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Observation of ionization instability of intense laser pulses

Description: Theoretical analysis and preliminary experiment on ionization instability of intense laser pulses in ionizing plasmas are presented. The ionization instability is due to the dependence of the ionization rate on the laser intensity and scatters the laser energy off the original propagation direction.
Date: June 25, 1999
Creator: Alexeev, I; Antonsen, T M; Li, Y; Milchberg, H M & Nikitin, S
Partner: UNT Libraries Government Documents Department

Atomic electron correlations in intense laser fields

Description: Abstract. This talk examines two distinct cases in strong opbical fields where electron correlation plays an important role in the dynamic.s. In the first. example, strong coupling in a two-electron-like system is manifested as an intensity-dependent splitting in the ionized electron energy distribution. This two-electron phenomenon (dubbed continuum-continuum Autler-Townes effect) is analogous to a strongly coupled two- level, one-electron atom but raises some intriguing questions regarding the exact nature of electron-electron correlation. The second case examines the evidence for two-electron ionization in the strong-field tunneling limit. Although our ability to describe the one- electron dynamics has obtained a quantitative level of understanding, a description of the two (multiple) electron ionization remains unc
Date: September 3, 1998
Creator: Agostini, P A; DiMauro, L F; Kulander, K; Sheehy, B & Walker, B
Partner: UNT Libraries Government Documents Department

Laser desorption from and reconstruction on Si(100) surfaces studied by scanning tunneling microscopy

Description: Laser irradiated Si(100) surfaces were studied with an ultrahigh-vacuum scanning tunneling microscopy (STM) system. Our observations indicate that only the dimerized outermost atomic layer is removed if the laser fluence is below the melting threshold with a photon energy larger than the band gap. The newly exposed layer, surprisingly, did not have a dimerized atomic structure, but rather, resembled that of a bulk-terminated structure. The uncovered layer remained atomically smooth (no vacancies) even after 90% of the outermost layer was removed. A possible explanation of these observations is that atom removal occurs by a preferential breakage of the atomic bonds in defect sites. When the laser fluence was increased to levels above the melting threshold, extensive surface roughening occurs.
Date: July 1, 1995
Creator: Xu, Jun; Overbury, S.H. & Wendelken, J.F.
Partner: UNT Libraries Government Documents Department

Laser-solid interaction and dynamics of laser-ablated materials

Description: An annealing model is extended to treat the vaporization process, and a hydrodynamic model describes the ablated material. We find that dynamic source and ionization effects accelerate the expansion front of the ablated plume with thermal vaporization temperature. The vaporization process and plume propagation in high background gas pressure are studied.
Date: September 1995
Creator: Chen, K. R.; Neboeuf, J. N.; Wood, R. F.; Geohegan, D. B.; Donato, J. M.; Liu, C. L. et al.
Partner: UNT Libraries Government Documents Department

Absorption of electromagnetic waves near the critical density. Annual report, 1 December 1972--30 November 1973

Description: The program is an experimental study of the interaction of highintensity ruby laser radiation with hydrogen gas in a free-jet. In these experiments, both laser intensity and the plasma frequency can be varied. By monitoring several parameters, including incident and reflected power, x-ray emission and the kinematics of plasma growth, the existence of an anomalous threshold for coupling radiation into plasma energy should be determinable. A discussion of the status of the Calspan experiment and of the status of other anomalous coupling studies at laser frequencies is presented. (auth)
Date: January 1, 1973
Creator: Daiber, J.W.; Rehm, R.G. & Thompson, H.M.
Partner: UNT Libraries Government Documents Department

Absorption of electromagnetic rays near the critical density. Quarterly progress report, June 1, 1973--September 1, 1973

Description: An experimental study of the interaction of high-intensity ruby laser radiation with hydrogen gas in a free-jet is described. In these experiments both laser intensity and the plasma frequency can be varied. By monitoring several parameters, including incident and reflected power, x-ray emission and the kinematics of plasma growth, the existence of an anomalous threshold for coupling radiation into plasma energy should be determinable. A discussion of the experiment, including a design drawing of the experimental arrangement and the diagnostic instrumentation, is included. (auth)
Date: January 1, 1973
Creator: Daiber, J.W.; Rehm, R.G. & Thompson, H.M.
Partner: UNT Libraries Government Documents Department

High order harmonic generation in rare gases

Description: The process of high order harmonic generation in atomic gases has shown great promise as a method of generating extremely short wavelength radiation, extending far into the extreme ultraviolet (XUV). The process is conceptually simple. A very intense laser pulse (I {approximately}10{sup 13}-10{sup 14} W/cm{sup 2}) is focused into a dense ({approximately}10{sup l7} particles/cm{sup 3}) atomic medium, causing the atoms to become polarized. These atomic dipoles are then coherently driven by the laser field and begin to radiate at odd harmonics of the laser field. This dissertation is a study of both the physical mechanism of harmonic generation as well as its development as a source of coherent XUV radiation. Recently, a semiclassical theory has been proposed which provides a simple, intuitive description of harmonic generation. In this picture the process is treated in two steps. The atom ionizes via tunneling after which its classical motion in the laser field is studied. Electron trajectories which return to the vicinity of the nucleus may recombine and emit a harmonic photon, while those which do not return will ionize. An experiment was performed to test the validity of this model wherein the trajectory of the electron as it orbits the nucleus or ion core is perturbed by driving the process with elliptically, rather than linearly, polarized laser radiation. The semiclassical theory predicts a rapid turn-off of harmonic production as the ellipticity of the driving field is increased. This decrease in harmonic production is observed experimentally and a simple quantum mechanical theory is used to model the data. The second major focus of this work was on development of the harmonic {open_quotes}source{close_quotes}. A series of experiments were performed examining the spatial profiles of the harmonics. The quality of the spatial profile is crucial if the harmonics are to be used as the source ...
Date: May 1, 1994
Creator: Budil, K.S.
Partner: UNT Libraries Government Documents Department

Propagation of realistic beams in underdense plasma

Description: The effect of beam structure on propagation through underdense plasma is examined in two different examples. First, it is shown that the distribution of intensities within a laser beam affects how the beam deflects in the presence of transverse plasma flow. A detailed analysis of beam deflection shows that the rate scales linearly with intensity and plasma density, and inversely with plasma temperature. When the plasma flow is subsonic, the deflection rate is proportional to the ion damping decrement, and scales as M/(1 - M{sup 2}){sup 3/2}, where M is the transverse flow Mach number. When the plasma flow is supersonic, the deflection rate scales as 1/[M(M{sup 2} - 1){sup 1/2}]. Next, the effect of beam structure on channel formation by very intense laser beer is studied. A diffraction-limited beam with 40 TW of input power forms a channel through 4OOpm of plasma, whereas when this beam is phase aberrated, channel formation does not occur.
Date: November 10, 1997
Creator: Hinkel, D.E.; Williams, E.A.; Berger, R.L.; Powers, L.V.; Langdon, A.B. & Still, C.H.
Partner: UNT Libraries Government Documents Department

Observation of photoexcited emission clusters in the bulk of KDP and laser conditioning under 355-nm irradiation

Description: Defect clusters in the bulk of large KDP crystals are revealed using a microscopic fluorescence imaging system and CW laser illumination. Exposure of the crystal to high power 355-nm, 3-ns laser irradiation leads to a significant reduction of the number of observed optically active centers. The initially observed defect cluster concentration is approximately 10<sup>4</sup>-10<sup>6</sup> per mm<sup>3</sup> depending on the crystal growth method and sector of the crystal. The number of defect clusters can be reduced by a factor of 10<sup>2</sup> or more under exposure to 355-nm laser irradiation while their average intensities also decreases. Spectroscopic measurements provide information on the electronic structure of the defects.
Date: December 15, 1998
Creator: De Yoreo, J. J.; Demos, S. G.; Radousky, H. B.; Staggs, M. & Yan, M.
Partner: UNT Libraries Government Documents Department

Observation of resonant energy transfer between identical-frequency laser beams

Description: Enhanced transmission of a low intensity laser beam is observed when crossed with an identical-frequency beam in a plasma with a flow velocity near the ion sound speed. The time history of the enhancement and the dependence on the flow velocity strongly suggest that this is due to energy transfer between the beams via a resonant ion wave with zero frequency in the laboratory frame. The maximum energy transfer has been observed when the beams cross in a region with Mach 1 flow. The addition of frequency modulation on the crossing beams is seen to reduce the energy transfer by a factor of two. Implications for indirect-drive fusion schemes are discussed.
Date: December 9, 1998
Creator: Afeyan, B. B.; Cohen, B. I.; Estabrook, K. G.; Glenzer, S. H.; Joshi, C.; Kirkwood, R. K. et al.
Partner: UNT Libraries Government Documents Department

High resolution photoelectron images and D{sup +} photofragment images following 532-nm photolysis of D{sub 2}

Description: The non-resonant ionization and dissociation of D{sub 2} by intense 532-nm laser light is studied by a variation of the ``Ion Imaging`` technique called ``Velocity Mapping``. Images of the both the photoelectrons and D{sup +} photofragments are obtained and analyzed at two different laser intensities. Results are compared to previous studies and several differences are discussed.
Date: January 1, 1998
Creator: Chandler, D.W.; Neyer, D.W. & Heck, A.J.
Partner: UNT Libraries Government Documents Department

Femtosecond laser postionization of sputtered and laser desorbed atoms

Description: This paper examines the photoionization efficiency of a femtosecond laser ionization source on several atomic species. Use of femtosecond laser ionization pulses to photoionize the desorbing flux from a sample surface is examined. Example of mass spectra produced is given using 248 nm pulse on sputtered Au. Al and Mo have widely different ionization potentials and show that 248 nm pulses are more efficient at photoionization than longer wavelengths; this is enhanced for Mo with high ionization potential. It is concluded that efficient photoionization occurs for atoms with ionization potentials low enough for two-photon ionization to occur.
Date: June 1, 1995
Creator: Pellin, M.J.; Lykke, K.R. & Calaway, W.F.
Partner: UNT Libraries Government Documents Department

Plasma Heating in Highly Excited GaN/AlGaN Multiple Quantum Wells

Description: Plasma Heating in Highly Excited GaN/AIGaN Multiple Quantum @@lvEu Wells w f + 1998 %p, K. C. Zeng, R. Mair, J. Y. Liz and H. X. Jiang a) ` fabrication and understanding of MQW lasers [2-5]. For the design of these lasers, one on RT optical studies. Our results revealed that in the GaN/AIGaN MQWS, plasma heating strongly effects the carrier distribution between the confined and unconfined band-to-band and fke excitonic transitions [7]. In the MQW sample under low the unconfined states as determined from the band structure. sample under high Lxc, we varied the excitation intensity by one order of magnitude from 0.110 to IO. The carrier density is estimated to be about N=1012/cm2 (at UC= 0.1 Io) to 1013/cm2 (at 1=== l.). We plotted the PL spectra for four representative excitation fimction of injected carrier density N (open squares). The ratio starts at a value of about 18% for N=1012/cm2 (& = O. lb), and reaches a value over 64 `XO for N=1013/cm2 (& = regions is a loss to optical gain. The carrier density is ve~ high in our experiment and an electron-hole plasma (EHP) state is expected. Because the carrier transfer process plasma temperature. The laser pump energy is about 4.3 eV, which is far above the energy band gap of the sample studied here. This may result in a hot carrier population carrier densities and plasma temperatures. Using a phenomenological expression based The calculated ratio of carriers in the unconfked to the confined states (Ima~ kf) as a finction of carrier density at different temperatures are plotted in Fig. 3 (solid lines). The figure shows that the experiment results can only be explained by plasma heating of the injected carriers at high & ( TP > TJ. The transparency carrier densities for GaN/AIXGal.XN MQW structures ...
Date: October 9, 1998
Creator: Botchkarev, A.; Chow, W.W.; Jiang, H.X.; Lin, J.Y.; Mair, R.; Morkoc, H. et al.
Partner: UNT Libraries Government Documents Department

Practical Point-to-Point Free-Space Quantum Key Distribution over 1/2 KM

Description: We have demonstrated point-to-point single-photon quantum key distribution (QKD) over a free-space optical path of {approximately}475 m under daylight conditions. This represents an increase of &gt;1,000 times farther than any reported point-to-point demonstration, and &gt;6 times farther than the previous folded path daylight demonstration. We expect to extend the daylight range to 2 km or more within the next few months. A brief description of the system is given here. The QKD transmitter, a.k.a. ''Alice'' (Fig. 1), consists of three thermoelectrically cooled diode lasers, a single interference filter (IF), two optical attenuators, two linear polarizers, two non-polarization beam-splitters (BSs), and a 27x beam expander. The two data-lasers' (dim-lasers') wavelengths are temperature controlled and constrained by the IF to {approximately}773 {+-} 0.5 nm, while the transmitted wavelength of the bright-laser (timing-laser) is {approximately}768 nm; the data-lasers are configured to emit a weak pulse of approximately 1 ns duration. The transmitter incorporates no active polarization switching--a first in QKD.
Date: February 1, 1999
Creator: Buttler, W.T.; Hughes, R.J.; Kwiat, P.G.; Lamoreaux, S.K.; Morgan, G.L. & Peterson, C.G.
Partner: UNT Libraries Government Documents Department

Improved Spatial Filter for high power Lasers

Description: A new pinhole architecture incorporates features intended to reduce the rate of plasma generation in a spatial filter for high-energy laser pulse beams. An elongated pinhole aperture is provided in an apertured body for rejecting off-axis rays of the laser pulse beam. The internal surface of the elongated aperture has a diameter which progressively tapers from a larger entrance cross-sectional area at an inlet to a smaller output cross-sectional area at an outlet. The tapered internal surface causes off-axis rays to be refracted in a low density plasma layer that forms on the internal surface or specularly reflected at grazing incidence from the internal surface. Off-axis rays of the high-energy pulse beam are rejected by this design. The external surface of the apertured body adjacent to the larger entrance cross-sectional area at the inlet to the elongated aperture is angled obliquely with respect to the to direction of the path of the high-energy laser pulse beam to backscatter off-axis rays away from the high-energy pulse beam. The aperture is formed as a truncated cone or alternatively with a tapered square cross-section. The internal surface of the aperture is coated with an ablative material, preferably high-density material which can be deposited with an exploding wire.
Date: June 1, 1998
Creator: Estabrook, Kent G.; Celliers, Peter M.; Murray, James E.; DaSilva, Luiz; MacGowan, Brian J.; Rubenchik, Alexander M. et al.
Partner: UNT Libraries Government Documents Department

Near-Field Spectroscopy of Selectively Oxidized Vertical Cavity Surface Emitting Lasers

Description: Selectively oxidized vertical cavity surface emitting lasers (VCSELS) have been studied by spectrally resolved near field scanning optical microscopy (NSOM). We have obtained spatially and spectrally resolved images of both subthreshold emission and lasing emission from a selectively oxidized VCSEL operating at a wavelength of 850 nm. Below threshold, highly local high gain regions, emitting local intensity maxima within the active area, were observed; these were found to serve as lasing centers just above threshold. Above threshold, the near field spatial modal distributions of low order transverse modes were identified by spectrally analyzing the emission; these were found to be complex and significantly different from those measured in the far field.
Date: December 9, 1999
Partner: UNT Libraries Government Documents Department

Final Report: Ultra Fast Regeneratively Amplified Titanium Sapphire Laser System, April 1, 1995 - March 31, 1997

Description: A regeneratively-amplified picosecond laser system for studying light-activatable energy-transducing molecules has been constructed with the equipment finds in this multiuser shared equipment grant. Manuscripts describing the use of this apparatus to study the transient release of heat by biomolecules using transient thermal gratings are in preparation.
Date: March 31, 1997
Creator: Schenck, Craig C.
Partner: UNT Libraries Government Documents Department

2003 Laser Diagnostic in Combustion Conference

Description: The GRC Laser Diagnostics in Combustion aims at bringing together scientists and engineers working in the front edge of research and development to discuss and find new ways to solve problems connected to combustion diagnostics. Laser-based techniques have proven to be very efficient tools for studying combustion processes thanks to features as non-intrusiveness in combination with high spatial and temporal resolution. Major tasks for the community are to develop and apply techniques for quantitative measurements with high precision e.g of species concentrations, temperatures, velocities and particles characteristics (size and concentration). These issues are of global interest, considering that the major part of the World's energy conversion comes from combustion sources and the influence combustion processes have on the environment and society.
Date: September 10, 2004
Creator: Allen, Mark G.
Partner: UNT Libraries Government Documents Department

Analysis of output surface damage resulting from single 351 nm, 3 ns pulses on sub-nanosecond laser conditioned KD2PO4 crystals

Description: We observe that by conditioning DKDP using 500 ps laser pulses, the bulk damage threshold becomes essentially equivalent to the surface damage threshold. We report here the findings of our study of laser initiated output surface damage on 500 ps laser conditioned DKDP for test pulses at 351 nm, 3 ns. The relation between surface damage density and damaging fluence (r(f)) is presented for the first time and the morphologies of the surface sites are discussed. The results of this study suggest a surface conditioning effect resulting from exposure to 500 ps laser pulses.
Date: October 31, 2007
Creator: Jarboe, J; Adams, J J & Hackel, R
Partner: UNT Libraries Government Documents Department