Journal Searches
|
Published under the names: B. Walker, B.C. Walker, Barry Walker, and Barry C. Walker with institution address
zip codes 92093, 19716, 11973, and 92106. For those who are interested the following Web of Science
statistics apply as of 2006: |  |
Title
Authors
SOURCE Volume, pages, Year
Book Publisher / Series / ISSN or ISBN
Abstract (* cited 10 or more times,** cited more than 30 times, *** cited more than 100 times)
Ionization of methane in strong and ultrastrong relativistic fields
Palaniyappan S, Mitchell R, Sauer R, Ghebregziabher I, White SL, Decamp MF, Walker BC
PHYSICAL REVIEW LETTERS 100, 183001 (2008)
The photoionization of methane is reported for intensities up to 10(19) W/cm(2) with linear and circular polarized light. While fragmental ions (e.g., CH3+, CH+, C+, C2+) created from 10(14) W/cm(2) to 10(15) W/cm(2) are formed by Coulomb explosion, ionization to form C3+ and C4+ involves Coulomb explosion and tunneling ionization. In ultrastrong fields, removal of a carbon K-shell electron from methane proceeds via tunneling and rescattering ionization, without the influence of molecular channels. Photoelectrons from methane at 10(19) W/cm(2) extend up to kinetic energies of 0.6 MeV.
Impact of coherence in radiation from ultrahigh-field atomic ionization
Ghebregziabher I, Walker BC
PHYSICAL REVIEW A 77, 023417 (2008)
We quantify interference effects on radiation by calculating the angle- and energy-resolved Larmor radiation from atomic ionization in the focus of ultraintense field. Our calculations use a Monte Carlo classical and semiclassical tunneling probability current models of ionization for intensities in the range of 10(16) to 10(20) W/cm(2). For nonrelativistic intensities, whether the radiation from the photoionization is treated coherently or incoherently, classically or semiclassically, leads to a negligible effect on radiation. For relativistic intensities, coherently summing across the tunneling ionization probability current decreases the radiation by an order of magnitude when compared to classical ionization or incoherent summation of the radiation from the tunneling ionization probability current. The interference effect is most pronounced for high-energy photons since ionization and the electron quiver may be 1 mu m and extends over multiple radiation wavelengths.
Time-resolved momentum imaging system for molecular dynamics studies using a tabletop ultrafast extreme-ultraviolet light source
E. Gagnon, A.S. Sandhu, A. Paul, K. Hagen, A. Czasch, T. Jahnke, P. Ranitovic, C.L. Cocke, B. Walker, M.M. Murnane, H.C. Kapteyn
REVIEW OF SCIENTIFIC INSTRUMENTS 79, 063102 (2008)
We describe a momentum imaging setup for direct time-resolved studies of ionization-induced molecular dynamics. This system uses a tabletop ultrafast extreme-ultraviolet EUV light source based on high harmonic upconversion of a femtosecond laser. The high photon energy around 42 eV allows access to inner-valence states of a variety of small molecules via single photon excitation, while the sub-–10-fs pulse duration makes it possible to follow the resulting dynamics in real time. To obtain a complete picture of molecular dynamics following EUV induced photofragmentation, we apply the versatile cold target recoil ion momentum spectroscopy reaction microscope technique, which makes use of coincident three-dimensional momentum imaging of fragments resulting from photoexcitation. This system is capable of pump-probe spectroscopy by using a combination of EUV and IR laser pulses with either beam as a pump or probe pulse. We report several experiments performed using this system.
Effect of focal geometry on radiation from atomic ionization in an ultrastrong and ultrafast laser field
Isaac Ghebregziabher and Barry C. Walker
PHYSICAL REVIEW A 76, 023415 (2007)
We use a tunneling–Monte Carlo model to calculate the dynamics and emitted Larmor radiation from electrons ionized in an ultrashort and ultrastrong pulsed laser focus over the intensity range from 1017 to 1020 W/cm2. We find the spatial variation of a laser field affects the radiation and can no longer be neglected at laser intensities leading to relativistic effects. We identify three regimes for the interaction as a function of the ratio of the single cycle quiver amplitude of the photoelectron to the laser focus waist. Adopting a one-dimensional or plane wave approximation when the laser driven excursion of the photoelectron exceeds the focus waist overestimates the total radiated energy by as much as an order of magnitude. Despite this, the spectral amplitude of the highest-energy photons from ionization in a laser focus is comparable to the plane wave case for excursions up to the beam waist since the laser focus imparts an extra boost of speed for electrons exiting the focus. Full spatial and temporal integration that includes the ionization of charge states before the peak of the pulse do not differ significantly from results that include only the radiation from ionization of the charge state at the peak of the laser field.
Enhanced high-order harmonic generation from Xe, Kr, and Ar in a capillary discharge
B. A. Reagan, T. Popmintchev, M. E. Grisham, D. M. Gaudiosi, M. Berrill, O. Cohen, B. C. Walker, M. M. Murnane, J. J. Rocca, and H. C. Kapteyn
PHYSICAL REVIEW A 76, 013816 (2007)
We report the use of a preionized medium created by a capillary discharge to extend the cutoff photon energy in high-order harmonic generation. The observed enhancements result from a combination of reduced ionization energy loss and reduced ionization-induced defocusing of the driving laser. We observe harmonic emission from Xe up to a photon energy of 160 eV, and we extend this technique to other noble gases, observing photons with energies up to 170 eV from Kr and 275 eV from Ar. The discharge plasma also provides a means to spectrally tune the harmonics by tailoring the initial level of ionization of the medium. Our results are interpreted using a hydrodynamic-atomic physics model of the discharge plasma. This work demonstrates that capillary discharges are a versatile and general method for generating harmonics, in particular from ions. Finally, this approach should be scalable to efficiently generate coherent light at much shorter wavelengths, in combination with phase-matching techniques.
Many Electron Ionization Processes in Strong and Ultrastrong Fields
A. DiChiara, I. Ghebregziabher, S. Palaniyappan, E. L. Huskins, A. Falkowski, D. Pajerowski, B. C. Walker
Progress in Ultrafast Intense Laser Science VI, Springer Series in Chemical Physics 85, Yamanouchi, K.; Chin, S.L.; Agostini, P.; Ferrante, G. (Eds.) (in press 2007).
Total ionization yields are reported for the noble gases in ultrastrong fields. The intensity range studied is from 1015 W/cm2 to 1018 W/cm2 with ionization charge states from one to twelve. Sequential ionization processes are modeled by tunneling ionization and shown to be accurate within a factor of two near saturation. Nonsequential, multielectron ionization is observed for most species and involves the correlation of up to four electrons; furthermore, correlated ionization is observed to occur for electrons initially in different atomic shells. A semiclassical, 3D relativistic rescattering model is compared to the data with limited success. The model shows the qualitative behavior of nonsequential ionization but fails to accurately predict the yields for many higher charge states. In neon the model accounts for 15% of the observed nonsequential ionization but in xenon only 1% is accounted for. The laser magnetic field does not play a role in many nonsequential ionization processes and a rescattering deflection parameter is presented to predict when magnetic field effects will impact rescattering processes such as high harmonic generation and nonsequential ionization.
High Harmonic Generation from Ions in a Capillary Discharge Plasma Waveguide
D.M. Gaudiosi, B. Reagan, T. Popmintchev, M. Grisham, M. Berrill, O. Cohen, B.C. Walker,
M.M. Murnane, H.C. Kapteyn, J.J. Rocca
Optics and Photonics News, p 44 December 2006
Self-assembly of epitaxial monolayers for vacuum wafer bonding
I. Altfeder, B. Huang, I. Appelbaum, B.C. WalkerAPPLIED PHYSICS LETTERS 89, 223127 (2006)
We show that self-assembled epitaxial metal monolayers can be used for hetero-integration of mismatched semiconductors, leading to simultaneously low interfacial resistance and high optical transparency. Lattice-mismatched wafers of Si(100) and Si(111) were bonded at room temperature in situ after vacuum deposition of a single atomic layer of Ag. The interfacial resistance was measured to be 3.9 ×10-4 Ohm·cm2 and the optical transmission of the interface at 2500 nm is approximately 98%. We discuss the important role of electron confinement in ultrathin Ag layers as a possible contributor to the bonding energy.
Fringe-free, background-free, collinear third-harmonic generation frequency-resolved optical gating measurements for multiphoton microscopy
R. Chadwick, E. Spahr, J.A. Squier, C.G. Durfee, B.C. Walker, D.N. Fittinghoff
OPTICS LETTERS 31, 3366 (2006).
A background-free, fringe-free, form of frequency-resolved optical gating using the third harmonic signal generated from a glass coverslip is used to characterize 100 fs pulses at the focus of a 0.65 NA objective.
Emergence from nonrelativistic strong-field rescattering to ultrastrong-field laser-atom physics: A semiclassical analysis
S. Palaniyappan, I. Ghebregziabher, A. DiChiara, J. MacDonald, and B. C. Walker
PHYSICAL REVIEW A 74, 033403 (2006).
We discuss a rescattering deflection parameter to gauge the progression from strong- to ultrastrong-field rescattering dynamics. Our analytical and numerical treatments of rescattering quantify the change from nonrelativistic, dipole approximation strong-field dynamics to an ultrastrong-field interaction where v/c =/ 0 and Blaser =/ 0. At the threshold between these two regimes the Lorentz deflection reduces the returning electron flux at the core and suppresses rescattering related phenomena. Both radiation and nonsequential ionization from rescattering are studied. We show the ultrastrong-field regime can occur at intensities as low as 1014 W/cm2 for mid-IR (2 µm to 5 µm) wavelengths or as high as 1018 W/cm2 for UV radiation.
High-order harmonic generation from ions in a capillary discharge
D.M. Gaudiosi, B. Reagan, T. Popmintchev, M. Grisham, M. Berrill, O. Cohen, B.C. Walker, M.M. Murnane, H.C. Kapteyn, and J.J. Rocca
PHYSICAL REVIEW LETTERS 96, 203001 (2006).
We demonstrate a significant extension of the high-order harmonic cutoff by using a fully-ionized capillary discharge plasma as the generation medium. The preionized plasma dramatically reduces ionization-induced defocusing and energy loss of the driving laser due to ionization. This allows for significantly higher photon energies, up to 150 eV, to be generated from xenon ions, compared with the 70 eV observed previously. We also demonstrate enhancement of the harmonic flux of nearly 2 orders of magnitude at photon energies around 90 eV when the capillary discharge is used to ionize xenon, compared with harmonic generation in a hollow waveguide. The use of a plasma as a medium for highorder harmonic generation shows great promise for extending efficient harmonic generation to much shorter wavelengths using ions.
Multielectron ultrastrong laser field ionization of Arn+,Krm+, and Xel+ (n=8, m=9, l=12,) at intensities from 1015W/cm2 to 1018 W/cm2
S Palaniyappan, A DiChiara, I Ghebregziabher, E L Huskins, A Falkowski, D Pajerowski and B C Walker
J. Phys. B: At. Mol. Opt. Phys. 39, S357-S369 (2006)
Ionization yields are reported for Ar, Kr and Xe in ultrastrong fields from 1015 W/cm2 to 1018 W/cm2. Non-sequential ionization (NSI) is shown to be a robust and general feature in ultrahigh field ionization. NSI yields measured are consistent with the trends predicted by a rescattering model, but as one proceeds to higher Z atoms more NSI is observed than predicted theoretically. Additional recollision mechanisms that may need to be considered in future theories of ultrastrong field–atom interactions include ‘chain’ NSI, NSI from excited states of the atom (e.g. Rydberg states or inner-shell holes) and the possibility of ultrastrong field enhanced recollision/impact processes.
Impact of the laser magnetic field on recombination and bremsstrahlung radiation from atomic ionization rescattering in ultraintense fields
I. Ghebregziabiher, S. Palaniyappan, J. MacDonald, B.C. Walker
Phys. Rev. A 73, 033419 (2006).
Recombination and bremsstrahlung radiation yields from strong field rescattering of
9×1014 W/cm2 (Ne+) to 3×1017 W/cm2
(Ne8+) photoelectrons show the laser magnetic field alters the rescattering
dynamics for fields beyond one-atomic unit of field (3×1016 W/cm2).
At 3×1016 W/cm2, the deflection of the rescattering wave begins
to exceed its transverse width and the yields of photons with energies above 100 hartree
of energy (2.7 keV) are attenuated. Radiation at 3×1017 W/cm2
(Ne8+) is suppressed by 102 for the highest energy photons near
1000 hartree and by 105 for lower energy in the range of 10–100 hartree.
Ultrastrong field ionization of Nen+ (n <= 8): Rescattering and the role of the magnetic field
Palaniyappan S, DiChiara A, Chowdhury E, Falkowski A, Ongadi G, Huskins EL, Walker BC
Phys. Rev. Lett. 94, 243003 (2005)
Ne+ to Ne8+ ionization yields in 10^(14) W/cm^(2) to 10^(18) W/cm^(2) laser fields are reported
over a 10^(9) dynamic range. A 3D relativistic rescattering model incorporating (e,2e) and (e,3e)
electron impact ionization, single- and double-excitation is compared to the data. For double
ionization the agreement is excellent; however, for higher charge states the model accounts for
only 15% of multielectron nonsequential ionization. Rescattering is not affected by the laser
magnetic field until 10^(17) W/cm^(2).
Cross-shell multielectron ionization of xenon by an ultrastrong field
A. DiChiara, S. Palaniyappan, A.F. Falkowski, E.L. Huskins and B.C. Walker
J.PHYS.B 38, Letter L183-L190 (2005)
Ultrastrong field ionization of Xe from 1014 W cm-2 to 1018 W cm-2 is measured for states as high as Xe+10.
Ionization of all charge states involves correlated multielectron processes in addition to the normal
"sequential" field ionization mechanism, where electrons are ionized independently, one after another.
Correlated "nonsequential" multielectron ionization has been observed even between electrons in different
shells. Measurements of sequential ionization agree with a semiclassical tunneling ionization model,
but a relativistic rescattering model accounts for only 1% of the observed nonsequential multielectron
yield.
Larmor Radiation from the Ultra-Intense Field Ionization of Atoms
E.A. Chowdhury, I. Ghebregziabiher and B.C. Walker
J.PHYS.B 38, 517 (2005)
The angle- and energy-resolved Larmor radiation from atomic ionization in the focus of ultra-intense laser
field is calculated using a semi-classical, trajectory ensemble model of ionization. We find that including
the quantum nature of the atomic ionization decreases the radiation yield by an order of magnitude compared
to a classical electron calculation due to interference effects in the extended probability of the electron
wavefunction and the quantum nature of tunnelling ionization. The evolution of the radiation from non-relativistic
to relativistic continuum dynamics is presented as a function of the laser intensity with Larmor radiation not
becoming prominent until 1019 W cm-2. For the ionization of Na10+ at a density of 1015 atoms cm-3 and a peak-focused
laser intensity of 1020 W cm-2, the radiation is highly directional at an angle of 45° from the laser electric field
with photon energies out to 500 eV.
Electron momentum states and bremsstrahlung radiation from the ultraintense field ionization of atoms
Chowdhury EA, Ghebregziabher I, MacDonald J, Walker BC
OPTICS EXPRESS 12, 3911 (2004)
Relativistic continuum dynamics for electrons from the ionization of atoms in an ultraintense
(1017 W/cm2 to 1020 W/cm2)
laser focus are analyzed using a semi-classical wavelet model.
The results quantify the energy and angle resolved photoionization yields due to the developing
relativistic dynamics in ultraintense fields. Using the final state momentum, the bremsstrahlung
radiation yield is calculated and shows a linear relationship between the radiation cutoff and the
laser intensity. At 1020 W/cm2 photons with energies
out to 10 MeV should be observed. The results
are quantitatively comparable to the observed angle resolved photoelectron spectra of current
ultraintense laser-atom experiments. The results show the azimuthal angular distributions becoming
more isotropic with increasing intensity.
TEM00 Terawatt Amplification Using Micro-optical Spatial Mode Conversion
A. DiChiara, E. Chowdhury, G. Ongadi, B.C. Walker, R. Tamosaitis
OPTICS LETTERS 28, 2106 (2003).
Micro-optic technology is used in a terawatt multipass Ti:sapphire amplifier to
convert high multimode, 532nm radiation from an unstable resonator Nd:YAG laser
into a TEM00 amplified output without sacrificing the amplified to pump energy
conversion efficiency. Experimental measurements and Fourier analysis of the
spatial mode show a 3.8-fold increase in the peak irradiance and order of
magnitude improvement in the spatial contrast.
Multielectron Ionization Processes In Ultrastrong Laser Fields
E.A. Chowdhury, B.C. Walker
JOURNAL OF OPTICAL SOCIETY OF AMERICA B 20, 109-112, (2003).
The ionization of neon was studied in strong (1014 to 1016W/cm2) and
ultrastrong (1017 to 1018 W/cm2) laser fields.
Measurements of ionization yields in ultrastrong fields reveal that they are dominated by sequential tunneling ionization
of the ion charge states. The rescattering mechanism, identified with the generation of
high-order harmonics and multiple electron ionization in strong fields, is modeled for
ultrastrong fields and is shown to be reduced by orders of magnitude when compared with
strong-field ionization. The results from the model are consistent with the experimental
results and indicate that the reduced core size for ions and the Lorentz force in ultrastrong
fields combine to reduce rescattering in ultrastrong fields.
UPC Bar Codes In The Information Age
B.C. Walker
PROBLEM-BASED LEARNING CLEARINGHOUSE # 83784258444 (2002).
(Retrievable from https://www.mis4.udel.edu/Pbl/viewIndex.jsp?id=83784258444)
A wealth of data is tracked with bar codes and the desire for more information has never been greater. The technical
limitations of storing information using ?high-density? optical barcodes is addressed. Fundamental optical principles are used and the
real world application is emphasized.
Color Rendering, Perception, and Light: The Textile Case of Different Dyes
B.C. Walker
PROBLEM-BASED LEARNING CLEARINGHOUSE # 82644403083 (2002).
(Retrievable from https://www.mis4.udel.edu/Pbl/viewIndex.jsp?id=82644403083)
Color rendering impacts billions of dollars of commerce ranging from the color consistency of fabric for
clothing to the color ink accuracy in a desktop printer. Students investigate the physics of color absorption and the spectral
properties of light sources. The physiology of color perception is also discussed.
Beyond the TV Tube
B.C. Walker
PROBLEM-BASED LEARNING CLEARINGHOUSE #72447444316 (2002).
(Retrievable from https://www.mis4.udel.edu/Pbl/viewIndex.jsp?id=72447444316)
Students are asked to compare CRT, LCD, and plasma display technology. After evaluating the merits and physics behind
each device, they decide which should be invested in with R&D money for the TV/computer display market in 3 years.
0.09 terawatt pulses with a 31% efficient, kilohertz repetition rate Ti:sapphire regenerative amplifier
J.Z.H. Yang, B.C. Walker
OPTICS LETTERS 26, 453-455, (2001).
We present an ultrafast regenerative amplifier that increases the energy of a laser pulse from 300 pJ to
6 mJ and produces average powers up to 9 W in a TEM00 spatial mode. The efficiency of the amplifier is 31%. As an ultrafast
amplifier, the system produces 0.09 TW peak powers with 4 mJ pulses having a 41 fs to 46 fs FWHM duration.
Generation of Hard X-rays by Ultrafast Terawatt Lasers
T. Guo, C. Spielmann, B.C. Walker, C.P.J. Barty
REVIEW OF SCIENTIFIC INSTRUMENTS 72, 41-47, (2001).
A compact, table-top terawatt Ti: Sapphire laser driven, ultrafast hard X-ray source for
time-resolved X-ray diffraction studies is described. With a copper target the energy conversion
efficiency from laser photons (800 nm) to copper K X-ray radiation (1.54 Å) is 0.008 %. The optimal
laser intensity for generating these X-rays is 1E18 W/cm2, lower than the highest laser intensity
available (5E18 W/cm2) from the laser system. These results are consistent with a theoretical model
proposed on the basis that the X-rays are produced as a result of laser driven electron ionization of
core level electrons of Cu atoms at near room temperature. This source also provides features such as
ultrashort pulse duration, extremely small source size, variable wavelengths, high peak spectral
brightness, and the potential for multiple beam line experiments. X-ray diffraction patterns from
GaAs single crystals and amorphous Ni films recorded with this source are presented.
``Nonrelativistic'' ionization of the L-shell states in argon by a ``relativistic'' 1019 W/cm2 laser field
Enam A. Chowdhury, C. P. J. Barty, BC Walker
PHYSICAL REVIEW A 6304, 2712, (2001)
The field ionization cross sections for the L shell states in argon are presented as measured with
pulsed laser radiation at an intensity of up to 1019 W/cm2. For these ultrahigh intensities, the
photoelectron continuum dynamics have been shown to be relativistic. However, the measured charge
state yields for Ar+9 to Ar+16 compare favorably to numerical solutions of the nonrelativistic
Schrodinger equation and to a popular ADK/WKB tunneling ionization model. The results are interpreted
within a two-step, strong-field ionization model where the initial tunneling ionization process is
dominated by nonrelativistic effects while the photoelectron continuum dynamics are strongly
relativistic.
Ionization of Argon n=2 (Ar+9 to Ar+16) by a "Relativistic" Laser Field
Barry C. Walker, Enam A. Chowdury, C. Toth, K.R. Wilson, C.P.J. Barty
MULTIPHOTON PROCESSES, AIP Press, New York (ISBN 1-56396-946-7), 667-672 (2000)
The high field photoionization cross sections for the (n=2) shell of argon were measured with
pulsed 800 nm radiation (27± 5 fs duration) at an intensity of 1.5´ 1019 W/cm2 ± 1´ 1019 W/cm2. The
pondermotive energy for an electron at these field intensities is approximately 1 MeV. The best
agreement with a calculated AC tunneling model was at an intensity of 0.6´ 1019 W/cm2. The difference
between experimental and calculated relative charge state yields is less than an order of magnitude
for Ar +9 to Ar +14 and Ar +16.
Ultrafast coherent and incoherent X-ray generation by inner shell atomic processes induced by 25 fs, 1J pulses of high power CPA lasers.
Cs. Tóth, D.Kim, Barry C. Walker, T.Guo S.H.Son, C.W.Siders, A.Cavalleri, C.P.J.Barty:
LASER PHYSICS 10, 513-520, (2000)
A new class of keV X-ray lasers based on specific inner-shell atomic transitions is analyzed. It
is shown that population inversion between inner-shell vacancy levels of medium-Z elements can be
created using only electron collisional ionization, or by using fast, incoherent X-ray pumping if the decay of the lower states is mediated by Coster-Kronig or super-Coster-Kronig processes. The requirements of extremely fast energy deposition on a target in order to compete with the inherently fast (0.1 to 20 fs) atomic processes can be fulfilled by use of an ultrahigh peak power laser system with ultrashort pulse duration. A state-of-the-art chirped pulse amplification (CPA) laser system delivering more than one Joule of optical energy in less than 25-fs with 10 Hz repetition rate is described. The laser system has also been used to produce high-brightness, narrow-linewidth incoherent X-rays that are optimal for time dependent X-ray diffraction studies. The design of sandwiched multilayer target structure and possible experimental configurations for effective production of coherent X-ray radiation are suggested. Preliminary experimental results of ultrafast 25 fs, high intensity (1019 W/cm2) excitation oflayered metal targets show anomalous enhancement of specific Ti lines in the 2-14 nm wavelength range.
A 50-EW/cm 2 Ti:sapphire laser system for Studying relativistic light-matter interactions
Barry Walker, C.Toth , D.Fittinghoff, T.Guo, D.Kim, C.Rose-Petruck, J.Squier, K.Yamakawa, K.Wilson, C.P.J.Barty
OPTICS EXPRESS 5, 196-202, (1999)
A 10-Hz repetition rate, 60-TW peak power, Ti:sapphire laser system was developed for use in
experiments where relativistic effects dominate the physics. The temporal, spectral, energy and
spatial characteristics of the laser pulses were measured in single shot format. The pulse duration
ranged from 22 fs to 25 fs and the pulse energy averaged 1.3 J. Atomic photoionization measurements
quantified the peak intensity of the laser pulse in situ. The measurements indicated an intensity of
at least 5*1019 W/cm2 was produced.
Theoretical And Experimental Spectral Phase Error Analysis For Pulsed Laser Fields
Walker B, Toth C, Fittinghoff D, Guo T
JOURNAL OPTICAL SOCIETY AMERICA B 16, 1292-1299, (1999)
Distortions in pulsed laser fields are analyzed using the root-mean-square of intensity weighted
spectral phase deviations. This method quantifies pulse errors independent of pulse duration and can
be applied to both simple, "transform limited" pulses and complex, "shaped" pulses. A good linear
relationship exists (fit correlation = 0.95) between the analyzed phase deviations and temporal pulse
distortion measures. In contrast, a common Taylor series analysis showed a fit correlation of only 0.5
with temporal measures. Alternative methods, such as the pulse FWHM, were determined to be less
general measures of pulse distortion and, in modeling of spectral phase errors, were shown to have
the potential of being misleading.
Picosecond-Milliangstrom Lattice Dynamics Measured By Ultrafast X-Ray Diffraction
Rose-Petruck C, Jimenez R, Guo T, Cavalleri A, Siders C, Raksi F, Squier J, Walker BC, Wilson KR, Barty CPJ
NATURE 398, 310-312, (1999)
Many processes fundamental to chemistry, physics, materials science, and biology, including the
dynamics of chemical bonds, crystal lattice dynamics and transition state passage occur on ultrafast
(femtosecond and picosecond) time scales. Although ultrafast optical spectroscopy provides access to
the relevant time scales, it does not directly provide structural information. Ultrafast x-ray
diffraction promises the direct measurement of atomic motions during such processes. In recent years,
various methods of generating ultrashort x-ray pulses have been developed and substantial progress has
been made toward ultrafast, time resolved structural measurements. Recently, a sub-picosecond
diffraction response to ultrashort optical excitation has been observed. In this letter, we present
an ultrafast x-ray diffraction study of optically excited GaAs which provides quantitative information
on ultrafast lattice dynamics in the bulk of the semiconductor after surface deposition from a 30
femtosecond optical pulse. Our experimental results are closely matched by a theoretical calculation,
based on the known thermo-elastic properties of GaAs, of 100 ps coherent acoustic phonon propagation
induced by rapid thermal expansion of the surface.*
Hybrid Vacuum-Atmosphere Compressors For Ultrafast, High-Peak Power Chirped-Pulse Amplification
Walker BC, Squier J, Fittinghoff DN, Rose-Petruck C, Barty CPJ
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 4, 441-444, (1998)
Theoretical and experimental results are presented for ultrafast, chirped-pulse amplifier
compressors that contain only the final, high-peak power grating in vacuum. The designs are compact
and the adjustments of the compressor gratings are at atmosphere. The optical aberrations and
dispersion of the system are found to be negligible for the compression of 25-fs pulses. The
application of the hybrid vacuum-atmosphere compressor is robust with respect to a broad range of
wavelengths and optical configurations.
Single- And Multiple-Electron Dynamics In The Strong-Field Tunneling Limit
Sheehy B, Lafon R, Widmer M, Walker B, DiMauro LF, Agostini PA, Kulander KC
PHYSICAL REVIEW A 58, 3942-3952, 1998
Evolution of atomic ionization into the strong- field limit offers the opportunity to study the
fundamentals of atom-laser interaction. In this study, we report on high precision measurements of
the ion and electron distributions from laser-excited helium and neon atoms which reflect the changing
into the pure tunneling regime. The experiments present evidence of both single- and two-electron
ionization. These data provide a direct quantitative test of various theories of strong-field
ionization. We show that a relatively simple semiclassical model which includes a description of a
field-driven electron elastically rescattering from an accurate ion core potential reproduces the
measured electron distributions for both atoms. However, using this model to calculate e-2e inelastic
rescattering yields cross sections which are incompatible with the measured two-electron ionization. *
Pulse Characterization Techniques For Sub 30 Fs Femtosecond Terawatt Lasers
Toth C, Fittinghoff D, Walker BC, Squier J, Barty CPJ
ULTRAFAST PHENOMENA XI 151, 282-288, 1998
SPRINGER-VERLAG / CHEMICAL PHYSICS / 3-540-65430-5
Dispersion Considerations In Ultrafast CPA Systems
Fittinghoff DN, Walker BC, Squier JA, Toth CS, Rose-Petruck C, Barty CPJ
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 4, 430-440, 1998
A basic assumption underlies many designs for chirped-pulse amplification (CPA) of ultrashort
pulses, This assumption is that a Taylor's series expansion of the dispersive delay is well behaved
in the sense that each phase order in the expansion produces an effect on the pulse that is
significantly smaller than the effect of the previous order, This work investigates this assumption
both qualitatively and quantitatively. We show quantitatively that the requirements for achieving
sub-20-fs pulses are much more stringent than for 100-fs pulses. We find that when the basic
assumption holds, a chirped pulse amplification (CPA) system may be designed by zeroing each order
in succession, but that zeroing may not work well for some systems that are not well behaved. For
these cases minimizing the overall dispersion becomes necessary. We discuss some common optical
components including bulk materials, expanders, and compressors and show that they generally satisfy
the basic assumption. We also discuss some optical components or effects that may not satisfy the basic
assumption such as dielectric mirrors or etalons, and describe means of limiting the problems
associated with them when designing ultrafast CPA systems. Finally, we discuss the problem of
optimizing a CPA system in the laboratory, and describe a new polarization-gate (PG)
frequency-resolved optical gating (FROG) arrangement that is based on thin-film polarizers and
that allows accurate measurements of the phase as well as the intensity with minimal dispersive
effects.
Ultrahigh Peak Power Lasers In The 10-Fs Regime
Barty CPJ, Guo T, LeBlanc C, Raksi F, RosePetruck C, Squier J, Walker BC, Wilson KR
APPLICATIONS OF HIGH FIELD AND SHORT WAVELENGTH SOURCES 1997, 7, 246-248
OPTICAL SOCIETY OF AMERICA / TECHNICAL DIGEST / 1-55752-489-0
Techniques for high-fidelity, high repetition rate amplification of 10-fs-range pulses to
ultrahigh peak powers are presented. A 70 TW-level Ti,sapphire system incorporating these ideas
is described and requirements for the extension to petawatt power levels are outlined.
X-Ray Diffraction Study Of Laser-Matter Interactions With An Ultrafast Table-Top X-Ray Source.
Guo T, Rose-Petruck C, Jimenez RX, Squier JA, Walker BC, Wilson KR, Barty CPJ
IN SITU PROCESS DIAGNOSTICS AND INTELLIGENT MATERIALS PROCESSING 502, 77-82, 1997
MATERIALS RESEARCH SOCIETY / MATERIALS CHARACTERIZATION THIN FILMS AND INTERFACES / 1-55899-407-6
X-ray diffraction, employing a table-top, laser driven x-ray source, has been used to investigate
laser-matter interactions with simultaneous picosecond and subatomic range distance resolution.
The x-ray source, consisting of a table-top terawatt laser system and a moving Cu wire target
apparatus, generates 5 * 10^10 photons/(4 pi steradians s) of Cu K_alpha radiation. The lattice
dynamics of the 111 planes of GaAs single crystals has been studied after the crystal is exposed to
intense femtosecond laser pulses. The diffraction results have yielded information about the
timescales of the lattice dynamics in the picosecond range and an upper limit for the width of
the x-ray pulses. Initial strain, defined as the percentage of lattice distortion resulting from
the laser illumination, is as high as 0.25 % and is followed by an exponential decay with a time
constant of 150 ps. Increases in the diffraction intensity after the laser irradiation have also
been observed, likely due to a transition from the dynamic to kinematic diffraction associated with
degradation of the crystal.
Single And Multiple Electron Dynamics In The Strong Field Limit
Sheehy B, Walker B, Lafon R, Widmer M, Gambhir A, DiMauro LF, Agostini P, Kulander KC
MULTIPHOTON PROCESSES 154, 106-117, 1996
INSTITUTE OF PHYSICS PUBLISHING LTD / 0951-3248
High precision photoelectron energy and angular distributions in helium and neon atoms for a
broad intensity range reflect the change in the continuum dynamics that occurs as the ionization
process evolves into the pure tunneling regime. Elastic rescattering of the laser- driven free
electron from its parent ion core leaves a distinct signature on the spectra, providing a direct
quantitative test of the various theories of strong field multiphoton ionization. We show that
it takes a relatively complete semi-classical rescattering model to accurately reproduce the
observed photoelectron distributions. However, the calculated inelastic rescattering rate fails
to reproduce the measured nonsequential double ionization yields.
Aberration-Free Stretcher Design For Ultrashort- Pulse Amplification
Cheriaux G, Rousseau P, Salin F, Chambaret JP, Walker B, Dimauro LF
OPTICS LETTERS 21, 414-416, 1996
A novel aberration-free pulse stretcher design is presented. This system permits the stretching
of a 30-fs pulse to 300 ps and recompression to a duration of 33 fs, limited by the spectral
clipping.*
Elastic Rescattering In The Strong Field Tunneling Limit
Walker B, Sheehy B, Kulander KC, DiMauro LF
PHYSICAL REVIEW LETTERS 77, 5031-5034, 1996
High precision measurements of helium photoelectron energy and angular distributions for a broad
intensity range reflect the change in the continuum dynamics that occurs as the ionization process
evolves into the pure tunneling regime. Elastic rescattering of the laser- driven free electron
from its parent ion core leaves a distinct signature on the spectra, providing a direct
quantitative test of the various theories of strong field multiphoton ionization. We show that
it takes a relatively complete semiclassical rescattering model to accurately reproduce the
observed distributions. *
Sub-20-Fs Multiterawatt Lasers And X-Ray Applications
Barty CPJ, Guo T, LeBlanc C, Raksi F, RosePetruck CG, Squier JA, Walker BC, Wilson KR, Yakovlev VV, Yamakawa K
X-RAY LASERS 151, 282-288, 1996
INSTITUTE OF PHYSICS PUBLISHING LTD / 0951-3248
Techniques for the generation of multiterawatt, sub-20-fs optical pulses are outlined.
Novel, ultrashort duration, electron pumped x-ray laser concepts are also described.
Observation Of Continuum-Continuum Autler-Townes Splitting - Reply
Walker B, Kaluza M, Sheehy B, DiMauro LF, Agostini P, Trahin M
PHYSICAL REVIEW LETTERS 77, 203-203, 1996
Phase-Control In The 2-Color Photodissociation Of HD+
Sheehy B, Walker B, Dimauro LF
PHYSICAL REVIEW LETTERS 74, 4799-4802, 1995
We report on the use of the relative phase between a two color optical field (1053 and 527 nm)
to coherently control the photodissociation of the HD+ molecule. The field induced spatial asymmetry
in the fragment spectrum shows a strong phase dependence, while no selectivity si produced in nearly
isoenergetic channels. The positively charged fragments are observed to preferentially dissociate in
a direction counterintuitive to classical expectations. **
Observation Of Continuum-Continuum Autler-Townes Splitting
Walker B, Kaluza M, Sheehy B, Agostini P, Dimauro Lf
PHYSICAL REVIEW LETTERS 75, 633-636, 1995
Photoelectron energy spectra resulting from two- photon ionization of calcium atoms by 180 fs
pulses have been studied as a function of wavelength (380-405 nm) and intensity (9-900 GW/cm(2)).
When the wavelength is tuned near the 4s --> 4p or 4s --> 5s ionic core transition, the photoelectron
peaks display a characteristic splitting proportional to the field strength and assignable to a
two-electron continuum-continuum Autler-Townes effect. Spectra obtained by an essential- state model
involving three coupled continua are compared to the experiment.*
U(P), 3U(P), 11U(P) - Above-Threshold Ionization Revisited
Yang B, Schafer KJ, Walker B, Kulander KC, Dimauro LF, Agostini P
ACTA PHYSICA POLONICA A 86, 41-50, 1994
Angular distributions of very high energy photoelectrons from Xe and Kr, excited by a 50 ps,
1054 nm laser, are presented. In Xe strong, narrow structures 45-degrees off the polarization axis
appear on above-threshold ionization peaks within a limited energy range centered around 9U(P),
where U(P) is the intensity-dependent ponderomotive energy. Under the same conditions the effect
is much weaker in krypton. These structures are discussed using a very simple classical model as
well as sophisticated single active electron calculations and the Keldysh-Faisal- Reiss theory. We
conclude these structures are the result of single-electron dynamics involving rescattering of a
tunneling component of the continuum wave function.
Precision-Measurement Of Strong-Field Double-Ionization Of Helium
Walker B, Sheehy B, Dimauro LF, Agostini P, Schafer KJ, Kulander KC
PHYSICAL REVIEW LETTERS 73, 1227-1230, 1994
The production of He+ and He-2+ by a 160 fs, 780 nm laser has been measured over an unprecedented
12 orders of magnitude in counting range. Enhanced double electron emission, called nonsequential
(NS) ionization. was observed over an intensity range where the single ionization dynamics is evolving
from multiphoton to pure tunneling. The NS yield is found to scale with the ac-tunneling rate for
the neutral, even when tunneling is not the dominant ionization pathway. A rescattering mechanism
fails to predict the observed NS threshold or magnitude. ***
Search For Strong Field Direct Two Electron Ionization Using Coincidence Spectroscopy
Agostini P, Mevel E, Breger P, Walker B, Yang B, DiMauro LF
SHORTWAVELENGTH V, PHYSICS WITH INTENSE LASER PULSES 17, 78-82, 1993
OPTICAL SOCIETY OF AMERICA / 1-55752-298-7
Angular distributions of very high energy photoelectrons from Xe and Kr, excited by a 50 ps,
1054 nm laser, are presented. In Xe strong, narrow structures 45-degrees off the polarization axis
appear on above-threshold ionization peaks within a limited energy range centered around 9U(P),
where U(P) is the intensity-dependent ponderomotive energy. Under the same conditions the effect
is much weaker in krypton. These structures are discussed using a very simple classical model as
well as sophisticated single active electron calculations and the Keldysh-Faisal- Reiss theory. We
conclude these structures are the result of single-electron dynamics involving rescattering of a
tunneling component of the continuum wave function.
Intensity-Dependent Scattering Rings In High-Order Above-Threshold Ionization
Yang Br, Schafer KJ, Walker B, Kulander KC, Agostini P, Dimauro LF
PHYSICAL REVIEW LETTERS 71, 3770-3773, 1993
Angular distributions of high energy photoelectrons from Xe and Kr, excited by a 50 ps, 1.05
mum laser, are presented. In Xe, strong, narrow rings approximately 45-degrees off the polarization
axis appear in a limited energy range centered around 9U(p), where U(p) is the ponderomotive
energy. This effect is much weaker in Kr. Single active electron calculations agree well with
these observations. We conclude that the rings result from single-electron ionization dynamics,
most likely involving rescattering from the ion core of the tunneling component of the continuum
wave function. **
Double-Ionization In The Perturbative And Tunneling Regimes
Walker B, Mevel E, Yang Br, Breger P, Chambaret Jp, Antonetti A, Dimauro LF, Agostini P
PHYSICAL REVIEW A 48, R894-R897 , 1993
We have studied the doubly charged ion yields and electron energy spectra (normal and coincidence)
for double ionization of atoms in two different ionization regimes (perturbative and tunneling). In
both cases, the double-ionization rates appear to be anomalously large in some intensity range and
strongly reduced by circular polarization. It is argued that these similar behaviors must result from
different physical mechanisms. **
Restriction Endonuclease And Genetic-Mapping Studies Indicate That The Vegetative Genome Of The Temperate, Salmonella- Specific Bacteriophage, Epsilon-15, Is Circularly-Permuted
Mcconnell M, Walker B, Middleton P, Chase J, Owens J, Hyatt D, Gutierrez H, Williams M, Hambright D, Barry M, Sage S, Fuller G, Birdwell M, Rydelski M, Risley S, Kat B
ARCHIVES OF VIROLOGY 123, 215-221, 1992
Data from physical and genetic mapping studies show that the vegetative genome of the temperate,
Group E 1 Salmonella bacteriophage, epsilon(15), is circularly-permuted. Preliminary evidence suggests
that the circular permutation of the epsilon(15) genome is non- random.
Anomalous Intensity-Dependent Vibrational Distributions Of Oxygen Molecules In A Nonresonant Laser Field - A Molecular Perspective
Walker B, Saeed M, Breeden T, Yang B, Dimauro Lf
PHYSICAL REVIEW A 44, 4493-4498, 1991
We report on the results of nonlinear ionization and dissociation of gas-phase oxygen molecules
in an intense (10(11-13) W/cm2) laser field. The investigations were performed with the second- and
third-harmonic wavelengths of two different laser systems, a 50-ps Nd,YLF (where YLF denotes yttrium
lithium fluoride) and a 10-ns Nd,YAG (where YAG represents yttrium aluminum garnet). Measurements
include energy resolved photoelectron and mass spectroscopy. We report on an interesting observation
made with excitation of the neutral with green (approximately 2.3 eV) photons, in which the
vibrational distribution of the molecular ion shows a strong intensity dependence. We have analyzed
this intensity-dependent behavior to be a result of a complex interplay between different resonant
and nonresonant electronic channels, which to first approximation does not require dynamical
Stark shifting of states. A model is presented that describes this behavior and results in the
determination of the molecular constants for the intermediate states.
