Laser Remediation of Threats Posed by Small Orbital Debris

The continually increasing amount of orbital debris in near Earth space poses an increasing challenge to space situational awareness. Recent collisions of spacecraft caused abrupt increases in the density of both large and small debris in near Earth space. An especially challenging class of threats is that due to the increasing density of small (1 mm to 10 cm dimension) orbital debris. This small debris poses a serious threat since: (1) The high velocity enables even millimeter dimension debris to cause serious damage to vulnerable areas of space assets, e.g., detector windows; (2) The small size and large number of debris elements prevent adequate detection and cataloguing. We have identified solutions to this threat in the form of novel laser systems and novel ways of using these laser systems. While implementation of the solutions we identify is challenging we find approaches offering threat mitigation within time frames and at costs of practical interest. We base our analysis on the unique combination of coherent light specifically structured in both space and time and applied in novel ways entirely within the vacuum of space to deorbiting small debris. We compare and contrast laser based small debris removal strategies using ground based laser systems with strategies using space based laser systems. We find laser systems located and used entirely within space offer essential and decisive advantages over groundbased laser systems

Transient polarization dynamics in a CO2_2 laser

We study experimentally and theoretically the polarization alternation during the switch-on transient of a quasi-isotropic CO$_2$ laser emitting on the fundamental mode. The observed transient dynamics is well reproduced by means of a model which provides a quantitative discrimination between the intrinsic asymmetry due to the kinetic coupling of molecules with different angular momenta, and the extrinsic anisotropies, due to a tilted intracavity window. Furthermore, the experiment provides a numerical assignment for the decay rate of the coherence term for a CO$_2$ laser.Comment: 14 pages, 6 figures, submitted to Opt. Com

Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

Aquifer denitrification is among the most poorly constrained fluxes in global and regional nitrogen budgets. The few direct measurements of denitrification in groundwaters provide limited information about its spatial and temporal variability, particularly at the scale of whole aquifers. Uncertainty in estimates of denitrification may also lead to underestimates of its effect on isotopic signatures of inorganic N, and thereby confound the inference of N source from these data. In this study, our objectives are to quantify the magnitude and variability of denitrification in the Upper Floridan Aquifer (UFA) and evaluate its effect on N isotopic signatures at the regional scale. Using dual noble gas tracers (Ne, Ar) to generate physical predictions of N<sub>2</sub> gas concentrations for 112 observations from 61 UFA springs, we show that excess (i.e. denitrification-derived) N<sub>2</sub> is highly variable in space and inversely correlated with dissolved oxygen (O<sub>2</sub>). Negative relationships between O<sub>2</sub> and δ<sup>15</sup>N<sub>NO3</sub> across a larger dataset of 113 springs, well-constrained isotopic fractionation coefficients, and strong <sup>15</sup>N:<sup>18</sup>O covariation further support inferences of denitrification in this uniquely organic-matter-poor system. Despite relatively low average rates, denitrification accounted for 32 % of estimated aquifer N inputs across all sampled UFA springs. Back-calculations of source δ<sup>15</sup>N<sub>NO3</sub> based on denitrification progression suggest that isotopically-enriched nitrate (NO<sub>3</sub><sup>–</sup>) in many springs of the UFA reflects groundwater denitrification rather than urban- or animal-derived inputs

The influence of phase-modulation on femtosecond time-resolved coherent Raman spectroscopy

The influence of phase-modulation on femtosecond time-resolved coherent Raman scattering is investigated theoretically and experimentally. The coherent Raman signal taken as a function of the spectral position shows unexpected temporal oscillations close to time zero. A theoretical analysis of the coherent Raman scattering process indicates that the femtosecond light pulses are amplitude and phase modulated. The pulses are asymmetric in time with more slowly decaying trailing wings. The phase of the pulse amplitude contains quadratic and higher-order contributions

Nuclear Magnetic Resonance and Hyperfine Structure

Contains reports on six research projects

Grain Boundary Induced Magneto-Far Infrared Resonances in Superconducting YBa2_2Cu3_3O7−δ_{7-\delta } Thin Films

Spectral features induced by 45$^{\circ }$ in-plane misoriented grains have been observed in the far infrared magneto-transmission of YBa$_2$Cu$_3$O$_{7-\delta }$ thin films. Two strong dispersive features are found at 80 and 160 $cm^{-1}$ and a weaker one at 116 $cm^{-1}$. The data can be well represented by Lorentzian oscillator contributions to the conductivity. Several possible interpretations are discussed. We conclude that the resonances are due to vortex core excitations.Comment: Latex file (14 pages) + 4 Postscript figures, uuencode

Nuclear Magnetic Resonance and Hyperfine Structure

Contains reports on five research projects

FAST CARS: Engineering a Laser Spectroscopic Technique for Rapid Identification of Bacterial Spores

Airborne contaminants, e.g., bacterial spores, are usually analyzed by time consuming microscopic, chemical and biological assays. Current research into real time laser spectroscopic detectors of such contaminants is based on e.g. resonant Raman spectroscopy. The present approach derives from recent experiments in which atoms and molecules are prepared by one (or more) coherent laser(s) and probed by another set of lasers. The connection with previous studies based on "Coherent Anti-Stokes Raman Spectroscopy" (CARS) is to be noted. However generating and utilizing maximally coherent oscillation in macromolecules having an enormous number of degrees of freedom is much more challenging. This extension of the CARS technique is called FAST CARS (Femtosecond Adaptive Spectroscopic Techniques for Coherent Anti-Stokes Raman Spectroscopy), and the present paper proposes and analyses ways in which it could be used to rapidly identify pre-selected molecules in real time.Comment: 43 pages, 21 figures; replacement with references added. Submitted to the Proceedings of National Academy of Science

Coherent phenomena in semiconductors

A review of coherent phenomena in photoexcited semiconductors is presented. In particular, two classes of phenomena are considered: On the one hand the role played by optically-induced phase coherence in the ultrafast spectroscopy of semiconductors; On the other hand the Coulomb-induced effects on the coherent optical response of low-dimensional structures. All the phenomena discussed in the paper are analyzed in terms of a theoretical framework based on the density-matrix formalism. Due to its generality, this quantum-kinetic approach allows a realistic description of coherent as well as incoherent, i.e. phase-breaking, processes, thus providing quantitative information on the coupled ---coherent vs. incoherent--- carrier dynamics in photoexcited semiconductors. The primary goal of the paper is to discuss the concept of quantum-mechanical phase coherence as well as its relevance and implications on semiconductor physics and technology. In particular, we will discuss the dominant role played by optically induced phase coherence on the process of carrier photogeneration and relaxation in bulk systems. We will then review typical field-induced coherent phenomena in semiconductor superlattices such as Bloch oscillations and Wannier-Stark localization. Finally, we will discuss the dominant role played by Coulomb correlation on the linear and non-linear optical spectra of realistic quantum-wire structures.Comment: Topical review in Semiconductor Science and Technology (in press) (Some of the figures are not available in electronic form

Small bowel enteroclysis with magnetic resonance imaging and computed tomography in patients with failed and uncertain passage of a patency capsule

<p>Abstract</p> <p>Background</p> <p>Video capsule enteroscopy (VCE) has revolutionized small bowel imaging, enabling visual examination of the mucosa of the entire small bowel, while MR enteroclysis (MRE) and CT enteroclysis (CTE) have largely replaced conventional barium enteroclysis. A new indication for MRE and CTE is the clinical suspicion of small bowel strictures, as indicated by delayed or non-delivery of a test capsule given before a VCE examination, to exclude stenosis. The aim of this study was to determine the clinical value of subsequent MRE and CTE in patients in whom a test capsule did not present itself in due time.</p> <p>Methods</p> <p>Seventy-five consecutive patients were identified with a delayed or unnoticed delivery of the test capsule. Seventy patients consented to participate and underwent MRE (44) or CTE (26). The medical records and imaging studies were retrospectively reviewed and symptoms, laboratory results and imaging findings recorded.</p> <p>Results</p> <p>Lesions compatible with Crohns disease were shown by MRE in 5 patients, by CTE in one and by VCE in four, one of whom had lesions on MRE. In patients without alarm symptoms and findings (weight loss, haematochezia, anaemia, nocturnal diarrheoa, ileus, fistula, abscess and abnormal blood tests) imaging studies did not unveil any such lesion. VCE's were performed in only 20 patients, mainly younger than 50 years of age, although no stenotic lesion was shown by MRE and CTE. In the remaining 50 patients no VCE or other endoscopic intervention was performed indicating that the referring physician was content with the diagnostic information from MRE or CTE.</p> <p>Conclusion</p> <p>The diagnostic value of MRE and CTE is sufficient for clinical management of most patients with suspected small bowel disease, and thus VCE may be omitted or at least postponed for later usage.</p