We present simple formulas for the diffraction efficiencies of a binary phase grating that performs array illumination with ultrashort laser pulses. Using scalar diffraction theory, we formulated the efficiencies as a function of pulse spectral width by Fourier-transforming the complex-modulated frequency spectra of diffracted pulses in the far-field region. From the analytical simulations, we found that pulse array uniformity departs from unity as the spectral width increases, or the pulse duration decreases, thereby limiting the attainable split counts. This finding can be considered in the design of gratings for delivering controlled amounts of pulse energies to diffraction orders of interest.
We propose a cascade optical system for multifocusing ultrashort pulse beams. In this system, diffractive and refractive subsystems that are optically coupled in cascade correct chromatic aberrations, a phase plate compensates for angular dispersions, and material dispersions are removed by pre-chirping the input pulse. Achromaticity of the system is essential for simultaneous compensation of spatio-temporal pulse distortions. We designed a system by applying the aberration correction conditions derived from an ABCD-matrix analysis. The designed system was evaluated with 20-fs pulses by characterizing the transmitted pulses in beam width and pulse duration to verify the proposed distortion compensation scheme. This technology enables high-throughput ultrafast laser processing.
In this paper, we demonstrate an improved way of providing direct access to the carrier-envelope phase (CEP) in low-energy pulses (nearly 1 μJ) by using the interference between the second harmonic (SH) and the third harmonic (TH) from a solid surface at the same time. The important point is that the spectra of these two harmonics overlap in the UV region. So, the interference signals between the SH and TH from the surface of a solid do not include a constant offset phase component due to the linear dispersion in the SHG crystal and can be used to measure the CEP directly. We describe CEP measurement based on the interference between
SH and TH. The result showed that the CEP fluctuation in our system was within ±π/6 even without a slow
feedback loop. A slow feedback loop, which compensates for an additional CEP drift caused in the amplifier and
the compressor, suppressed the CEP fluctuation within ±π/12 for hours.
Vertical III-V compound semiconductor nanowires grown on Si(111) surfaces have been attracting interest for
application to opto-electronic integrated circuits (OEICs). In nanowire growth, heterostructures in the axial and radial
direction can be obtained by combining different materials with different growth conditions. These effects should make
it possible to fabricate complicated and functional three-dimensional structures in a bottom-up manner. These advances
should lead to new types of nanodevices. We describe the formation of several heterostructures using GaP-based
nanowires on Si(111). The catalysts used were Au particles obtained from Au colloids. We obtained GaP/GaAs/GaP
nanowires bent at thinned GaAs nodes, InP egg-like structures in GaP nanowires, core-multishell Ga(In)P/GaAs(or air-gap)/
GaP nanowires with flat tops, and GaAs/AlInAs capped GaInAs nanowires for long-wavelength photon emission.
These structures were successively grown on vertical GaP nanowires on Si(111) substrates.
We have demonstrated the sub-10-ps time-resolved measurement of the x-ray absorption fine structure (XAFS)
in laser-excited Si foil by using a femtosecond laser-produced plasma soft x-ray as a probe. We observed a rapid
change and recovery in the absorption structure near its LII,III edge induced by 100-fs laser pulse irradiation
when the laser intensity was in the 109-1010 W/cm2 range. When the incident laser intensity was of the order of
1012 W/cm2, which is higher than the damage threshold, the extended x-ray absorption fine structure (EXAFS)
signals clearly revealed inter atomic distance expansion and structural disordering as well as a change in the
electronic structure caused by the production of liquid Si. We also describe our recent results on spatio-temporally
resolved soft x-ray absorption in an expanding ablated particle cloud from aluminum that was heated with a
1014-W/cm2, 100-fs laser pulse by using an imaging system for time-resolved soft x-ray absorption spectroscopy.
Time-integrated spatially resolved emission spectra of soft x-rays were obtained in the case of 100-fs-laser irradiation of tantalum and aluminum targets by controlling the time intervals between the main pulse and artificial pre-pulse. The intensities of the main pulse and pre-pulse were fixed to 1.3x1016 and 2.0x1014 W/cm2. X-ray intensities were enhanced more than several ten fold at the optimal pulse-separation time of 2 ns. We confirmed that the main emission area for each target was less than 50-μm long from the target surface. In contrast to the small size of main emission area, we observed that line emissions from the aluminum target expanded far from the target.
High-density plasma created near a solid surface by an intense femtosecond laser pulse emits ultrashort x-ray pulses that are synchronized with the laser pulse. We show the spectral and temporal properties of broadband soft-x-rays emitted from a femtosecond laser-produced plasma on a metal surface. The soft x-ray emission was increased about 20-fold by fabricating an array of nanocylinders on a gold surface. We demonstrate the cross-correlation measurement of soft x-ray pulse duration in the femtosecond region by using an optical field-induced ionization process in Kr gas. We used a 10-ps soft x-ray pulse to demonstrate the time-resolved absorption measurement of optically excited silicon near its LII,III edge. We also employed a picosecond soft x-ray to measure the spatiotemporal evolution of ablated particles in femtosecond-laser-produced aluminum plasma.
High-density plasma created near a solid surface by an intense femtosecond laser pulse emits ultrashort x-ray pulses that are synchronized with the laser pulse. We show the spectral and temporal properties of broadband soft x-rays emitted from a femtosecond laser-produced plasma on a metal surface. The soft x-ray emission was increased about 20-fold by fabricating an array of nanocylinders on a gold surface. We demonstrate the cross-correlation measurement of soft x-ray pulse duration in the femtosecond region by using an optical field-induced ionization process in Kr gas. We used a 10-ps soft x-ray pulse to demonstrate the time-resolved absorption measurement of optically excited silicon near its LII,III edge. We alo employed a picosecond soft x-ray to measure the spatiotemporal evolution of ablated particles in femtosecond-laser-produced aluminum plasma.
The invention of high-power ultra short pulse lasers has opened way to investigations aimed at creation of a new type of bright x-ray source for different applications including material science and time resolved x-ray diffraction for biology. The conversion efficiency of the laser energy incident onto a solid target into the x-ray emission depends on many factors, including the temporal profile of laser pulse. We report here the results of our theoretical and experimental investigations of the line x-ray emission from layer solid targets irradiated by ultra short laser pulses. The parameters of laser pre-pulse and target thickness are optimized in order to get the maximum laser energy conversion into the emission in the selected x-ray line. Multi-layer foils are proposed in order to increase the energy of K-α line emission
from laser plasma simultaneously with shortening of x-ray pulse up to hundred femtoseconds. The emission is studied, both experimentally, and by means of analytical model and numerical simulations.
High density plasmas created near a solid surface by a femtosecond laser pulse emit ultrashort x-ray pulses that are synchronized with the laser pulse. We show the spectral and temporal properties of broadband soft x-rays emitted from a femtosecond laser-produced on a metal surface. The minimum pulse duration was < 5 ps. We achieved an approximately 20-fold enhancement of the soft x-ray emission by fabricating an array of nanocylinders on a gold surface. We measured the duration of the soft x-ray emitted from the laser produced plasma by the cross-correlation method using an optical field-induced ionization process in Kr gas. Utilizing a 10-ps soft x-ray pulse, we measured the time- resolved soft x-ray absorption of optically excited silicon near its LII,III edge. We found that laser-pulse irradiation caused a more than 10% increase in the soft x- ray absorption near the edge, which recovered within 20 ps. From experimental results, we assume the origin of this absorption change to be the bandgap renormalization of Si. We also employed picosecond soft x-rays to measure the spatiotemporal evolution of ablated particles in Al plasma created by a 100-fs laser pulse.
The soft x-ray generation properties for both flat targets and structured targets such as nanohole-alumina and Au- nanocylinder are evaluated. The experimental results for flat metal targets have revealed the fundamental properties of soft x-ray such as broadband continuum spectra and short pulse duration of less than 3 ps. By adopting structured targets such as nanohole-alumina and Au-nanocylinder targets, a more than 30 fold enhancement of x-ray generation yield is achieved compared with that for flat targets of the same materials with a slight increase of pulse duration, less than 20 ps. Then, the duration of soft x-ray pulse from femtosecond (fs) laser produced W plasma was measured by using cross-correlation method. In the experiment, we measured the transmission spectra of short soft x-ray pulse produced by main fs optical pulses through ionized Kr gas by probe fs optical pulses as a function of the time delay between main and probe fs optical pulses. The pulse duration of 4 ps was observed for soft x-ray at 15.6 nm of W plasma. The time-resolved measurement of the inner-shell absorption change of Si during the irradiation with a high-intensity fs optical pulse is achieved by using a picosecond soft x-ray pulse as a probe pulse in pump-probe experiments. A more than 5% increase in the absorption of Si membrane at near the LII,III edge (around 100 eV) was observed. The recovery time of the absorption change was measured to be about 20 ps. From these experimental results, this absorption change is assumed to be the bandgap renormalization of Si.
We propose a cross-correlation technique for measuring the shape of an ultrashort soft x-ray-pulse using the rapid change in the Kr+ population that occurs during optical field-induced ionization. By calculating the time evolution of the Kr charge states during ionization, we showed that the increase in the Kr+ population operates as 'switch', and the transient state of Kr+ during the sequential ionization operates as 'sampling gate' for measuring a soft x-ray-pulse shape. The temporal resolution of this technique is expected to overcome the limitation imposed by the ionizing laser pulse duration as a result of the ultrafast nature of optical field-induced ionization. Using the 'switch' operation, we measured a soft x-ray-shape pulse are 15.6 nm emitted from W plasma produced by a 100-fs laser pulse. Assuming a Gaussian temporal profile, we found the soft x-ray-pulse duration to be about 4 ps. This result is in good agreement with the duration measured with an x-ray streak camera thus configuring the feasibility of this 'switch' operation. The 'sampling gate' operation will be useful for directly measuring the original pulse shape of a femtosecond soft x-ray.
High-density plasmas created near a solid surface by a femtosecond laser pulse emit ultrashort x-ray pulses that are synchronized to the laser pulse. The spectral and temporal properties of x-rays emitted from a femtosecond laser-produced on metal surface are shown. The minimum pulse duration was less than 3 ps. More than a 30-fold enhancement of soft x-ray emission was achieved by fabricating an array of nanoholes on an alumina surface. Utilizing a 10-ps soft x-ray pulse, we demonstrated time-resolved soft x-ray absorption measurement of optically excited silicon near its LII,III edge. We found that a laser pulse irradiation caused a more than 10% increase in soft x-ray absorption near the edge, which recovered within 20 ps.
Femtosecond laser-produced plasma are bright ultrafast x-ray sources. Since the short x-ray pulses from such plasmas are synchronized to laser pulses, femtosecond laser-produced plasmas are important sources to probe the ultrafast response of optically excited materials. Conversion efficiency has to be improved so that laser-produced parameters, as a prepulse, one can improve x-ray emissions from laser-produced plasma. We report here the result of experimental and theoretical investigations of the line x- ray emissions form aluminum plasma created by a pair of femtosecond laser pulses in order to show the optimum energy conversion into selected x-ray lines.
Line x-ray emission from short-pulse laser-produced plasmas is studied via 1D and 2D hydrodynamics models coupled with planar atomic physics post-processor. Laser energy conversion efficiency to He-(alpha) line emission is shown to be significantly enhanced when laser prepulse precedes the main pulse at an optimum pulse separation. The calculated optimum pulse separation of 1.5-2 ns as well as emitted energy and spectra compare well with the experiment.
High-density plasmas created near a solid surface by a femtosecond laser pulse emit ultrashort x-ray pulses that are synchronized to the laser pulse. In the first part of this paper, the spectral and temporal properties of the x- ray emitted from plasma created on aluminum film by a femtosecond laser pulse are shown. The minimum pulse duration was < 3 ps as measured by an x-ray streak camera. The energy conversion efficiency, from laser pulse into soft x-ray at 14 +/- 0.05 nm, was 10-6 - 10-5. More than a 30-fold enhancement in soft x-ray emission was achieved by fabricating an array of nanoholes on an alumina surface. In the latter half, we demonstrate time-resolved absorption measurement in the soft x-ray region by means of pump-probe spectroscopy. Using a 10-ps x-ray pulse, we measured time-resolved absorption of optically-pumped silicon near its LII,III edge. We found that laser-pulse irradiation caused a more than 10% increase in soft x-ray absorption near the edge, which means that the transition of electrons in inner shells was rapidly modulated by excitation of valence electrons. The absorption change recovered within 20 ps.
The inversion of high-power ultra short pulse lasers has opened way to investigations aimed at creation of a new type of bright x-ray source for different applications including material science and time resolved x-ray diffraction for biology. The conversion efficiency of the laser energy incident onto a solid target into the x-ray emission depends on many factors, including the temporal profile of laser pulse. We report here the results of our theoretical and experimental investigations of the line x-ray emission from solid targets irradiated by ultra short laser pulses. The parameters of laser pre-pulse are optimized in order to get the maximum laser energy conversion into the emission in the selected x-ray line.
Table-top x-ray lasers driven by a commercial terawatt (TW) femtosecond laser in gases such as N2, O2, SF6, and Kr are investigated. X-ray lasing to the ground state in low-charged nitrogen and oxygen ions is successfully demonstrated using a commercial TW femtosecond laser system by optical-field ionization. Small signal gain coefficients of 9.6 cm-1 for the NIII 3s(2S)-2p)2P) transition at 45.2 nm and 11.7 cm-1 for the OIII 2p3s(3P)-2p2(3P) transition at 37.4 nm x-ray lasers are achieved for a linearly-polarized 100-fs pump laser pulse of 25 mJ. Four models that includes the monopole collisional excitation and radiation trapping effects in neon-like S and nickel-like Kr ions were built and gains for neon-like S 3p-3s and nickel-like Kr 4d-4p transitions were calculated in SF6 and Kr gases with different gas pressures. Calculations yield positive prospects for table- top neon-like S and nickel-like Kr ion x-ray lasers driven by a 1-TW circularly-polarized 100-fs laser pulse.
The soft x-ray emission from plasma on a gold-doped glass target produced by a 130-fs Ti:sapphire laser pulse was studied at a laser intensity range of 1015 - 1016 W/cm2. To evaluate the effects of gold clusters in glass, we compared gold-doped glass with different gold cluster sizes while keeping the gold densities the same (approximately 0.001 vol%). We found that larger gold clusters led to an increase in soft x-ray emission integrated in the range from 5 to 20 nm. Integrated soft x- ray emission from a gold-doped glass target reached about 50% of that from a 1-micrometers thick gold film target. The gold clusters in glass enhanced the soft x-ray emissions about four times compared with a host glass which contained no gold clusters. The pulse durations of soft x-rays emitted from gold-doped glass targets were measured to be 4 - 7 ps, which were slightly shorter than those from a gold target. Due to the low metal density and high plasma formation threshold, particulate deposition, which is a serious obstacle in applying laser-produced plasma x-rays to practical use, was greatly reduced by the use of a gold- doped glass target. From these points of view, femtosecond laser-produced plasma x-ray emitted from metal-doped glass show promise as an ultrafast soft x-ray source.
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