Dramatic Raman Gain Suppression in the Vicinity of the Zero Dispersion Point in Gas-Filled Hollow-Core Photonic Crystal Fiber

In 1964 Bloembergen and Shen predicted that Raman gain could be suppressed if the rates of phonon creation and annihilation (by inelastic scattering) exactly balance. This is only possible if the momentum required for each process is identical, i.e., phonon coherence waves created by pump-to-Stokes scattering are identical to those annihilated in pump-to-anti-Stokes scattering. In bulk gas cells, this can only be achieved over limited interaction lengths at an oblique angle to the pump axis. Here we report a simple system that provides dramatic Raman gain suppression over long collinear path-lengths in hydrogen. It consists of a gas-filled hollow-core photonic crystal fiber whose zero dispersion point is pressure-adjusted to lie close to the pump laser wavelength. At a certain precise pressure, generation of Stokes light in the fundamental mode is completely suppressed, allowing other much weaker nonlinear processes to be explored.Comment: 4 pages, 5 figure

New device for continuous-wave THz emission: large area emitter

We discuss two different approaches to overcome the power limitations of CW THz generation imposed to conventional photomixers. The increase in power achievable by using arrays of AEs is studied. Then ?large area emitters? are proposed as an alternate approach to overcome the power limitations. In this antenna-free new scheme of photomixing, the THz radiation originates directly from the acceleration of photo-induced charge carriers generated within a large semiconductor area. The quasi-continuous distribution of emitting elements corresponds to a high-density array and results in particularly favorable radiation profiles

From arrays of THz antennas to large-area emitters

Arrays of coherently driven photomixers with antenna (antenna emitter arrays, AEAs) have been evaluated as a possibility to overcome the power limitations of individual conventional photomixers with antenna (?antenna emitters?, AEs) for the generation of continuous-wave (CW) THz radiation. In this paper, ?large area emitters? (LAEs) are proposed as an alternative approach, and compared with AEAs. In this antenna-free new scheme of photomixing, the THz radiation originates directly from the acceleration of photo-induced charge carriers generated within a large semiconductor area. The quasi-continuous distribution of emitting elements corresponds to a high-density array and results in favorable radiation profiles without side lobes. Moreover, the achievable THz power is expected to outnumber even large AEAs. Last not least, the technological challenge of fabricating LAEs appears to be significantly less demanding

Parametrisch Kontrollierte Raman-Streuung in Hohlkernfasern

When a laser pump beam of sufficient intensity is incident on a Raman-active medium such as hydrogen gas, a strong Stokes signal, red-shifted by the Raman transition frequency Ω, is generated. This is accompanied by the creation of a “coherence wave” of synchronized molecular oscillations with a wavevector determined by the optical dispersion. Within its lifetime, this coherence wave can be used to shift by Ω the frequency of an arbitrary third optical signal, provided phase-matching is satisfied. The work presented in this thesis investigates how the unique interplay of gas dispersion and modal fiber dispersion in kagomé-type hollow-core photonic crystal fiber (kagomé-PCF) can be tailored to achieve perfect phase-matching under various conditions, making the system suitable for different applications such as frequency or mode conversion. Compared to established techniques, where crossed-beam configurations have to be used, the use of kagomé-PCF enables a fully collinear propagation regime, which along with the tight modal confinement drastically reduce the required pump intensities while still achieving record high conversion efficiencies. We show that in standard kagomé-PCF, broadband phase-matched frequency shifting by Ω is possible in the whole spectral range from the ultraviolet to the near-infrared by operating only with the fundamental LP01 fiber mode. Optionally intermodal scattering can be utilized to simultaneously modify the transverse spatial pattern of the generated light field in a fully controllable way. Furthermore, a novel hybrid waveguide design extends the spectral range of operation even down to the THz-frequency range, making the system a suitable platform for a new type of THz-source. Beyond that, we demonstrate strong suppression of the Raman gain by phase-matching the coherence wave generated by the pump/Stokes fields to the corresponding pump/anti-Stokes transition, an effect which was already predicted in 1964, but so far only observed phenomenologically. The experimental results are complemented by theoretical discussion assisted by numerical analysis that apart from bringing in-depth understanding of the observed phenomena, might also help to design and optimize further schemes like Raman frequency combs, Raman lasers or Raman spectroscopy systems.Wenn ein ausreichend intensiver Pumplaserstrahl auf ein Raman-aktives Medium wie etwa Wasserstoff trifft, entsteht ein ausgeprägtes Stokes Signal, welches um die Raman Übergangsfrequenz Ω rotverschoben ist. Dieser Vorgang wird begleitet von der Anregung einer „kohärenten Welle“ (Englisch: coherence wave) von synchronen molekularen Schwingungen mit einem Wellenvektor der durch die optische Dispersion festgelegt ist. Innerhalb ihrer Lebenszeit, erlaubt die coherence wave die Frequenz eines beliebigen, dritten optischen Signals um Ω zu verschieben - unter der Vorraussetzung, dass eine korrekte Phasenanpassung gegeben ist. Diese Arbeit untersucht, wie das einzigartige Zusammenspiel von Gas- und Modendispersion in Kagomé-Hohlkernfasern genutzt werden kann, um unter verschiedenen Bedingungen perfekte Phasenanpassung zu erreichen. Dies ermöglicht die Nutzung dieses Systems für Anwendungen wie zur Frequenz- oder Modenkonversion. Verglichen mit etablierten Techniken (wie einer gekreuzten Strahlführung) erlauben Kagomé-Fasern vollständige kollineare Strahlführung und eine stark gebundene Mode. Dadurch reduziert sich enorm die benötigte Pumpstrahlintensität bei gleichzeitig unerreicht hoher Konversionseffizienz. Wir zeigen, dass in gewöhnlichen Kagomé-Fasern eine breitbandige, phasenangepasste Frequenzverschiebung um Ω im ganzen Spektralbereich vom Ultravioletten bis hin zum nahen Infraroten möglich ist. Dabei verwenden wir ausschließlich die fundamentale LP01 Mode. Alternativ können wir intermodale Streuung verwenden, um die transversale Struktur des erzeugten Lichtfelds in kontrollierter Weise zu verändern. Außerdem erweitert ein entwickeltes Konzept für einen neuartigen Hybridwellenleiter den nutzbaren Frequenzbereich bis in den THz-Frequenzbereich. Dieser Wellenleiter stellt daher eine Technologieplatform für neuartige THz-Quellen dar. Des Weiteren weisen wir eine ausgeprägte Unterdrückung der Raman-Vertärkung nach, indem die durch die Pumpe/Stokes Felder generierte coherence wave an den Pumpe/anti-Stokes Übergang phasenangepasst wird. Dieser Effekt wurde bereits 1964 vorhergesagt, aber bisher nur phänomenologisch nachgewiesen. Die experimentellen Ergebnisse werden durch eine theorethische Diskussion, sowie durch numerische Analysen komplementiert. Dies ermöglicht ein tiefgehendes Verständnis der beobachteten Phänomene und kann helfen weitere Anwendungen wie Raman-Frequenzkämme, Raman-Laser oder Raman-Spektroskopiesysteme zu entwickeln oder zu verbessern

A gold-nanotip optical fiber for plasmon-enhanced near-field detection

A wet-chemical etching and mechanical cleaving technique is used to fabricate gold nanotips attached to tapered optical fibers. Localized surface plasmon resonances (tunable from 500 to 850 nm by varying the tip dimensions) are excited at the tip, and the signal is transmitted via the fiber to an optical analyzer, making the device a plasmon-enhanced near-field probe. A simple cavity model is used to explain the resonances observed in numerical simulations