The deep space network, volume 15

The DSN progress is reported in flight project support, TDA research and technology, network engineering, hardware and software implementation, and operations. Topics discussed include: DSN functions and facilities, planetary flight projects, tracking and ground-based navigation, communications, data processing, network control system, and deep space stations

Field-resolved studies of ultrafast light-matter interaction

The fastest light-matter interactions between electrons and optical laser pulses occur on attosecond timescales below the half-cycle oscillation period of the electric field. The investigation of such ultrafast processes and ultimately their control, therefore, requires field-resolved measurements. In this work, the understanding of well-established and newly emerging sub-cycle-resolved techniques for the characterization of optical pulses and ultrafast light-induced processes is expanded and the application of the methods includes gases, bulk solids, and nanostructures. In the first part, the mechanism behind the macroscopic current generation in optical field-induced photocurrent measurements in gases is studied theoretically and experimentally. A rigorous model is developed that connects the measured current to the microscopic movement of charge carriers and includes scattering with atoms and the interaction of charges via the Coulomb force. The model is validated against an extensive set of experiments which measure the carrier-envelope-phase dependent strong-field photoemitted current induced on a pair of electrodes surrounding the focus of a few-cycle laser pulse. The role of the mean-free path as well as the Coulomb interaction is identified. The model provides a fundamental understanding of the signal generation mechanism in photoconductive sampling of laser pulses which had been missing before and which will allow to identify fundamental limitations and strategies for further optimization of the detection. The second set of experiments aims at the transient change of the reflectivity after excitation with a near-infrared pump pulse. Electro-optic sampling is used for the field-resolved characterization of the mid-infrared probe pulses which covers a wavelength range from below 3 micron to above 6 micron. Measurements on semiconductors are performed and dynamics occuring on the femtosecond to the picosecond timescale after photoexcitation are studied. The demonstrated experiments represent an important milestone in pushing field-sampling methods from the THz into the PHz domain. The investigation of the generation of isolated attosecond pulses in the extreme ultra-violet photon energy range using high-harmonic generation (HHG) in noble gases is the topic of the third section. The focus is put on the overdriven regime, where the driving laser undergoes severe reshaping due to plasma effects. Experimentally, attosecond streaking is used to demonstrate isolated attosecond pulses for the first time in this regime. Theoretically, the phasematching mechanism in this regime is studied using extensive numerical simulations. An extension of conventional phasematching expressions is introduced which describes the contribution of the HHG dipole phase due to the blue-shift of the driving laser. The results are important for a complete understanding of HHG phasematching and might help to find routes towards more efficient HHG in the water window. Finally, attosecond measurements on metal nanotips are presented. The attosecond field-resolved characterization of the nanoscale near-fields on a nanotip and the response function using attosecond streaking is demonstrated. Moreover, another field-reconstruction method based on the modulation of the strong-field photocurrent is used for the measurement of the enhanced near-fields at the nanotip apex and different aspects of the methods are studied. Combining the latter approach with the concept of the nanotip as nanoscale localized field sensor, the attosecond characterization of an orbital angular momentum beam in free-space below the diffraction limit is demonstrated. These results pave the way towards nanoscale attosecond field-resolved measurements on generic nanostructures.Die schnellsten Prozesse der fundamentalen Licht-Materie-Wechselwirkung zwischen Elektronen und sichtbaren Laserpulsen finden auf der Attosekunden-Zeitskala statt, unterhalb der halben Schwingungsperiode des elektrischen Feldes. Feldaufgelöste Messungen der beteiligten Laserpulse sind daher für die Untersuchung solch schneller Prozesse und ihrer Kontrolle unabdingbar. In der vorliegenden Arbeit wird das fundamentale Verständnis von etablierten als auch neu aufkommenden subzyklen-aufgelösten Messtechniken erweitert mit Anwendungen in Gasen, Festkörpern und Nanostrukturen. Im ersten Teil wird der Mechanismus hinter der Erzeugung makroskopischer Ströme in den Messungen feld-induzierter Photoströme experimentell und theoretisch untersucht. Die Entwicklung eines rigorosen Models wird präsentiert, das die gemessenen Ströme mit der mikroskopischen Bewegung der Ladungsträger verknüpft. Es beinhaltet außerdem die Streuung an Atomen und die Coulomb-Wechselwirkung. Das Model wird in einer Reihe von umfassenden Experimenten bestätigt, in denen die Abhängigkeit der Ströme von der Phase der Trägerwelle zur Einhüllenden des Laserpulses gemessen werden. Zur Messung der Ströme wird ein Elektrodenpaar, das den Fokus eines intensiven Wenigzyklen-Pulses in verschiedenen Gasen umgibt, verwendet. Der Einfluss der mittleren freien Weglänge und der Ladungs-Wechselwirkung wird aufgeklärt. Das Model liefert ein fundamentales Verständnis der Signalerzeugung in der auf Photoleitung beruhenden Messung von Laserpulsen. Dies erlaubt die Identifizierung der fundamentalen Grenzen und eröffnet Wege zur Optimierung der Messmethode. Die zweite Reihe von Experimenten hat die Messung der transienten Änderung der Reflektivität nach der Anregung durch einen Pumppuls in nahen Infrarotbereich zum Gegenstand. Elektro-optisches Sampling wird verwendet für die feldaufgelöste Charakterisierung der Probepulse im mittleren Infrarotbereich, von unter drei bis über sechs Mikrometern Wellenlänge. Messungen an Halbleitern werden durchgeführt und die Dynamik, die auf der Zeitskala von Femtosekunden bis Pikosekunden nach der Photoanregung stattfindet, wird untersucht. Die gezeigten Experimente sind ein wichtiger Meilenstein für die Erweiterung feldaufgelöster Messtechniken vom THz- in den PHz-Frequenzbereich. Die Erzeugung von isolierten Attosekundenpulsen im extrem ultravioletten Wellenlängenbereich durch hohe Harmonische (HHG) in Edelgasen ist das Thema des dritten Abschnitts. Der Schwerpunkt liegt auf der Untersuchung des übersteuerten Regimes, in dem der Anregungslaser im Erzeugungsprozess eine starke Umformung durch Plasmaeffekte erfährt. Auf experimenteller Ebene wird die Attosekunden Streaking Technik angewendet, um zum ersten Mal die Erzeugung isolierter Attosekundenpulse unter diesen Umständen nachzuweisen. Anhand umfangreicher numerischer Simulationen wird der Mechanismus der Phasenanpassung in diesem Regime theoretisch untersucht. Eine Erweiterung der konventionellen analytischen Beschreibung der Phasenanpassung wird eingeführt, die den Beitrag der Dipolphase der hohen Harmonischen aufgrund der Blauverschiebung des Anregungslasers berücksichtigt. Die Ergebnisse sind von grundlegender Bedeutung für ein vollständiges Verständnis der HHG-Phasenanpassung und helfen möglicherweise dabei, Wege zu effizienter HHG-Erzeugung im sogenannten Wasserfenster zu finden. Zuletzt werden Attosekundenmessungen an metallischen Nanospitzen vorgestellt. Die feldaufgelöste Charakterisierung von Nahfeldern an einer Nanospitze auf der Attosekunden- und Nanometerskala und der entsprechenden Antwortfunktion durch die Attosekunden Streaking Methode werden demonstriert. Darüber hinaus wird eine andere Technik zur Feldrekonstruktion auf die Messung der verstärkten Nahfelder an dem Ende der Nanospitze angewandt. Verschiedene Aspekte der Messmethode, die auf der Modulation des durch Starkfeldemission erzeugten Photostroms beruht, werden untersucht. Schließlich wird die Kombination der demonstrierten Methodik mit dem Konzept der Nanospitze als nanolokalisiertem Feldsensor demonstriert. Damit wird die zeitlich und räumlich aufgelöste Charakterisierung eines frei propagierenden Laserstrahls mit Drehimpuls auf der Attoekunden-Skala und unterhalb des Beugungslimits gezeigt. Die Resultate ebnen den Weg hin zu feldaufgelösten Messungen im Nanometer-Attosekunden Bereich an beliebigen Nanostrukturen

Compressive Sensing Applications in Measurement: Theoretical issues, algorithm characterization and implementation

At its core, signal acquisition is concerned with efficient algorithms and protocols capable to capture and encode the signal information content. For over five decades, the indisputable theoretical benchmark has been represented by the wellknown Shannon’s sampling theorem, and the corresponding notion of information has been indissolubly related to signal spectral bandwidth. The contemporary society is founded on almost instantaneous exchange of information, which is mainly conveyed in a digital format. Accordingly, modern communication devices are expected to cope with huge amounts of data, in a typical sequence of steps which comprise acquisition, processing and storage. Despite the continual technological progress, the conventional acquisition protocol has come under mounting pressure and requires a computational effort not related to the actual signal information content. In recent years, a novel sensing paradigm, also known as Compressive Sensing, briefly CS, is quickly spreading among several branches of Information Theory. It relies on two main principles: signal sparsity and incoherent sampling, and employs them to acquire the signal directly in a condensed form. The sampling rate is related to signal information rate, rather than to signal spectral bandwidth. Given a sparse signal, its information content can be recovered even fromwhat could appear to be an incomplete set of measurements, at the expense of a greater computational effort at reconstruction stage. My Ph.D. thesis builds on the field of Compressive Sensing and illustrates how sparsity and incoherence properties can be exploited to design efficient sensing strategies, or to intimately understand the sources of uncertainty that affect measurements. The research activity has dealtwith both theoretical and practical issues, inferred frommeasurement application contexts, ranging fromradio frequency communications to synchrophasor estimation and neurological activity investigation. The thesis is organised in four chapters whose key contributions include: • definition of a general mathematical model for sparse signal acquisition systems, with particular focus on sparsity and incoherence implications; • characterization of the main algorithmic families for recovering sparse signals from reduced set of measurements, with particular focus on the impact of additive noise; • implementation and experimental validation of a CS-based algorithmfor providing accurate preliminary information and suitably preprocessed data for a vector signal analyser or a cognitive radio application; • design and characterization of a CS-based super-resolution technique for spectral analysis in the discrete Fourier transform(DFT) domain; • definition of an overcomplete dictionary which explicitly account for spectral leakage effect; • insight into the so-called off-the-grid estimation approach, by properly combining CS-based super-resolution and DFT coefficients polar interpolation; • exploration and analysis of sparsity implications in quasi-stationary operative conditions, emphasizing the importance of time-varying sparse signal models; • definition of an enhanced spectral content model for spectral analysis applications in dynamic conditions by means of Taylor-Fourier transform (TFT) approaches

Aeronautical engineering: A continuing bibliography with indexes (supplement 255)

This bibliography lists 529 reports, articles, and other documents introduced into the NASA scientific and technical information system in June 1990. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Structural Effects on Nonadiabatic Photocyclization in ortho-Arenes

Nonadiabatic photocyclization is the chemical dynamic relevant to the function of many photoswitchable materials as well as photochemical synthesis of polyaromatic hydrocarbons by cyclodehydrogenation. ortho-arenes are an under-studied class of molecular photoswitch owing to their low cyclized product stabilities that otherwise provide a unique opportunity for the experimentalist to study photocyclization mechanisms in detail. Through the use of time-resolved absorption spectroscopy on femtosecond to microsecond timescales the entire photocyclization process can be monitored from “birth” to “death,” i.e. ring-fusion to ring-fission. Following ultraviolet photoexcitation OTP undergoes cyclization to form DHT. Although global spectral analysis with simple kinetic models adequately fits spectral dynamics, signatures of DHT formation are obscured by spectral overlap with the excited-state that has a lifetime of ~3 picoseconds. Thermal ring-reopening of DHT to regenerate OTP occurs with a 38 nanosecond lifetime and an activation energy of 0.27 eV. Following the study of OTP a variety of other ortho-arenes were examined by systematic substitution, including phenyl substituted analogs, 1,2,3-triphenylbenzene, ortho-quaterphenyl and hexaphenylbenzene, as well as boron-nitrogen substituted analogs, including hexaphenylborazine and 1,2:3,4:5,6 tris(o,o’-biphenylylene) borazine. Generally these substitutions increased the excited-state lifetime relative to OTP due to an increase in either electronic delocalization or structural hindrance within excited-state geometries while the stability of the corresponding photoproducts decreased relative to DHT due to entropic effects. A notable exception is hexaphenylbenzene, which exhibits a 2 microsecond lifetime for ring-reopening of the photoproduct tetraphenyl-DHT that is a consequence of entropic stabilization due to increased phenyl-phenyl steric interactions that constrain thermally activated relaxation to the transition state. Furthermore, excited hexaphenylborazine decays within 3 picoseconds due to the localized electronic character of the borazine ring. No direct spectroscopic observation of cyclization was observed for any boron-nitrogen substituted system due to the increase in charge localization that reduces the stability of the conjugated DHT photoproduct. More recent experiments utilizing pump-repump-probe spectroscopy have determined that the observed excited-state decay is kinetically decoupled from photocyclization, which occurs in less than 200 femtoseconds. The results presented in this thesis provide insight for the improvement of photoswitch and photosynthetic efficiency through the generalization of these structure-dynamics relationships

Aeronautical engineering: A continuing bibliography with indexes (supplement 240)

This bibliography lists 629 reports, articles, and other documents introduced into the NASA scientific and technical information system in May, 1989. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Extreme Ultraviolet Measurements of Thermal and Elastic Dynamics In Nanostructured Media

Nanofabrication today spans from atomic level precision to hierarchically organized structures reaching up to microns. Nanoscale material properties are profoundly different from their bulk counterparts, and when combined with metamaterial approaches, systems can be engineered with properties unavailable in naturally occurring substances. These effects have applications from nanoelectronics, to thermoelectrics, to nanoparticle-based cancer therapies. However, the full capabilities of these materials have not yet been realized due to the difficulty of studying functional nanosystems. In this thesis, I study the properties of nanoscale materials at their intrinsic length and time scales, via the diffraction of extreme ultraviolet (EUV) beams, generated coherently using tabletop high harmonic generation. First, using periodic nanoline gratings, I systematically explore size- and spacing-dependence in the ultrafast cooling of nanoscale heat sources. We find that, though nanoscale heat sources generally cool much slower than the bulk diffusive prediction, they can be brought within a factor of two of the efficient, diffusive prediction simply by bringing them closer together. I then use similar nanoline transducers to study nanoscale acoustic waves, and thus extract the elastic tensor of isotropic, amorphous films, down to 11nm thickness. We find that hydrogenating these films to a critical level of broken bonds causes a divergence towards incompressible behavior, which could also mitigate thickness-dependent changes in the films&rsquo; mechanical properties. I next demonstrate our technique can measure thermal and elastic dynamics in 3D silicon metalattices, a promising thermoelectric material. To measure even more general samples, I finally extend this EUV nanometrology technique into a non-contact modality. First, I implement an optical transient grating excitation to study micron-scale thermal transport in 2D nanoparticle-molecular arrays. We measure an effective thermal conductivity for the arrays that is three orders of magnitude lower than bulk gold. Second, I participate in EUV transient grating experiments at the FERMI free electron laser to directly excite deep nanoscale thermal and elastic dynamics, and design a similar experiment for tabletop EUV light sources.</p