as the semiconductor and aluminium as the metallic contact material on two adjacent cleavage faces. The emission from the edge can be tentatively explained as the result of an avalanche breakdown effect in the semiconductor due to the high electric fields between the narrow contacts with distances of about 100 nm.
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The elastic scattering efficiency from subwavelength-size surface holes or protrusions depends sensitively on the dielectric properties of their immediate environment. Scanning near-field optical microscopy exploits this effect to create... more
The elastic scattering efficiency from subwavelength-size surface holes or protrusions depends sensitively on the dielectric properties of their immediate environment. Scanning near-field optical microscopy exploits this effect to create optical images whose resolution is not restricted by the diffraction limit. Typically, 20 nm laterally and, when operated in a topographic mode, 0.1 nm in height can be resolved. Images were obtained both in transmission and reflection.
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The concept of surface-enhanced spectroscopy is applied to the detection of small changes in the optical properties of the microenvironment of a single small scattering object. Apertures of a diameter of 0.18–0.48 μm in thin films of... more
The concept of surface-enhanced spectroscopy is applied to the detection of small changes in the optical properties of the microenvironment of a single small scattering object. Apertures of a diameter of 0.18–0.48 μm in thin films of silver or gold supported by a glass slide are used as microprobes to detect the properties of an adjacent liquid. Resonant conditions for enhanced light scattering are obtained, which allow for a very sensitive measurement of refractive-index differences of the order of 10−4 in a small volume determined by the aperture size. Enhanced quenching of scattering from an aperture by dissolved dye molecules is introduced as a new type of surface-enhanced spectroscopy. Conditions were found that lead to an enhancement of fluorescence intensity of a Rhodamine solution in the probe volume by a factor of 40. It is suggested that these enhanced processes be used to detect time-dependent processes in the vicinity of the apertures such as fluctuations in the population of the small number of dye molecules within the probe volume of the apertures.
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On the basis of the previous considerations [E. G. Bortchagovsky and U. C. Fischer, J. Chem. Phys. 117, 5384 (2002)] we analyze here the shape of the spectral lines created in transmission spectra by thin films deposited on a substrate.... more
On the basis of the previous considerations [E. G. Bortchagovsky and U. C. Fischer, J. Chem. Phys. 117, 5384 (2002)] we analyze here the shape of the spectral lines created in transmission spectra by thin films deposited on a substrate. We demonstrate that such lines have as a rule the shape of an asymmetrical Fano line with the minimum shifted
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Research Interests: Chemistry, Carotenoids, Medicine, Biological Sciences, Vitamin A, and 15 morePhysical sciences, Light, Bacteriorhodopsin, Retinal, CHEMICAL SCIENCES, Lysine, Amino Acid Profile, Hydrogen-Ion Concentration, Protons, Halobacterium, Apoproteins, Isomerism, Ultraviolet Rays, protonation, and Chemical Phenomena
Experimental results are interpreted, which were obtained by tip enhanced Raman Spectroscopy (TERS) in a specific SNOM (Scanning Near-Field Optical Microscope) STM (Scanning Tunneling Microscope) configuration with a tunnel gap. The... more
Experimental results are interpreted, which were obtained by tip enhanced Raman Spectroscopy (TERS) in a specific SNOM (Scanning Near-Field Optical Microscope) STM (Scanning Tunneling Microscope) configuration with a tunnel gap. The interpretation is performed in terms of a classical physical model of a photon as a vacuum phonon polariton. The metal tip is considered as a conductive infinitely thin shell of a diamagnetic electron gas. The stationary single electron tunnel current of an electron tunnel passage frequency f tunnel carries not the charge e but is shown to carry an energy hf e and an isotropic orbital angular momentum h/4π which corresponds to an isotropic transversal electron spin density of an angular frequency 2π 2 f e .Raman Spectroscopy, Electron Spin, Photon Spin, relativistic Quantum mechanicsThe electron spin current corresponds to an isotropic azimuthal inertial torque which acts on the photon by an increase of its energy and torque. Neglecting the influence o...
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We recently performed experiments on the transfer of photon spin to electron orbital angular momentum. For an interpretation of the experimental results we used a classical electrodynamic model of the photon as a propagating... more
We recently performed experiments on the transfer of photon spin to electron orbital angular momentum. For an interpretation of the experimental results we used a classical electrodynamic model of the photon as a propagating electromagnetic solitary wave which is developed in detail here. A linearly polarized monochromatic photon is considered as a propagating solitary electromagnetic wave of finite energy hf which carries an angular momentum h/2pi with the frequency f and Plancks constant h. This model has, apart from being a tool for an interpretation of our experimental results, far reaching consequences of fundamental relevance and guides us to an outline to a unified quantum theory of electromagnetism and gravitation including an explanation of the electrodynamic origin of the quantized charge of an electron.
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A general concept for parallel near-field photochemical and radiation-induced chemical processes for the fabrication of nanopatterns of a self-assembled monolayer (SAM) of (3-aminopropyl)triethoxysilane (APTES) is explored with three... more
A general concept for parallel near-field photochemical and radiation-induced chemical processes for the fabrication of nanopatterns of a self-assembled monolayer (SAM) of (3-aminopropyl)triethoxysilane (APTES) is explored with three different processes: 1) a near-field photochemical process by photochemical bleaching of a monomolecular layer of dye molecules chemically bound to an APTES SAM, 2) a chemical process induced by oxygen plasma etching as well as 3) a combined near-field UV-photochemical and ozone-induced chemical process, which is applied directly to an APTES SAM. All approaches employ a sandwich configuration of the surface-supported SAM, and a lithographic mask in form of gold nanostructures fabricated through colloidal sphere lithography (CL), which is either exposed to visible light, oxygen plasma or an UV–ozone atmosphere. The gold mask has the function to inhibit the photochemical reactions by highly localized near-field interactions between metal mask and SAM and ...