A vacuum-free ultrafast laser-based x-ray source is demonstrated. Hard x-rays up to 80KeV are generated from Cu, Mo, Ag, Sn, and Ge targets in a laminar helium flow surrounded by atmosphere using tightly focused 33fs, 3mJ laser pulses. X-ray spectra, conversion efficiencies, and source sizes are presented. Six-fold efficiency improvement is observed, over similar sources found in the literature . Source sizes determined for Cu and Mo show distinct dependences on laser pulse energy. It is also shown that the Cu source size has no dependence on the presence of the spectral band around the 8KeV K-shell lines.
©2008 Optical Society of America
X-rays have been widely used in industry, medicine, and many kinds of research, including crystallography, diffraction, microscopy, spectroscopy, imaging, and other analysis. In a typical x-ray source, electrons are accelerated from a cathode to a target anode using high voltage. When the energetic electrons interact with the bulk target material, bremsstrahlung and characteristic line x-ray photons are generated. Bremsstrahlung radiation arises due to deflection and deceleration of electrons in the target, resulting in a continuous x-ray spectrum. Line emission occurs when inner shell electrons in the anode are ejected by collisions and relaxation produces characteristic photons. The line spectra are narrowband and related to the local electronic configuration of the material. Due to the strong interaction of these bombarding electrons with air, x-ray generation has generally been restricted to the vacuum environment. The goal of the research discussed in this paper is to develop a hard x-ray source that works without any vacuum, by using an ultrafast laser to excite electrons indigenous to various target materials in microscopic regions bathed in a gentle helium flow.
Any material that might be chosen as an x-ray source already contains the electrons needed to generate x-rays characteristic of the source material. Furthermore, it is well-known that such indigenous electrons can be strongly accelerated by intense laser pulses that produce and interact with plasma at the surface of a source material through resonance absorption, vacuum heating, J⃗×B⃗. heating , etc. Many of the hot electrons of such an interaction leave the dense plasma region having been driven forward into the bulk material or backward into the vacuum. Consequently, strong space-charge fields develop in and around the plasma. The electric fields can draw back a significant portion of the backward-emitted electrons, many of which penetrate into the “cold” target. These energetic electrons generate bremsstrahlung and characteristic x-rays as do the energetic electrons of conventional sources.
Ultrafast lasers have been known since before the inception of Chirped Pulse Amplification (CPA) [3,4] to have the capability to power x-ray sources [5–8] as described above. However, such sources are still operated chiefly in vacuum because the generation of energetic electrons within the region of a focused laser pulse, incident in atmospheric pressure, is generally limited by the breakdown of air in front of the target material. Atmospheric breakdown  causes self-induced defocusing, plasma absorption and scattering, reducing intensity at the target. Laser-intensity profile fluctuations may also be enhanced by the pre-generated air plasma. The most important cause of intensity loss is the wavefront distortion imposed by the fore-plasma. Previous work has demonstrated the generation of x-rays in a re-circulating helium environment using 40fs laser pulses , which however, still requires a chamber to maintain the helium environment.
In order to generate x-rays in an atmospheric pressure environment one needs to minimize the effects of the fore-plasma on the laser intensity at the target surface. The following issues must be considered. (1) Minimization of the propagation distance in the distorting plasma in conjunction with production of a minimal focal spot and maximal intensity requires a small Rayleigh range, and therefore, a high-numerical-aperture focusing optic. Furthermore, the onset of ionization can be delayed and the ionized path length can be correspondingly reduced by increasing the ionization intensity threshold of the experimental environment. (2) Minimization of the refractive index perturbation within the fore-plasma requires a minimal plasma electron density. The plasma refractive index is given by
where ω is the laser frequency, and γ=(1-v 2/c 2)-1 2, the electron relativistic Lorentz factor. The electron plasma frequency is given by: ωpe=(4πnee 2/me)1/2, where me and e are the mass and charge of the electron, and ne is plasma electron density. For a given laser wavelength and intensity, the only parameter which can be controlled during experiments is the electron density. Selecting a gas with the fewest number of electrons per atom or molecule in the environment accomplishes this goal.
We implement these considerations using a 50-mm diameter f/1.2 60°-off-axis paraboloidal mirror, aided by a deformable mirror , to focus laser pulses in a gently flowing helium environment near the opening of a delivery nozzle . This setup tightly focuses laser pulses to a 1.1µm 1/e2 beam radius with a nominally 5µm Rayleigh range (zr), which minimizes the length of laser-interaction with the ionized helium fore-plasma. Helium has only two electrons to release when it is fully ionized, yielding the smallest effect of refractive index change of any gas. While hydrogen has the same number of electrons per molecule, its lower ionization threshold induces greater effect. Helium also has a significantly higher ionization threshold than air. Low density helium is singly ionized at ~1015W/cm2 and fully ionized at ~1016W/cm2 [12,13]. At atmospheric pressure, collisional effects may modify the actual threshold for helium ionization under ultrafast laser irradiation. Even with these effects, helium is expected to retain the highest breakdown threshold. Figure 1 shows the iso-intensity contour of laser beam at 1015 and 1016 W/cm2 near the focus. With the target installed at the beam waist, the axial distances from onset of singly and doubly ionized regions to focus are 120µm and 390µm, respectively. For comparison, if an f/2 optic is used, these numbers are nearly tripled, and for f/5 the increase in distorting path length approaches a factor of 20.
Another advantage of working in a helium environment is that helium and air block the energetic debris generated in the laser-target interaction. This protects the focusing optic from contamination and ablative machining. Laser-based generation of x-rays in vacuum generally requires a debris shield that must be replaced periodically; in some cases as often as every few minutes of operation. Without the difficulty of working with a vacuum chamber, and without the debris issues, x-ray sources are much easier to use and the experimental setup is greatly simplified.
We have recently demonstrated hard x-ray generation from Cu, Mo, Ag, Sn, and Ge, in laminar flowing helium at atmospheric pressure (without the use of an external chamber). The Mo source has also been applied to imaging and diffraction experiments. In section II of this paper, we describe a regenerative amplification laser system (regen) useful for x-ray generation, featuring excellent stability and intensity contrast. In section III, the experimental setup for x-ray generation is described, with experimental results discussed in section IV. We conclude with a discussion of the prospects for vacuum-free x-ray sources in section V.
2. High contrast regenerative amplification laser system
In experiments studying the interaction of intense laser pulses with solids, prepulses and amplified stimulated emission (ASE) arriving in advance of the main pulses can generate unwanted plasma expansion capable of affecting the main interaction of the intended pulse with the target material. It is desirable that a baseline for prepulse and ASE intensities be set below the breakdown threshold of target material and that intentional prepulse energy be made available to control plasma scale-length. It has been shown that ASE contrast may be improved by the use of an amplifier with a minimal numerical aperture [14,15]. Implementing this concept, we utilize a regenerative CPA system constructed according to the layout shown in Fig. 2. Seed pulses from a 90nm-bandwidth oscillator are selected at 500Hz and amplified to ~1µJ by a 6-pass preamplifier. 2.5mJ pulses from a Spectra-Physics Evolution-X laser pump this preamplifier. The preamplified pulses are then focused to a spot size of ~50µm through a saturable absorber (3mm-thick Hoya IR85) to improve pulse contrast . The cleaned pulses are then stretched to ~40ps and coupled into the regen. A cryogenically cooled Ti:sapphire crystal sealed inside a vacuum chamber and pumped from both ends by a total energy of 28mJ (Spectra-Physics Evolution X residual and Evolution 30) provides gain. Horizontally polarized seed pulses (100nJ, 40ps) enter the regen by an s-polarized reflection from a thin film plate polarizer and pass through a Pockels cell where they are switched to vertical polarization for minimal loss in the regen cavity. With two passes through the gain medium per round trip, the small signal gain and the round trip cavity loss are 1.8 and 17%, respectively. After 10 round trips 8.6mJ of the pulses are switched to horizontal polarization by the Pockels cell and dumped by s-polarized reflection off the second plate polarizer.
The regen cavity is 10.2m long corresponding to a round-trip time of 34.1ns. The cavity mode size resulting from the use of 3.00m radius-of-curvature mirrors matches the seed beam size at the crystal—1.04mm (1/e2 Gaussian diameter). The pump beams are focused to a slightly larger size of 1.1mm (1/e2 Gaussian diameter). An amplified average power of 4.3W, with short-term (10min.) stability of less than 0.25%, results.
After being dumped, the amplified beam passes through a collimating telescope and a Glan-laser polarizer that strips vertically polarized prepulses leaked by the output polarizer each round trip. All regen designs suffer from the existence of these prepulses. They can be a few percent of the main pulse; however, they can be largely suppressed by the use of a polarizer, given that their polarization is orthogonal to that of the main pulse. There is also a weaker identically polarized prepulse on every round trip produced by residual polarization rotation in the cavity. This configuration limits the strongest of these prepulses to ~2×10-4 of the main pulse with proper adjustment of the intra-cavity Pockels cell alignment. The beam is then passed through a second Pockels cell (with crossed Glan-laser polarizers), improving the contrast by another four orders of magnitude and ensuring that the prepulses occurring at multiples of the regen round trip time are below the level of 10-8.
A grating pair compresses the output pulses to 33fs FWHM with 4mJ energy. The pulse duration is measured by SPIDER (Spectral Phase Interferometry for Direct Electric field Reconstruction). This is longer than the transform-limited pulse duration of 28fs, owing to uncompensated high-order dispersion, where the Pockels cell KD*P provides the dominant term.
An additional improvement to the stated pulse contrast is gained by the fact that the prepulses have passed through less material and will not be as well compressed as the main pulse. The nearest, and least-decompressed, prepulse passes through a difference of 12mm of Ti:sapphire, 20mm of KD*P, and 14.8mm of fused silica. For a main pulse bandwidth of 33nm compressed to its transform limit (28fs FWHM), the nearest prepulse would theoretically be compressed to ~180fs, improving the intensity contrast by a factor of ~6. Also, the nanosecond ASE contrast was measured to be better than 4.5×10-11 using a third-order correlator with a dynamic range of >1012.
The laser beam is expanded to a diameter of 50mm by a mirror-based telescope located 30cm after the compressor. It is subsequently focused by an f/1.2 gold-coated paraboloidal mirror with the aid of a 47mm-diameter silver coated deformable mirror (Xinetics Inc). The perimeter intensity at this limiting aperture is ~10% of peak intensity. The deformable mirror corrects the mild wave-front distortions of the laser system and pre-compensates for the more severe aberrations caused by the focusing optic. After optimizing the figure of the deformable mirror in air with strongly attenuated laser pulses, a 1.1µm Gaussian radius is measured at the experimental focus. At full power, the pulse energy on the target is about 3mJ. The estimated peak on-target intensity is 6×1018W/cm2 (if focused in vacuum). However, upon consideration of the uncertainties in measurements of pulse duration, pulse energy, and focal intensity profile, as well as slight day-to-day variation in alignment, we estimate that at least 3×1018W/cm2 is consistently available.
3. X-ray generation without a chamber: setup and measurement configuration
To greatly simplify the use of x-ray sources we develop the experimental setup shown in Fig. 3, which allows us to apply these laser pulses to generate x-rays in open space without an enclosure . For radiation protection the source is shielded by an easily placed set of plastic and lead pieces that allow great freedom to reconfigure the source and experimental geometries. Focused p-polarized laser pulses are incident at 45° on target materials in a region where helium gas is released at a low rate of flow. A ½×2mm gas nozzle is mounted on a 3-dimensional translation stage and positioned ~3mm below the laser focus. With helium flowing at a rate of 0.3 to 1 liter/min, the nozzle position is adjusted to minimize the audible effect of breakdown in the absence of a target. As the degree of interaction with the weaker atoms and molecules composing air is reduced, the spectral blueshifting  of the pulses transmitted through the ionizing focus is minimized. Viewed through a BG39 color filter, the intensities of yellow and green colors generated in the focal region are thus reduced. Laser-induced breakdown spectroscopy (LIBS) of air reveals 3 strong lines of nitrogen II (500.5nm, 567.9nm, and 777.5nm) as shown in Fig. 4 (a), and, with the introduction of helium, several lines of helium I appear (447.1nm, 501.6nm, 587.6nm, 166.7nm, and 706.5nm) as shown in Fig. 4 (b). The minimization of the nitrogen lines can be used to finely adjust the nozzle position. The small nitrogen peaks observable in the latter spectrum, indicate that the buoyant helium is still allowing some air into the focal zone even at the optimal nozzle position. It is possible that when the target is set in the focal plane, more air mixes with the flowing helium in the focal zone, because the helium entrains air between its own flow and the target surface. A larger nozzle aperture with an optimized flow rate could produce a purer helium environment in the focal area but might also consume more helium. The energy loss of the laser beam passing, with ionization, through the flowing helium is measured to be 5%.
After the helium nozzle position is optimized, a target is installed in the focal plane. Though the focal intensity of the pre-pulse may exceed the target breakdown threshold in these experiments, neither the pre-expanding target plasma nor the electrons emitted by that plasma have the capability to alter the wavefront quality of the main pulse in the fore-plasma. The metallic targets used in our experiments are 10-mm-thick polished disks of Cu, Mo, Ag, and Sn, and the Ge target is a 500µm thick wafer. All targets are 100mm in diameter. The Ge wafer is affixed to a 10mm-thick optical glass flat. The target is installed on a 50mm mirror mount, which itself is fastened to a precision spindle which is part of a motor-driven <r,θ,z> stage. This target manipulator automatically moves the target under computer control to present fresh area for each laser shot. Target surfaces must be replenished after 30 to 120 minutes of exposure, depending on the degree of separation between subsequent shots. Care is taken to align the target due to the short Rayleigh range of the focus.
Laser generated x-ray spectra are recorded by an x-ray spectrometer consisting of a cadmium telluride (CdTe) detector and a multi-channel analyzer (Amptek). The spectrometer is calibrated against an independently recorded 241Am spectrum and fixed at 145–220cm from the x-ray source at 23° off target surface. A 200µm-diameter lead pinhole is placed 3cm in front of the lead-shielded detector to limit the spectrometer aperture, enabling single-photon-counting mode in the presence of very high photon flux. X-ray fluorescence caused by electron bombardment of materials outside the immediate focal zone is reduced by positioning a pair of strong deflecting magnets between the source and the detector. In addition to the 200µm-diameter lead pinhole in front of the x-ray detector, the aforementioned distance of 145cm~220cm is required before the spectrometer to avoid pile-up effects due to the high x-ray flux. The x-ray spectra of all targets are corrected for the transmission of air . A small number of erroneous counts are registered below ~7KeV arising from CdTe escape events or fluorescence from the intervening air or the lead aperture. These counts are exaggerated by the large absorption correction; hence data are not shown in this region. The data above 30KeV are uncorrected because the transmission coefficient is >95% in this situation. Between 6.7 and 80KeV, the CdTe detector efficiency is nearly flat and >95% according to the manufacturer. Finally, spectral signal is integrated over 2 minutes to obtain adequate photon counting statistics.
X-ray source size measurements of the stated elements, and experimental determination of source size scaling with the laser pulse energy are carried out using knife-edge projection measurements  at an angle of 23° from the target surface in the plane of incidence. An indirect-detection x-ray camera (Roper Scientific-XSI) collects images of a <110> cleaved edge of a piece of 500µm-thick GaAs crystal. Transmission of the GaAs crystal is 0.25% at 25.2KeV, which is the highest Kα photon energy (Sn) produced by the targets tested. In our data analysis, the crystal is considered to be opaque. The camera is configured with a 1:1 fiber-coupled scintillator (Hamamatsu, J6677-01). While the pixel size of the camera is 20×20µm2, the resolution of the x-ray camera system is measured to be 7 line-pairs/mm at 33% contrast ratio for lead resolution bars on the face of the scintillator. The magnification, defined as the ratio between the source-scintillator distance and the source-edge distance, ranges between 38.8 and 42.9 in the source size measurements.
4. Results and discussion
4.1 Hard x-ray spectra
The characteristic Kα and Kβ line emissions and broadband bremsstrahlung emission for all targets are visible in the spectra shown in Fig. 5. From these data, estimations are made of the energy conversion efficiency (CE) from incident laser pulse energy to total x-ray energy (summed over the range between 6.7KeV to 80KeV), and from laser to Kα line emission. In the calculation, it is assumed that x-ray emission has a nearly isotropic distribution as confirmed by angularly resolved measurements using a cadmium zinc telluride (Amptek) x-ray detector . The CEs and photon fluxes shown in Table I, are for x-rays emitted into the 2π sr solid angle in the front of the targets.
The Kα x-ray flux of ~1.2×1010 photons per second into 2π sr for Cu, driven by 1.5W is comparable to a source operating in helium with a brass-wire target , where an incident laser power of 8W produces approximately 1010 photons per second into 2π sr including all the emitted Kα lines (Fe, Cu, and Zn). The six-fold increase in efficiency in the present work may be attributed to a combination of three leading factors: the quality of focus afforded by the use of a deformable mirror; the sharpness of the focus (f/1.2 vs. f/1.5) and its influence on propagation length in ionized helium; or the influence of repetition rate (0.5kHz vs. 2kHz) and helium transport on debris.
The introduction, in front of the targets, of the small lead shielding box used in previous Cu x-ray measurements  reduces CEs by 10–50% without effecting spectral shape: the reduction for Cu being 50%. The reason for this reduction seems to be related to the partial obstruction of debris, and the consequent effect on the ionization threshold near focus and on wavefront quality.
In order to make further comparisons with experiments done in vacuum using different laser energies, we need to scale the CEs with the changes in pulse energy. The CEs in vacuum for these 5 targets were reported with 0.8mJ, 22fs pulses on the target . The CE scaling laws for pulse energy, E, (CEtot∝E 1.59 and CEKα∝E 1.50) were determined for Mo  Based on these results, vacuum CEs for the current laser parameters are extrapolated and compared with the present helium CEs by the ratios shown in Table I. These ratios range from 11 to 109.
Some of the reasons for this large range may rest in differences in target surface preparation for Ag, Mo, and Sn—these being commercially prepared surfaces for the earlier vacuum experiments  and surfaces polished abrasively to a nearly specular surface for the present study. Going from vacuum to helium, the loss of total x-ray efficiency is larger than that of Kα, i.e., the broadband bremsstrahlung emission weakens more than does the line emission, indicating a cooler plasma in helium. Little effect can be attributed to the difference in integration range used to determine the total x-ray yield, because the signal measured between the 3.8KeV cutoff of the vacuum experiment and the 6.7KeV cutoff of the current one represents only 5% of the total vacuum x-ray signal. This is within the margin of error for these experiments.
Among the causes for lower efficiency, compared with the vacuum case, is firstly the intensity loss caused by the helium fore-plasma before the laser pulses arrive at the targets. Using laser-induced-breakdown spectroscopy (LIBS) to observe the fore-plasma (Fig. 4 (b)), we see that the helium atoms are fully ionized, and weak nitrogen lines are still visible, indicating that the helium in the focal zone is not pure. The presence of some concentration of air molecules in the focal zone dramatically lowers the ionization threshold of the experimental environment and increases the electron density of the distorting fore-plasma. The reduction of the breakdown threshold might be sensitive even to small concentrations of low-ionization-threshold species, especially if collisional ionization dominates within the ultrashort pulse duration. In addition to the air molecules and atoms, N2, O2, Ar, and CO2 that can be measured by LIBS, residual debris from previous laser shots can further increase the fore-plasma distortions. All of these effects are essentially absent in vacuum, where ballistic transport rapidly carries debris away from the focal zone and the other species are absent a priori. The distorting plasma modifies the size of laser focal spot, changing its energy distribution, enhancing intensity fluctuations on the beam profile, and therefore, modifying the intensity distribution. As mentioned earlier, absorption and scattering of the incident light in the fore-plasma is not expected to exceed 5%. Secondly, wavefront distortions at the airhelium boundary may compound the plasma-induced distortion. Thirdly, in the vacuum experiments, short wavelengths generated by the plasma and filtered by 3-mm-thick BG39 glass are used to optimize the target position at the focal plane , while in the flowing helium experiments, the short wavelength signal is strongly affected by the ionization-blueshifted spectrum of the fundamental. Thus, it is possible that in the vacuum case, the target position with respect to the focal zone is more accurately controlled than in the flowing helium case.
4.2 Source size measurement
The procedure described in Refs. [11,19] is followed to infer the x-ray source sizes from the projection images of GaAs edges. The source sizes are presented in table I where the horizontal (H) refers to dimensions within the plane of incidence and the vertical (V) to those perpendicular to it. The vertical sizes range from 9 to 17µm, and the horizontal sizes from 12 to 28µm. The ellipticity of all the sources is consistent with the cited measurements, where the horizontal sizes are also larger than the vertical when viewed at 23° and 45° off the target surfaces.
In order to explore the spectral dependence of source size, we measured the Cu source with different filters. The source sizes shown in Table II are measured with 48-µm-thick Cu, 25-µm Zn, and 33.8-µm Fe foils fixed in front of the CCD camera. Within the statistical deviation of the data, we observe no source-size variation. The respective transmissions of these filters are shown in Fig. 6. Notably, the Fe filter removes the strong Kα and Kβ line emissions around 8KeV, yielding a measurement of bremsstrahlung source size. Yet, we know from the Cu x-ray spectrum that the energy in these lines account for 71% of the total x-ray energy. This much line emission energy added to bremsstrahlung emission would be sufficient to affect the source size measurement if the K-shell line source size were significantly different from that of the bremsstrahlung.
In the Ref. , the authors observed a decrease in the contrast of diffraction fringes formed at a distance from a knife-edge with increased laser pulse energy. These Fresnel diffraction experiments employed a similar ultrafast-laser-based Si x-ray source in vacuum. This suggested to the authors that an increase in source size occurs with the increasing pulse energy. This is confirmed here by the use of the knife-edge technique to measure the source size as a function of pulse energy for Cu and Mo targets. Increasing the incident pulse energy from 0.42 to 2.9mJ (with corresponding laser intensities from 0.9×1018 to 6.2×1018 W/cm2), the source sizes shown in Fig. 7 are obtained. For both targets, and at all pulse energies, the sources exhibit an elliptical shape, with the horizontal size being larger than the vertical. For the Cu target, the vertical size gradually increases from 5 to 9µm as the horizontal does, from 8 to 12µm. For the Mo source, the size at 0.42mJ is 6×7µm2 while for the higher pulse energies, the source sizes are ~9×13µm2-exhibiting more of a step-like behavior.
To evaluate these differing results, it may be useful to consider a recent study  that discusses energy transport in material surrounding laser excitation of Al. Based on irradiation of an Al target under a similar range of conditions, the authors were led to suggest that ballistic radiative transport and/or energetic electron transport having been driven by transverse ponderomotive forces and influenced by strong magnetic fields must play a more significant role than do thermal electron diffusion or radiative diffusion, in spreading energy from the irradiated area. Further investigation is needed to elucidate the x-ray source size broadening with respect to pulse energy.
We develop a vacuum-free, ultrafast, laser-based, hard x-ray source, with up to 80KeV hard x-rays. The source operates in a laminar helium flow in atmosphere through the interaction of 33fs, 3mJ laser pulses with Cu, Ge, Mo, Ag, and Sn targets. Measured x-ray spectra, source sizes, and conversion efficiencies are reported. The x-ray conversion efficiencies are between 1 and 10% of those expected in vacuum, while the source sizes remain comparable. 6-fold efficiency improvement is observed over previously published data for x-rays generated in an atmospheric helium environment.
Hard x-ray source sizes determined for Cu and Mo show distinct dependences on laser pulse energy. The size of the Cu source gradually increases with the laser pulse energy, whereas, the relationship between source size and pulse energy for the Mo target mimics a step function. It is also shown that, in this regime, the Cu source size has no dependence on the presence of the spectral band around the 8KeV K-shell lines.
The source in flowing helium is currently being used to provide diagnostics of fatigue evolution in Ni-based super-alloys and diffraction of individual dendrites in the same materials. The simplicity of operating this source with its small spot size and (still) high yield enhances our capability to apply x-ray measurement in practical applications such as crystal diffraction and high-resolution radiographic projection imaging, allowing greater range and agility in the arrangement of experiments. Based on our experience, we believe that the vacuum-free x-ray source will find use in many applications, such as, computerized tomography, microsecond to picosecond measurements of x-ray absorption (X-ray Absorption Fine Structure—XAFS, X-ray Absorption Near Edge Structure—XANES), time-resolved imaging and diffraction, cancer detection, solid-state physics, micro-circuit analysis, materials analysis, and plasma diagnostics. Finally, we note that further elucidation of the mechanisms by which sources operating in atmosphere may be improved is desirable.
This work is supported by the AFOSR (MURI FA9550-05-1-0416), the NSF (PFC PHY-0114336), and by The College of Engineering at University of Michigan.
References and links
1. F. Benesch, T. Lee, Y. Jiang, and C. G. Rose-Petruck, “Ultrafast laser-driven x-ray spectrometer for x-ray absorption spectroscopy of transition metal complexes,” Opt. Lett. 29, 1028–1030 (2004). [CrossRef] [PubMed]
2. S. C. Wilks and W. L. Kruer, “Absorption of ultrashort, ultra-Intense laser light by solids and overdense plasmas,” IEEE J. Quantum. Electron. 33, 1954–1068 (1997). [CrossRef]
4. G. A. Mourou, C. P. J. Barry, and M. D. Perry, “Ultrahigh-intensity lasers: physics of the extreme on a tabletop,” Phys. Today 51, 22–28 (1998). [CrossRef]
5. J. D. Kmetec, C. L. Gordon III, J. J. Macklin, B. E. Lemoff, G. S. Brown, and S. E. Harris, “MeV x-ray generation with a femtosecond laser,” Phys. Rev. Lett. 68, 1527–1530 (1992). [CrossRef] [PubMed]
6. A. Rousse, A. Antonetti, P. Audebert, A. Santos, F. Fallies, J. Geindre, G. Grillon, A. Mysyrowicz, and J. Gauthier, “Observation of x-ray fluorescence resulting from photoionization by a femtosecond laserproduced x-ray source,” J. Phys. B 27, L697–L701 (1994). [CrossRef]
7. B. Hou, J. A. Nees, W. Theobald, G. A. Mourou, L. M. Chen, J.-C. Kieffer, A. Krol, and C. C. Chamberlain, “Dependence of hard x-ray yield on laser pulse parameters in the wavelength-cubed regime,” Appl. Phys. Lett. 29, 2259–2261 (2004). [CrossRef]
8. J. C. Kieffer and J. P. Matte, H. Pépin, M. Chaker, Y. Beaudoin, T. W. Johnston, C. Y. Chien, S. Coe, G. Mourou, and J. Dubau, “Electron distribution anisotropy in laser produced-plasmas from x-ray line polarization measurements,” Phys. Rev. Lett. 68, 480–483 (1992).
9. D. Umstadter, “Relativistic laser-plasma interactions,” J. Phys. D: Appl. Phys. 36, R151–R165 (2003). [CrossRef]
10. O. Albert, H. Wang, D. Liu, Z, Chang, and G. Mourou, “Generation of relativistic intensity pulses at a kilohertz repetition rate,” Opt. Lett. 25, 1125–1127 (2000). [CrossRef]
11. B. Hou, J. Easter, K. Krushelnick, and J. A. Nees, “Generation of hard x-rays by femtosecond laser pulse interaction with Cu in laminar helium flow in ambient air,” Appl. Phys. Lett. 92, 161501 (2008). [CrossRef]
12. B. Walker, B. Sheehy, L. F. DiMauro, P. Agostini, K. J. Schafer, and K. C. Kulander, “Precision measurement of strong field double ionization of helium,” Phys. Rev. Lett. 73, 1227–1230 (1994). [CrossRef] [PubMed]
13. K. Y. Kim, I Alexeev, and H, M. Milchberg, “Single-shot measurement of laser-induced double step ionization of helium,” Opt. Express 10, 1563–1572 (2002). [PubMed]
14. C. Yanovsky, G. Felix, and Mourou, “Why ring regenerative amplification (regen)?” Appl. Phys. B 74, S181–S183 (2002). [CrossRef]
15. J. Easter, B. Hou, E. Power, and J. Nees, “Stable long-cavity regenerative amplifier with 10–11 ASE contrast,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science and Photonic Applications Systems Technologies, Technical Digest (CD) (Optical Society of America, 2007), paper JTuD6.
16. K.-H. Hong, B. Hou, J. A. Nees, E. Power, and G. A. Mourou, “Generation and measurement of >108 intensity contrast ratio in a relativistic kHz chirped-pulse amplified laser,” Appl. Phys. B 81, 447–457 (2005). [CrossRef]
17. P. Chessa, E. De Wispelaere, F. Dorchies, V. Malka, J. R. Marquès, G. Hamoniaux, P. Mora, and F. Amiranoff, “Temporal and angular resolution of the ionization-induced refraction of a short laser pulse in helium gas,” Phys. Rev. Lett. 82, 552–555 (1999). [CrossRef]
18. A. Thompson, X-ray Data Booklet (Lawrence Berkeley National Laboratory, Berkeley, CA2001), http://xdb.lbl.gov.
19. B. Hou, J. Nees, A. Mordovanakis, M. Wilcox, G. Mourou, L.M. Chen, J.-C. Kieffer, C.C. Chamberlain, and A. Krol, “Hard x-ray generation from solids driven by relativistic intensity in the wavelength-cubed regime,” Appl. Phys. B 83, 81–85 (2006). [CrossRef]
20. B. Hou, A. Mordovanakis, J. Easter, K. Krushelnick, and J. A. Nees, “Directional properties of hard x-ray sources generated by tightly focused ultrafast laser pulses,” submitted to Appl. Phys. Lett.
21. D. Boschetto, G. Mourou, A. Rousse, A. Mordovanakis, Bixue Hou, J. Nees, D. Kumah, and R. Clarke, “Spatial coherence properties of a compact and ultrafast laser-produced plasma KeV x-ray source,” Appl. Phys. Lett. 90, 011106, (2007). [CrossRef]
22. B. T. Bowes, H. Langhoff, M. C. Downer, B. Hou, J. Nees, and G. Mourou, “Femtosecond microscopy of radial energy transport in a micro-scale aluminum plasma excited at relativistic intensity,” Opt. Lett. 31, 116–118 (2006). [CrossRef] [PubMed]