Spectroscopy of Xenon and Xenon-Noble Gas Mixtures for Bose-Einstein Condensation of Vacuum-Ultraviolet Photons

Bose-Einstein condensation of photons has first been observed in 2010 by the group of Martin Weitz in Bonn. In the experimental scheme, a photon gas is confined to a wavelength-size microcavity filled with a liquid dye solution that exhibits a thermally equilibrated rovibronic level structure. The photon gas is subject to a thermal contact with the dye solution via repeated absorption and reemission cycles; thereby it is driven into thermal equilibrium. Once the photon number exceeds the system’s critical particle number, condensation sets in, with the ground state energy corresponding to the microcavity’s low-frequency cutoff, typically equivalent to around 580 nm wavelength. Besides other intriguing properties, a Bose-Einstein condensate of photons is a source of coherent and monochromatic light circumventing the need for an optically inverted active medium, setting it apart from a laser. Accordingly, a realization in the vacuum-ultraviolet spectral regime (100 nm - 200 nm wavelength) appears particularly alluring, as here the construction of lasers is difficult due to the high pump powers required to achieve population inversion. The present thesis aims at the exploration of xenon as a thermalization mediator for such an application, with this heaviest of all stable noble gases exhibiting a transition wavelength of 146.9 nm on its 5p6 → 5p56s transition. In dense environments, this gas forms transient quasimolecules, replicating several features of dye molecules, such as the emergence of a quasi-molecular manifold of energetic sublevels and an associated Stokes shift between the spectral profiles of absorption and emission. Experimental results of two-photon excitation spectroscopy of the 5p6 → 5p66p and 5p56p' transitions are reported, aiming at the identification of suitable pumping schemes for future vacuum-ultraviolet photon condensates. Both the gaseous and supercritical phases are covered, with sample pressures as high as 95bar. Additionally, results of an experimental scheme are presented, devised to increase the reabsorption of light emitted on the strongly red-shifted second excimer continuum around 172 nm wavelength by providing an auxiliary visible-spectral-range photon field. Corresponding absorption measurements involving a nondegenerate driving of the 5p6 → 5p56p two-photon transitions are reported. Further, results on the spectroscopy of mixtures between xenon and any of the other stable noble gases are presented; such heteronuclear mixtures are considered an alternative candidate for a thermalization mediator. In the samples explored here, xenon contributions are relatively small, around 100 ppm of total sample pressures of up to 90bar. For such samples, absorption and emission spectra are investigated with particular emphasis on the influence of varying xenon and noble gas contributions. The fulfillment of the Kennard-Stepanov relation is assessed, an important indicator for the suitability of a medium as thermalization mediator for photons.Die Bose-Einstein-Kondensation von Photonen wurde erstmalig im Jahr 2010 in der Arbeitsgruppe von Martin Weitz in Bonn beobachtet. Im Experiment wird ein Photonengas in einem Mikroresonator eingeschlossen, welcher eine Größe im Bereich der Wellenlänge aufweist und mit einer flüssigen Farbstofflösung gefüllt ist, welche eine thermisch equilibrierte rovibronische Niveaustruktur besitzt. Über wiederholte Absorptions- und Emissionszyklen steht das Photonengas in thermischem Kontakt mit der Farbstofflösung, wodurch es ins thermische Gleichgewicht überführt wird. Sobald die Photonenzahl die kritische Teilchenzahl des Systems überschreitet, tritt die Bose-Einstein-Kondensation ein, wobei die Grundzustandsenergie durch die untere Abschneidefrequenz des Mikroresonators gegeben ist, welche typischerweise einer Wellenlänge von 580 nm entspricht. Neben anderen faszinierenden Eigenschaften stellt ein Bose-Einstein-Kondensat aus Photonen eine Quelle von kohärentem und monochromatischem Licht dar. Dabei wird die Notwendigkeit eines optisch invertierten aktiven Mediums umgangen, wodurch das System von einem Laser abgegrenzt wird. Entsprechend erscheint eine Realisierung im vakuumultravioletten Spektralbereich (Wellenlängen zwischen 100 nm und 200 nm) besonders reizvoll, da hier die Konstruktion eines Lasers aufgrund der zum Erreichen einer Besetzungsinversion notwendigen hohen Pumpleistungen schwer ist. Die vorliegende Arbeit zielt auf die Untersuchung von Xenon als Thermalisierungsmedium für solch eine Anwendung ab, da dieses schwerste aller Edelgase eine Übergangswellenlänge von 146,9 nm auf seinem 5p6 → 5p56s Übergang aufweist. In dichten Umgebungen formt dieses Gas kurzlebige Quasimoleküle, deren Eigenschaften teilweise jenen von Farbstoffmolekülen ähneln. So bildet sich zum Beispiel eine quasimolekulare Mannigfaltigkeit energetischer Unterniveaus, welche zu einer Stokes-Verschiebung zwischen den spektralen Profilen von Absorption und Emission führt. Experimentelle Ergebnisse der Zweiphotonen-Spektroskopie der 5p6 → 5p66p und 5p56p' Übergänge werden präsentiert; das Ziel ist hierbei passende Anregungsschemata für zukünftige Photonenkondensate im vakuumultravioletten Spektralbereich zu identifizieren. Mit Probendrücken von bis zu 95 bar werden sowohl die gasförmige als auch die superkritische Phase abgedeckt. Zusätzlich werden Ergebnisse eines experimentellen Ansatzes vorgestellt, welcher konzipiert wurde um mithilfe eines zusätzlichen Feldes von sichtbaren Photonen die Reabsorption von Licht zu erhöhen, welches auf dem stark rotverschobenen zweiten Exzimerkontinuum im Wellenlängenbereich um 172 nm emittiert wurde. Entsprechende Absorptionsmessungen unter nicht-entarteter Anregung der 5p6 → 5p56p Zweiphotonen-Übergänge werden präsentiert. Des Weiteren wird über Ergebnisse zur Spektroskopie von Mischungen zwischen Xenon und jeweils einem der anderen stabilen Edelgase berichtet; solche heteronuklearen Mischungen werden als Alternativkandidat für ein Thermalisierungsmedium betrachtet. In den hier untersuchten Proben sind die Xenon-Beimischungen relativ klein, im Bereich von 100 ppm relativ zu Gesamtprobendrücken von bis zu 90 bar. Für solche Proben werden Absorptions- und Emissionsspektren untersucht; besonderes Augenmerk liegt hierbei auf dem Einfluss variierender Xenon- und Edelgas-Beimischungen. Es wird begutachtet inwieweit die Kennard-Stepanov-Relation erfüllt ist, welche einen wichtigen Indikator für die Eignung eines Mediums als Thermalisierungsmedium für Photonen darstellt

Kramers--Kronig relations and high order nonlinear susceptibilities

As previous theoretical results recently revealed, a Kramers-Kronig transform of multiphoton absorption rates allows for a precise prediction on the dispersion of the nonlinear refractive index $n_2$ in the near IR. It was shown that this method allows to reproduce recent experimental results on the importance of the higher-order Kerr effect. Extending these results, the current manuscript provides the dispersion of $n_2$ for all noble gases in excellent agreement with reference data. It is furthermore established that the saturation and inversion of the nonlinear refractive index is highly dispersive with wavelength, which indicates the existence of different filamentation regimes. While shorter laser wavelengths favor the well-established plasma clamping regime, the influence of the higher-order Kerr effect dominates in the long wavelength regime

Study of Formation and Decay of Rare-Gas Excimers by Laser- Induced Fluorescence

The aim of this chapter is to review the experimental and numerical techniques for the estimation of the laser-induced fluorescence (LIF) decay in rare gases using time-correlated single-photon counting. The advantages of single-photon counting technique are discussed by means of measurement uncertainty analysis. In addition, this chapter provides information concerning the application of this technique to filamentary dielectric barrier discharges (DBD) and radiation trapping of the resonant transitions

Two Photon Absorption Laser Induced Fluorescence for Fusion Class Plasmas

Neutral hydrogen particles play an important role in many fusion systems. The edge region of fusion plasmas is strongly influenced by these neutral particles and is of growing importance because of the challenges of plasma material interaction. A two photon absorption laser induced fluorescence diagnostic at West Virginia University has been constructed to measure the local density and velocity distribution of these neutral particles. The diagnostic measures the ground state of hydrogen isotopes by way of two photon absorption from the 1s to 3d state and subsequent single photon emission to the 2 p state. These measurements are absolutely calibrated by comparing the integrated emission spectra to that of a measurement performed on a known density of calibration gas and knowing the relative absorption cross sections for the two species. Measurements were performed on deuterium atoms in the Helicity Injected Torus with Steady Induction 3 and calibrated using the standard krypton calibration scheme. Measured neutral densities were well below predicted values and the measurement process identified a flaw in the krypton calibration scheme. A new calibration scheme using xenon gas was developed to eliminate any possibility of chromatic aberration through refractive optics. This new xenon calibration scheme required measurement of the relative absorption cross section between the 5p6 to 4p 57f to 5p55 d Xe scheme and the 4p6 to 4 p55p to 4p 55s Kr scheme, then comparison of the Xe to Kr relative cross section to the Kr to H relative cross section to determine the overall Xe to H relative absorption cross section. Doppler free two photon absorption laser induced fluorescence measurements were also performed on the compact helicon for waves and instabilities experiment (CHEWIE), for hydrogen, deuterium, and krypton neutrals. The Doppler free technique increased signal intensity and narrowed the measured spectral width of the absorption line. The Doppler free technique allows for higher sensitivity and faster data acquisition rates of neutral density measurements on high temperature systems. These experiments demonstrated the efficacy and improved the performance of the two photon absorption laser induced fluorescence diagnostic

Introducing many-body physics using atomic spectroscopy

Atoms constitute relatively simple many-body systems, making them suitable objects for developing an understanding of basic aspects of many-body physics. Photoabsorption spectroscopy is a prominent method to study the electronic structure of atoms and the inherent many-body interactions. In this article the impact of many-body effects on well-known spectroscopic features such as Rydberg series, Fano resonances, Cooper minima, and giant resonances is studied, and related many-body phenomena in other fields are outlined. To calculate photoabsorption cross sections the time-dependent configuration interaction singles (TDCIS) model is employed. The conceptual clearness of TDCIS in combination with the compactness of atomic systems allows for a pedagogical introduction to many-body phenomena.Comment: 15 pages, 6 figures, 1 table. The following article has been accepted by American Journal of Physic

Ion laser plasmas

The typical noble gas ion laser plasma consists of a high-current-density glow discharge in a noble gas, in the presence of a magnetic field. Typical CW plasma conditions are current densities of 100 to 2000 A/cm^2, tube diameters of 1 to 10 mm, filling pressures of 0.1 to 1.0 torr, and an axial magnetic field of the order of 1000 G. Under these conditions the typical fractional ionization is about 2 percent and the electron temperature between 2 and 4 eV. Pulsed ion lasers typically use higher current densities and lower operating pressures. This paper discusses the properties of ion laser plasmas, in terms of both their external discharge parameters and their internal ion and excited state densities. The effect these properties have on laser operation is explained. Many interesting plasma effects, which are important in ion lasers, are given attention. Among these are discharge nonuniformity near tube constrictions, extremely high ion radial drift velocities, wall losses intermediate between ambipolar diffusion and free fall, gas pumping effects, and radiation trapping. The current status of ion laser technology is briefly reviewed

Neutron Detection by Noble Gas Excimer Scintillation

The field of neutron detection has many essential applications, from nuclear reactor instrumentation, oil-well logging, radiation safety, and, in recent years, homeland security. Due to the shortage and increasing cost of the neutron absorber used in most conventional gas-filled proportional counters, there has been an increased motivation for the development of alternative methods of neutron detection that do not rely on 3He. Excimer-based neutron detection (END) is a potential alternative with many advantages, notably the lack of dependence on 3He. Similar to traditional proportional counters, END operates on the interaction of a neutron with a neutron absorbing nucleus (10B, 6Li, or 3He). The energetic charged particles produced in these reactions lose energy in the surrounding gas background and cause ionization and excitation of the noble gas molecules. The difference between END and traditional gas-filled detectors, which collect the ionized charge to produce a detectable signal, is the formation of noble gas excimers (Ar2*, Kr2*, or Xe2*). These excited dimers decay from an excited state back to ground level and emit far-ultraviolet (FUV) radiation in the form of photons which can be collected using a photomultiplier tube (PMT) or other photon detector. The most important advantage to these potential detectors is the fact that they do not rely on the use of 3He. The excimer scintillation yield from rare noble gases following the 10B neutron capture reaction in both 10B enriched BF3 gas and reticulated vitreous carbon foam (RVC) coated with a layer of B4C is the focus of this thesis. Experimental data were collected at the National Institute of Standards and Technology (NIST) and on a recently established thermal neutron beamline at the Maryland University Training Reactor (MUTR). The comparison of these data to data from previous thin-film experiments provides the groundwork for the continuation of future END work using these materials, which will be used to develop and optimize a deployable neutron detector based on excimer emission