210 research outputs found
Saturable absorption of femtosecond optical pulses in multilayer turbostratic graphene
No full textWe investigate the nonlinear transmission of a ∼280-layer turbostratic graphene sheet for near-infrared amplifier laser pulses (775 nm, Ti:sapphire laser) with a duration of 150-fs and 20-fs. Saturable absorption is observed in both cases, however it is not very strong, amounting to ∼13% transmittance change for the 20-fs (150-fs) pulses at a peak intensity of 30 GW/cm2 (4 GW/cm2). The dependence on incident peak intensity is reproduced well using a theoretical model for the time-dependent saturable absorption, where the excited carriers vacate the photo-excited energy range within 3-5 fs, which we attribute to energy redistribution due to carrier-carrier scattering. This is also supported by spectrally resolved measurements for the 20-fs pulses, which show a marked dependence of the degree of saturation on the photon energy. A key result is that the shorter pulses do not yield a lower saturation fluence, due to the combined effects of the broader excitation bandwidth, and the rapid and broad energy redistribution. We also predict the potential performance of multilayer graphene samples for removing pedestal and pre-pulse structure from ultrafast high-energy pulses
Investigation of nanometer scale charge carrier density variations with scattering-type scanning near-field microscopy in the THz regime
No full textNear-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
Spin-conserving carrier recombination in conjugated polymers
No full textThe ultimate efficiency of polymer light-emitting diodes is limited by the fraction of charges recombining in the molecular singlet manifold. We address the question of whether this fraction can principally exceed the fundamental limit set down by spin statistics, which requires the possibility of spin changes during exciton formation. Sensitized phosphorescence at 4?300 K enables a direct quantification of spin conversion in coulombically bound electron?hole pairs, the precursors to exciton formation. These are stabilized in external electric fields over times relevant to carrier transport, capture and recombination in devices. No interconversion of exciton intermediates between singlet and triplet configurations is observed. Static magnetic fields are equally unable to induce spin mixing in electroluminescence. Our observations imply substantial exchange splitting at all times during carrier capture. Prior statements regarding increased singlet yields above 25% merely on the basis of higher singlet than triplet formation rates should therefore be re-examined
Charge carrier photogeneration in conjugated polymer
No full textThe electric field-assisted charge carrier photogeneration in a Ladder-Type Methyl substituted Poly(Para-Phenylene) was investigated by studying the electromodulated ultrafast transient differential absorption spectra and kinetics. The field stimulated appearance of the polaron absorption and quenching of the singlet exciton absorption proves that dissociation of excitons into geminately bound electron-hole pairs occurs within the entire exciton lifetime and proceeds from the relaxed state. An influence of the optically created excitons and charge pairs on the Stark-shift of the absorption band was observed and interpreted as caused by the modification of the dielectric constant of the medium. The initial separation distance of the charge carriers and its evolution in time were determined from the Stark-shift dynamics. The initial separation distance was obtained being slightly larger than the distance between adjacent polymer chains
Terahertz single pixel imaging based on a Nipkow disk
No full textWe describe a terahertz single pixel imaging system based on a Nipkow disk. Nipkow disks have been used for fast scanning imaging systems since the first experimental television was invented in 1926. In our work, a Nipkow disk with 24 scanning lines was used to provide an axial resolution of 2 mm/pixel. We also show that by implementing a microscanning technique the axial resolution can be further improved to 0.5 mm/pixel. Imaging of several objects was demonstrated to show that this simple scanning system is promising for fast and/or real time terahertz imaging applications
Broadside-coupled triangular split-ring-resonators for terahertz sensing
No full textIn this study, broadside-coupled triangular split-ring-resonators are designed and simulated as THz sensors. Their double-side sensing capability provides sensitivity enhancement compared with singlesided sensors. The material to be detected is modeled as an over-layer on the structure. The ensuing change of the transmission resonances is investigated as a function in the thickness and permittivity of the substrate and of the one- or two-sided over-layer. The propagation direction of the incident wave also allows to select the excited mode (via the polarization of the radiation) which provides additional flexibility to the proposed sensor device by the possibility to operate it over a larger frequency range. Overall, the polarization dependency of the structure, the double-side sensing and the high sensitivity make such structures attractive as THz sensors
Responsivity at 0.27 THz of a heterostructure field effect transistor detector in a quasi-optical package
No full textWe have fabricated AlGaAs/InGaAs/AlGaAs heterostructure field effect transistors (HFET) with integrated on-chip antennas and we have measured their optical responsivity when mounted in a in-house developed quasi-optical package with a silicon substrate lens © 2013 IEEE
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