White light filamentation: tailoring & application for charge reversal of Ag3

Abstract

This thesis describes two research goals. First to extend the charge reversal (NeNePo) spectroscopy of small Ag clusters by adaptive coherent control strategies to enhance the excitation selectivity of a desired quantum state. Second, to improve the control tool by providing one octave of coherent frequencies, spanning from the VIS to the NIR region. For this purpose both high stability and reproducibility of the broadened spectrum are indispensable. This was realized by filamentation in atmospheric air through slightly focusing amplified fs laser pulses of about 1 mJ. In the development of this light source as a tool for investigations of ultrafast dynamics, the filamentation process itself was studied to learn more about the interplay between the Kerr effect and weakly ionized plasma. The characterization of different gases confirmed an analogous mechanism based on the initial action of self-phase modulation, causing spectral broadening around the laser fundamental and the retarded generation of an asymmetric spectral shoulder approaching the UV range. One explanation found in the literature for this pronounced broadening on the short wavelength side is a plasma blue shift which was also retraced in simulations for this work. Generally, gases with higher nonlinear refractive index require lower pulse energy and gas pressure, respectively, to achieve WL generation. The shape of spectral broadening around the fundamental strongly depends on the prechirp. In contrast, the VIS tail keeps its spectral shape and positive chirp, unless a strong prechirp completely terminates filamentation. Zero prechirping at high input energy or pressure, respectively, causes the highest nonlinearity and the pulse tends to first split temporally. Later, with still increased nonlinearity it also splits spatially. An alternative for tuning the filamentation conditions lies in adding He to the air which tends to halt the filamentation process. This quenching effect due to the low nonlinearity of He might be utilized to scale filamentation towards even higher energy throughput. However, with the right preconditions a reliable pulse compression down to 6.3 fs for filamentation in air was enabled with four bounces on chirped mirrors in the range from 600—900 nm. Currently, these are the shortest pulses generated via filamentation in air. Variable compression from 450—1000 nm can be achieved with the bare 4-f setup which actually corresponds to a telescope-grating compressor. Since the characterization of few optical cycles and octave exceeding bandwidths, respectively, is as demanding as its generation, a commercial solution is not available. By implementing a TG-FROG, bandwidths ranging from 380—950 nm were optically gated for the first time. This provided the basis for extensively studying the temporal and spectral properties of the WL. This FROG variant utilizes 4WM of the three incoming beams according to a forward box scenario and requires only BK 7 glass as the nonlinear medium. Several special features of this gate mechanism are very promising for a wide range of applications in ultrafast pulse characterization: First, it does not convert the frequency of the gated pulse (unlike SHG, THG or SFG), and therefore leads to intuitive traces avoiding ambiguities of other FROG methods. Second, it handles the entire transmission range of the nonlinear medium and is realized with standard optical components. The problem of choosing appropriate beam splitters to provide the three beams can be overcome by geometrical beam separation, as was successfully demonstrated for the measurement of the few cycle pulses. Furthermore, the combination of WL shaping and 4WM not only allows the characterization of very complex pulse forms but also the unique possibility of cross-correlation measurements in a single beam arrangement, which was named single beam TG-X-FROG. Its prerequisite is a controllable pulse shaping device. To assure this controllability from 450—950 nm, the WL shaper built in this work needed to be calibrated for every pixel due to the wavelength dependent operation of the liquid crystals in the spatial light modulator (SLM). In this manner, the realization of a double color pump probe excitation scheme was demonstrated likewise as a single beam setup. In this example, a short VIS pulse centered at 625 nm was time delayed with respect to a short NIR pulse centered at 800 nm by applying a linear phase ramp to the VIS part by the SLM. Since the third order optical nonlinearity of the 4WM process connotes a high sensitivity of the 4WM signal on the input pulse duration, it was utilized for adaptive pulse compression with the shaper. By this, optimizing the 4WM signal in a feedback loop experiment with an evolutionary algorithm turned out to be a straight forward approach for pulse compression of the filament output. This technique differs from others, in that knowledge of the phase of the incoming light field is not required and the optimized spectral phase function on the SLM defines a correcting reference from which analytical functions can be generated. As the other goal of the thesis was the extension of the NeNePo scheme by shaped pulses, an analytic approach was established first on the diatomic Ag2 before the free optimization of the WL was applied for Ag3NeNePo spectroscopy. The analytic approach covers the generation of pulse trains with variable sub pulse separation by amplitude shaping the laser fundamental spectrum. This delivered a cyclic driving of the photo detachment step, being subsequently probed by a single time delayed ionization pulse. Due to the Fourier relation between the time and frequency domains, a synchronization of the sub pulse separation to the vibrational period of the neutral Ag2 preferentially excites the vibrational eigenstates of the neutral species, leading to an enhanced localization of the wave packet which can be probed most efficiently. This experiment opens the door towards mode selective pumping and probing of the NeNePo process of more complex systems. When adding one atom to create the trimer Ag3, the nuclear dynamics gains complexity. In particular, the neutral Ag3 undergoes configurational changes from linear to an obtuse triangular structure by bending the outer atoms. The bending ends with an intra-molecular collision, followed by IVR which populates other vibrational modes. Since coherence effects after that collision were never identified in previous pump probe experiments, free optimization of WL pulses denotes a new and promising approach. The optimized supercontinuum exhibits two distinct spectral regions, each containing very short pulses or pulse sequences, respectively. The NIR spectral region from 750—950 nm was exclusively optimized to a compressed pulse when starting from phase noise. In contrast, the VIS part from 530—760 nm contains a rich structure apparent in all optimization experiments. The time scales of the spectrally and temporally modulated VIS sequences exhibit for the first time temporal structures far below the time scale of the IVR process. Thus they refrain from an intuitive interpretation in the aimed direction of finding indications for preserved coherences after the collision. In a similar sense Talbot annotated his Photogenic Drawings “They are impressed by Nature's hand; and what they want us jet of delicacy and finish of execution arises chiefly from our want of sufficient knowledge of her laws.” To further verify the excitation path to the cationic species, a collaboration with theory is in progress. However, the closed loop results for free phase optimization of the NeNePo process witness the first-time application of shaped WL, generated via filamentation, for coherent quantum control. Since the development of new thing leads to unexpected observations like the TG-X-FROG, the WL optimization results refrain from an intuitive interpretation in the aimed direction of finding indication for preseved coherences after the intramolecular collosion The unique property of TG FROG in combination with a octave spanning pulse shaper allows performing of pump probe excitation and cross-correlation within one single beam (TG-X-FROG). Besides ultra broadband operation, the capability of pulse compression to 6.3 fs is another striking property which is valuable for the generation of as pulses. The new developed setup is distinguished on one hand by the ease of realization, since the supercontinuum and the few cycle pulses are achieved in air and on the other hand in the high economy in optical components. The dispersion free TG-X-FROG consists of seven standard optical components only. Utilizing the output of the filamentation process as a light source emphasizes the generation of new wavelengths, blue shifted with respect to the inducing laser fundamental. The investigated TG-X-FROG mechanism was proven to gate frequencies which lie one octave below the grating pulses. Why not transfer this scheme to a fundamental wavelength of 400 nm? Although the frequency doubling from 800 to 400 nm reduces pulse energy, filamentation will benefit from the increased nonlinear index at shorter wavelengths and increasing cross sections for multi photon ionization at higher photon energy. The spectral operation for TG FROG is limited by the transmission range of the nonlinear medium which closes at about 190 nm in case of Fused Silica. Since reflective pulse shapers with mirrors are already available, the single beam pump probe excitation and the single beam TG-X-FROG can be applied for investigation on biological systems which typically absorb in the UV range. By this, the artists palette will be further extended for drawing new images of nature.

Diese Arbeit verfolgte zwei Ziele. Erstens die Erweiterung der Pump Probe Ladungsumkehrspektroskopie (NeNePo) an kleinen Silber Clustern mit geformten fs Pulsen. Zunächst wurde das Photodetachment des zweiatomigen Anions mittels variabler Pulszüge zum neutralen Ag2 etabliert, welches als einfachstes Modellsystem vibratorischer Kerndynamik zu sehen ist. Ein zeitverzögerter Probepuls löst ein weiteres Elektron ab und generiert das gemessene Ag2 Signal. Mit dem Durchscannen des Subpulsabstandes um die Vibrationsperiode des Neutralteilchens herum konnte die Fokussierung des Ag2 Wellenpaketes gesteuert werden, abzulesen an der Modulationstiefe des transienten Ag2 Signals. Um neue Untersuchungsmöglichkeiten zur Analyse der Kerndynamik des Ag3 Trimers zu eröffnen, bestand der zweite Teil der Arbeit in der Weiterentwicklung der kohärenten Kontrollmethode hin zu geformten fs-Weißlichtpulsen. Als Lichtquelle diente die Filamentierung verstärkter fs Pulse (0.6-1.2 mJ) überwiegend in atmoshärischer Luft. Die Steuerung der Filamentierungsbedingungen, die u.a. zu oktavüberschreitenden Spektren von 380-950 nm führten, wurde ebenso für Edelgase, wie für Sauerstoff untersucht. Generell zeichnete sich dabei ein einheitlicher Entstehungsmechanismus der stark unsymmetrischen Spektren ab. Die maßgebenden Effekte sind die spektrale Verbreiterung um die Fundamentale des Lasers aufgrund von Selbstphasenmodulation (SPM) bereits bei niedrigen Intensitäten, gefolgt von einer Plasmaflanke auf der Rückseite des Pulses, die eine kontinuierliche Verbreiterung bis hin zu UV Wellenlängen hervorruft. Im Rahmen der Filamentcharakterisierung zeigte sich eine starke Abhängigkeit des NIR Spektrums vom Chirp des erzeugenden Pulses, bei konstanter Charakteristik des VIS Anteils - sofern vorhanden. Letzteres ist ein weiterer Hinweis auf zeitliche Selbständerung der Propagationsbedingungen im Filament. Wenngleich die Auswirkungen solcher zeitlicher Selbständerungen wie z.B. self-steepening aus den Messdaten nicht direkt ablesbar waren, so konnte doch das zeitliche Aufspalten in multiple Subpulse bei hohen Gasdrücken bzw. Intensitäten beobachtet werden. Außerdem konnten die bis dato kürzesten, an Luft generierten Pulse demonstriert werden. Die Kürze von 6.3~fs gelang durch Filamentierung in der Laboratmosphäre und anschließender Kompression mittels zweier standard Chirp-Spiegel. Mit einem in dieser Arbeit entworfenen TG-FROG wurden die Pulse vermessen. Die Besonderheiten dieses TG-FROG bestehen in der minimalen Anzahl optischer Komponenten, der geometrischen Strahlteilung zur Vermeidung von Materialdispersion und in der Verwendung von BK 7 Glass als nichtlinearem Medium. Dieses ermöglicht die Charakterisierung von Pulslängen mit nur wenigen optischen Zyklen einerseits, sowie oktavübergreifender Spektren (380-950 nm) andererseits, durch einen einfachen und robusten Aufbau. Die volle Leistungsfähigkeit des Aufbaus entfaltet sich in Kombination mit dem Pulsformer, dessen Konzeption und Kalibrierung unabhängige Phasen- und Amplituden-Änderungen im Bereich von 450-1000 nm erlauben. Diese Kombination ermöglicht: (i) adaptive Pulskompression zu wenigen optischen Zyklen mittels genetischer Optimierung, (ii) die Realisierung eines Zwei-Farben-Pump-Probe Experiments mit einem einzigen Strahl und (iii) die Möglichkeit einer optischen Kreuzkorrelation des NIR und VIS Anteils ebenfalls innerhalb eines Strahls (TG-X-FROG). Letztgenannte Funktionalität ergibt sich aus der Entstehung des transienten Gitters durch den intensiven NIR Anteil, an welchem das im Allgemeinen schwächere VIS Spektrum gestreut wird. Das Zusammenspiel aller Bestandteile, die hohe Stabilität und das hervorragende Strahlprofil ermöglichten die erfolgreiche Implementierung in eine adaptive Rückkopplungsschleife, bei der ein evolutionärer Algorithmus die Ionenausbeute des Ladungsumkehrsignals von Ag3 in der Gasphase optimierte. Die erstmalige Anwendung geformter Weißlichpulse aus einem Plasmafilament in der Ultrakurzzeitspektroskopie im Rahmen eines ``closed loop'' Experiments lieferte neuartige Anregungssignaturen für die Ladungsumkehr des Ag3 Clusters. Dabei zeichnen sich sehr unterschiedliche Verhalten für den NIR und VIS Bereich aus den Spuren der optimierten Pulse ab. Wird der NIR Bereich als ein komprimierter Puls für multiphotonische Ionisation genutzt, ergeben sich im VIS Anteil sehr kurze, reichhaltige Strukturen, auf Zeitskalen die wesentlich kürzer sind als in bisherigen Experimenten beobachtet.