Nanoscale Optical Tomography using Volume-scanning Near-field Microscopy

The relationship between sample structure and data in volume-scanning backscattering mode near-field optical microscopy is investigated. It is shown that the three-dimensional structure of a dielectric sample is encoded in the phase and amplitude of the scattered field and that an approximate reconstruction of the sample structure may be obtained

Local characterization of the optical properties of annealed Au films on glass substrates

We present scanning near field microscopy and local spectroscopic characterisation of gold nanoparticles fabricated on glass sodalime cover slides. The nanoislands are fabricated by the thermal annealing of gold thin films. Results are presented for samples annealed at 300 °C, 400 °C, and 500 °C. We study the spectral dependence of the transmittance at the nanoscale level with respect to the nanoislands size, shape, and interparticle distance employing a Scanning Near-field Optical Microscopy

Theory of a magnetic microscope with nanometer resolution

We propose a theory for a type of apertureless scanning near field microscopy that is intended to allow the measurement of magnetism on a nanometer length scale. A scanning probe, for example a scanning tunneling microscope (STM) tip, is used to scan a magnetic substrate while a laser is focused on it. The electric field between the tip and substrate is enhanced in such a way that the circular polarization due to the Kerr effect, which is normally of order 0.1% is increased by up to two orders of magnitude for the case of a Ag or W tip and an Fe sample. Apart from this there is a large background of circular polarization which is non-magnetic in origin. This circular polarization is produced by light scattered from the STM tip and substrate. A detailed retarded calculation for this light-in-light-out experiment is presented.Comment: 17 pages, 8 figure

Investigation of nanometer scale charge carrier density variations with scattering-type scanning near-field microscopy in the THz regime

Near-field microscopy is a versatile technique for non-destructive detection of optical properties on the nanometer scale. Contrary to conventional microscopy techniques, the resolution in near-field microscopy is not restricted by the diffraction limit, but by the size of the probe only. Typically, wavelength-independent resolution in the range of few ten nanometers can be achieved. Many fundamental phenomena in solid states occur at such small length scales and can be probed by infrared and THz radiation. In the present work, nanoscale charge carrier distributions were investigated with near-field microscopy in classic semiconductors and state-of-the-art graphene field-effect transistors. A CO2 laser, the free-electron laser FELBE at the Helmholtz-Zentrum Dresden Rossendorf and a photoconductive antenna were applied as radiation sources for illumination of the samples. In the theoretical part of the work, the band model for charge carriers in semiconductors is briefly explained to derive typical charge carrier densities of such materials. The influence of the charge carriers to the light-matter interaction is introduced via the Drude model and evaluated for both infrared and THz radiation. In field-effect transistors, charge carrier density waves can occur when strong AC fields are coupled into the device. The phenomena in such transistors are introduced as a more complex material system. To describe the near-field coupling of the samples to the nanoscopic probe, the dipole model is introduced and extended for periodic charge carrier density, as elicited by low repetition-rate excitation lasers. Consequently, sidebands occur as new frequencies in the signal spectrum, allowing for a more sensitive probing of such transient processes. Experimental investigations of these sidebands were performed with a CO2 laser setup on a bulk germanium sample which was excited with femtosecond laser pulses. New frequencies up to the 8th sideband could be observed. The results show a characteristic near-field decay for all sidebands when the probe-sample distance is increased. A nanoscale material contrast in the sidebands signatures has been demonstrated via near-field scans on a gold / germanium heterostructure. Near-field signatures of graphene-field effect transistors have been examined utilizing FELBE. The results match the predicted behavior of charge carriers in such a device and in particular represent the first direct observations of the plasma waves. In collaboration with the group of Prof. Dr. Hartmut G. Roskos (Goethe-Universität Frankfurt), the plasma wave velocity in the graphene field-effect transistor has been derived via fitting to the model for two datasets on different devices from independent fabrications. The obtained velocity is in good agreement with literature values. The results promise the application of field-effect transistors as THz detectors and emitters and may lead to faster communication technology.:1 Introduction 2 Fundamentals 2.1 Semiconductors 2.2 Plasma Waves in Graphene Field-Effect Transistors 2.3 Near-Field Microscopy 2.3.1 Aperture-SNOM 2.3.2 Scattering-SNOM 2.4 THz Optics 3 SNOM-Theory 3.1 Dipole Model 3.2 Detection and Demodulation 3.3 Pump-induced Sidebands in SNOM 3.4 Field Enhancement by Resonant Probes 4 Near-Field Microscope Setups 4.1 FELBE THz SNOM 4.2 Pump-modulated s-SNOM 4.3 THz Time-Domain-Spectroscopy SNOM 5 Sideband Results 5.1 Pump-induced Sidebands in Germanium 5.2 Fluence Dependence 5.3 Higher-order sidebands 5.4 Oscillation Amplitude 5.5 Technical Aspects of the Sideband Demodulation 6 Field-Effect Transistors 6.1 Device Design 6.2 Data Analysis 6.3 Near-Field Overview Scans 6.4 Plasma Wave Examination 6.5 Conclusion 7 Discussion and Outlook A Appendix A.1 Scanning Probe Microscopy A.2 Atomic Force Microscope List of Figures BibliographyNahfeldmikroskopie ist eine vielseite Technik für das zerstörungsfreie Auslesen von optischen Eigenschaften auf der Nanoskala. Im Gegensatz zur konventionellen Mikroskopie ist die Auflösung nicht durch Beugungseffekte, sondern durch die Größe der genutzten Sonde begrenzt. Überlicherweise werden wellenlängenunabhängig Auflösungen von einigen zehn Nanometern erreicht. Viele fundamentale Prozesse in der Festkörperphysik treten auf Längenskalen dieser Größenordnung auf und können mit Infrarot- und THz-Strahlung untersucht werden. In dieser Arbeit wurden nanoskalige Ladungsträgerverteilungen mit Rasternahfeldmikroskopie untersucht, einerseits in klassischen Halbleitern, anderseits in state-of-the-art Graphen Feldeffekttransistoren. Zur Beleuchtung der Proben wurden ein CO2 Laser, der freie-Elektronen Laser FELBE am Helmholtz-Zentrum Dresden-Rossendorf und eine photoleitende Antenne verwendet. Im theoretischen Teil der Arbeit wird das Bändermodell für Ladungsträger in Halbleitern erklärt, um daraus typische Ladungsträgerdichten in diesen Materialien abzuleiten. Der Einfluss der Ladungsträger auf die Interaktion mit Strahlung wird durch das Drude-Modell eingeführt und für Infrarot- und THz-Strahlung abgeschätzt. In Graphen Feldeffekttransistoren können Ladungsträgerdichtewellen auftreten, wenn starke Wechselfelder in das Bauelement eingekoppelt werden. Die Prozesse in solchen Transistoren werden als komplexeres Materialsystem eingeführt. Um die Nahfeldkopplung der Proben an die Sonde zu beschreiben, wird das Dipol-Modell eingeführt und für periodische Ladungsträgerdichten erweitert, wie sie bspw. durch Pumplaser mit niedrigen Repetitionsraten erzeugt werden können. In der Folge entstehen Seitenbänder als neue Frequenzen im Signalspektrum, welche eine sensitivere Messung solcher transienten Prozesse ermöglichen. Experimentelle Untersuchungen des erweiterten Dipol-Modells wurden mit einem CO2 Laser Aufbau an einem Germaniumkristall durchgeführt, welcher mit Femtosekunden Laserpulsen angeregt wird. Neue Frequenzen im Spektrum konnten bis zu dem achten Seitenband beobachtet werden. Die Resultate zeigen den typischen Abfall des Nahfeldes, wenn der Abstand zwischen Sonde und Probe vergrößert wird. Ein Materialkontrast auf der Nanoskale im Seitenband-Signal konnte durch laterale Rasternahfeld-Scans auf einer Gold/Germanium Heterostruktur gezeigt werden. Die Nahfeldsignaturen der Graphen Feldeffekttransistoren wurden mit FELBE untersucht. Die Resultate stimmen mit dem vorausgesagtem Verhalten der Ladungsträger in einem solchen Bauteil überein und sind die erste direkte Beobachtung solcher Plasmawellen. In Kooperation mit der Gruppe um Prof. Dr. Hartmut G. Roskos (Goethe-Universität Frankfurt) wurde die Geschwindigkeit der Plasmawelle durch Regression der Daten berechnet. Dabei wurden zwei Datensätzen an Bauteilen von unabhängigen Fabrikationsprozessen genutzt. Die berechnete Geschwindigkeit ist in guter Übereinstimmung mit Literaturwerten. Die Resultate verheißen die Anwendung von Feldeffekttransistoren als THz Sender und Detektoren und könnten zu schnellerer Kommunikationstechnologie führen.:1 Introduction 2 Fundamentals 2.1 Semiconductors 2.2 Plasma Waves in Graphene Field-Effect Transistors 2.3 Near-Field Microscopy 2.3.1 Aperture-SNOM 2.3.2 Scattering-SNOM 2.4 THz Optics 3 SNOM-Theory 3.1 Dipole Model 3.2 Detection and Demodulation 3.3 Pump-induced Sidebands in SNOM 3.4 Field Enhancement by Resonant Probes 4 Near-Field Microscope Setups 4.1 FELBE THz SNOM 4.2 Pump-modulated s-SNOM 4.3 THz Time-Domain-Spectroscopy SNOM 5 Sideband Results 5.1 Pump-induced Sidebands in Germanium 5.2 Fluence Dependence 5.3 Higher-order sidebands 5.4 Oscillation Amplitude 5.5 Technical Aspects of the Sideband Demodulation 6 Field-Effect Transistors 6.1 Device Design 6.2 Data Analysis 6.3 Near-Field Overview Scans 6.4 Plasma Wave Examination 6.5 Conclusion 7 Discussion and Outlook A Appendix A.1 Scanning Probe Microscopy A.2 Atomic Force Microscope List of Figures Bibliograph

Hyperspectral imaging with scanning near-field optical microscopy: applications in plasmonics

We present the realisation of near-field spectroscopic measurements with fibre-tip-based scanning near-field microscopy. It allows the simultaneous acquisition of near-field images in a broad spectral range (400 nm to 1000 nm), thus recovering local spectroscopic information. This technique is essential in order to understand the resonant interaction of light with nanostructured material as the far-field and nearfield spectral response can differ significantly, e.g., in the case of plasmonic nanostructures. Several example applications of hyperspectral near-field imaging are given for visualisation of Bloch modes in plasmonic crystals and plasmon-assisted transmission through a slit. © 2010 Optical Society of America

Method for increasing sensitivity of shear-force distance control for scanning near-field microscopy

Scanning-near field optical microscopy requires a distance control mechanism. In most cases, it is based on the shear-force detection. In this paper we report how the performance of the shear-force detection based on the most common nonoptical approach, a Quartz tuning fork, can be improved. Our approach is based on exciting oscillations in just one arm of the fork, not two. This approach reduces the response time of the shear-force detection system. We also introduce an ultra-sensitive system with a long free fiber tip. © 1999 Elsevier Science B.V. All rights reserved