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To realize the ultrahigh intensity over 10^23 W/cm^2, we carried out the wavefront correction and tight focusing of the CoReLS petawatt laser. By the wavefront correction and tight focusing of the CoReLS petawatt laser with two-stage adaptive optics systems and an f/1.1 (f=300 mm) off-axis parabolic mirror, we obtained a near-diffraction-limited focal spot. The measured peak intensity was (1.1±0.1)×10^23 W/cm2, the first realization of the laser intensity over 10^23 W/cm^2. With the PW laser of intensity over 10^23 W/cm^2, we plan to explore strong field QED phenomena and proton/ion acceleration dominated by the RPA mechanism.

Laser wakefield electron acceleration (LWFA) is an emerging technology for the next generation of electron accelerators. As intense laser technology has rapidly developed, LWFA has overcome its limitations and has proven its possibilities to facilitate compact high-energy electron beams. Since high-power lasers reach peak power beyond petawatts (PW), LWFA has a new chance to explore the multi-GeV energy regime. In this article, we review the recent development of multi-GeV electron acceleration with PW lasers and discuss the limitations and perspectives of the LWFA with high-power lasers.

Angular dispersion observed in a nanosecond optical parametric chirped-pulse amplification (ns-OPCPA) amplifier adopted in the frontend of a multi-PW laser was analyzed. The theory on the angular dispersion, extended by including the wavefront rotation and the pulse front tilt of a strongly chirped laser pulse, revealed that the wavefront rotation is a major contributor to the angular dispersion, as compared to the pulse front tilt, in a ns-OPCPA amplifier. It was also shown that the wavefront rotation could be introduced by the phase mismatch and the noncollinear propagation angle in the noncollinear ns-OPCPA amplifier. The theoretical prediction was experimentally verified by measuring the angular dispersion of the ns-OPCPA frontend installed in the 20-fs, 4-PW Ti:Sapphire laser. We emphasize the importance of the proper characterization and control of the angular dispersion in the ns-OPCPA amplifier since the focus intensity of an ultrahigh power laser could be significantly reduced due to the spatiotemporal effect even for small induced angular dispersion.

Laser wakefield accelerators rely on relativistically moving micron-sized plasma cavities that provide extremely high electric field >100GV/m. Here, we demonstrate transverse shaping of the plasma cavity to produce controlled sub-GeV electron beams, adopting laser pulses with an axially rotatable ellipse-shaped focal spot. We showed the control capability on electron self-injection, charge, and transverse profile of the electron beam by rotating the focal spot. We observed that the effect of the elliptical focal spot was imprinted in the profiles of the electron beams and the electron energy increased, as compared to the case of a circular focal spot. We performed 3D particle-in-cell (PIC) simulations which reproduced the experimental results and revealed dynamics of a new asymmetric self-injection process. This simple scheme offers a novel control method on laser wakefield acceleration to produce tailored electron beams and x-rays for various applications., 5 pages, 5 figures

We demonstrate efficient multicycle terahertz pulse generation at 14.6 THz from large-area lithium niobate crystals by using high-energy (up to 2 J) femtosecond Ti:sapphire laser pulses. Such terahertz radiation is produced by phase-matched optical rectification in lithium niobate. Experimentally, we achieve maximal terahertz energy of 0.71 mJ with conversion efficiency of $\sim$0.04%. We also find that the effective interaction length for optimal terahertz conversion is fundamentally limited by terahertz absorption, laser pulse stretching by material dispersion, and terahertz-induced nonlinear cascading effects on the driving laser pulse., Comment: 4 figures

This paper demonstrates the design and development of a synchronized femtosecond laser pulse switching system and its applications in nano-patterning of transparent materials. Due to synchronization, we are able to control the location of each irradiated laser pulse in any kind of substrate. The control over the scanning speed and scanning step of the laser beam enables us to pattern periodic micro/nano-metric holes, voids, and/or lines in various materials. Using the synchronized laser system, we pattern synchronized nano-holes on the surface of and inside various transparent materials including fused silica glass and polymethyl methacrylate to replicate any image or pattern on the surface of or inside (transparent) materials. We also investigate the application areas of the proposed synchronized femtosecond laser pulse switching system in a diverse field of science and technology, especially in optical memory, color marking, and synchronized micro/nano-scale patterning of materials.

We demonstrate 0.71-mJ multicycle terahertz pulse generation at 14.8 THz from a bulk lithium niobate crystal with 80 TW femtosecond laser pumping. This terahertz radiation arises from large-area, phase-matched, optical rectification in lithium niobate.

The single-shot spatiotemporal characterization of an ultrahigh intensity laser pulse was performed using a multispectral wavefront sensor. For the measurement of the spatio-spectral electric field, a femtosecond laser pulse was spectrally modulated and separated by a Fabry-Perot etalon coupled with a grating pair, and its spatio-spectral electric field was measured with a wavefront sensor. The spatiotemporal electric field was reconstructed from the measured spatio-spectral electric field of a multi-PW laser pulse. We found that the spatiotemporal distortion could reduce the focused laser intensity by 15%, compared to the case of a diffraction-limited and transform-limited laser pulse.

We report on fabricating high-fill-factor plano-convex spherical and square micro-lens arrays on fused silica glass surface by using a CO2 laser-assisted reshaping technique. Initially, periodic micro-pillars are encoded on glass surfaces by means of a femtosecond laser beam, afterwards, the micro-pillars are polished several times by irradiating a CO2 laser beam on top of the micro-pillars. Consequently, a spherical micro-lens array with micro-lens size of 50 μm × 50 μm and a square micro-lens array with micro-lens size of 100 μm × 100 μm are formed on the surface of the fused silica glass. We also study the intensity distribution of light passing through the glass sample engraved with a spherical micro-lens array. The simulation result shows that the focal length of the spherical micro-lens array is 35 μm. Furthermore, we investigate the optical properties of glass samples with engraved micro-lens arrays. The proposed CO2-laser-based reshaping technique is simple and fast and shows promises for fabricating arrays of smooth micro-lenses in various transparent materials.

We present a proof of principle for onsite calibration of a radiochromic film (EBT3) using CR-39 as an absolute proton-counting detector and laser-accelerated protons as a calibration source. A special detector assembly composed of aluminum range filters, an EBT3 film, and a CR-39 detector is used to expose the EBT3 film with protons in an energy range of 3.65 MeV-5.85 MeV. In our design, the proton beam is divided into small beamlets and their projection images are taken on the EBT3 film and the CR-39 detector by maintaining a certain distance between the two detectors. Owing to the geometrical factor of the configuration and scattering inside the EBT3, the areal number density of protons was kept below the saturation level of the CR-39 detector. We also present a method to relate the number of protons detected on the CR-39 in a narrow energy range to protons with a broad energy spectrum that contribute to the dose deposited in the EBT3 film. The energy spectrum of protons emitted along the target normal direction is simultaneously measured using another CR-39 detector installed in a Thomson parabola spectrometer. The calibration curves for the EBT3 film were obtained in the optical density range of 0.01-0.25 for low dose values of 0.1 Gy-3.0 Gy. Our results are in good agreement with the calibrations of the EBT3 film that are traditionally carried out using conventional accelerators. The method presented here can be further extended for onsite calibration of radiochromic films of other types and for a higher range of dose values.

We present a highly efficient double plasma mirror (DPM) that provides ultrahigh-contrast multi-petawatt (PW) laser pulses with a temporal contrast ratio reaching 1 0 17 up to 160 ps and 1 0 12 up to 2 ps before the main pulse. The high reflectivity of 70%, along with the high-contrast enhancement factor of 700,000, was achieved from the DPM installed after the final stage of a 4 PW Ti:sapphire laser. The 4 PW laser was equipped with cross-polarized wave generation and optical parametric chirped-pulse amplification stages for initial high-contrast operation. The DPM operation was undertaken with conditions that did not modify the spatiotemporal profiles of incident multi-PW laser pulses. This highly efficient DPM with the high-contrast enhancement promises the utilization of multiple PMs as a practical rear end for upcoming tens of petawatt lasers to achieve ultrahigh temporal contrast.

We developed a two-stage optical parametric chirped-pulse amplification (OPCPA) amplifier with a double-sliced pump pulse. To utilize a single-longitudinal laser pulse with an 8-ns duration and a 532-nm central wavelength as a high-energy pump source efficiently, two 2-ns quasi-rectangular pump pulses were generated by double-slicing the green pulse with two Pockels cells. The energy and spectral width of the amplified laser pulse with an 820-nm central wavelength were maximized by optimizing the phase matching conditions in the OPCPA amplifier via a 3-D simulation. The amplified laser pulse had a broad bandwidth of 120 nm (FWHM) and a high energy of 75 mJ. And the amplified pulse had an energy stability of 2.14% (RMS) on account of the temporal jittering between the single-mode green pulse and the Pockels cells while the single-mode green pump laser had an energy stability of 1.1% (RMS).

International audience; Generation of high-quality electron beams from laser wakefield acceleration requires optimization of initial experimental parameters. We present here the dependence of accelerated electron beams on the temporal profile of a driving PW laser, the density, and length of an interacting medium. We have optimized the initial parameters to obtain 2.8 GeV quasi-monoenergetic electrons which can be applied further to the development of compact electron accelerators and radiations sources.

This paper reports the fabrication of various micro-lensed single-mode optical fibers through the use of an enhanced peak power $CO_2$ laser beam. The end faces of the optical fibers are exposed to the $CO_2$ laser beam to form convex, concave, and conical shape optical fiber tips. Peak power of the $CO_2$ laser beam was varied from 0.8 W to 1.5 W depending on the shape of the optical fiber tip. We also discover the dependence of the angle of the optical fiber tip on the rotation angle and the number of $CO_2$ laser irradiations. The angle shows an increasing trend with both these parameters. We achieve a wide range of lenticular fibers with end face angle varying from $4.47^{\circ}$ to $8.13^{\circ}$ . Furthermore, we investigate the emission pattern of light from the developed micro-lensed fibers. The proposed $CO_2$ laser based optical fiber reshaping technique shows great consistency, and thus is suitable for commercial applications.

A 20 fs, 4 PW Ti:sapphire laser with a repetition rate of 0.1 Hz was developed and its performance has been tested in laser-driven charged particle acceleration.

The experimental results that demonstrate the changes in the characteristics of laser-driven proton beams are presented along with numerical simulations, as the relativistic laser profile changes from Gaussian to Laguerre-Gaussian.

A high energy pulse with the energy of 3.5 J was spectrally broadened by a supercontinuum generation and it was recompressed from 31 fs to 14.5 fs by the dispersion compensation.

One of two petawatt Ti:Sapphire laser beamlines at CoReLS was upgraded to a high-contrast, 4 PW, 20 fs laser with a repetition rate of 0.1 Hz and its performance has been tested for multi-GeV electron acceleration.

Short-pulse x-ray/gamma-ray sources have become indispensable light sources for investigating material science, bio technology, and photo-nuclear physics. In past decades, rapid advancement of high intensity laser technology led extensive progresses in the field of radiation sources based on laser-plasma interactions - x-ray lasers, betatron radiation and Compton gamma-rays. Ever since the installation of a 100-TW laser in 2006, we have pursued the development of ultrashort x-ray/gamma-ray radiations, such as x-ray lasers, relativistic high-order harmonics, betatron radiation and all-optical Compton gamma-rays. With the construction of two PW Ti:Sapphire laser beamlines having peak powers of 1.0 PW and 1.5 PW in 2010 and 2012, respectively [1], we have investigated the generation of multi-GeV electron beams [2] and MeV betatron radiations. We plan to carry out the Compton backscattering to generate MeV gamma-rays from the interaction of a GeV electron beam and a PW laser beam. Here, we present the recent progress in the development of ultrashort x-ray/gamma-ray radiation sources based on laser plasma interactions and the plan for developing Compton gamma-ray sources driven by the PW lasers. In addition, we will present the applications of laser-plasma x-ray lasers to x-ray holography and coherent diffraction imaging. [references] 1. J. H. Sung, S. K. Lee, T. J. Yu, T. M. Jeong, and J. Lee, Opt. Lett. 35, 3021 (2010). 2. H. T. Kim, K. H. Pae, H. J. Cha, I J. Kim, T. J. Yu, J. H. Sung, S. K. Lee, T. M. Jeong, J. Lee, Phys. Rev. Lett. 111, 165002 (2013).

Physical processes in laser–matter interaction used to be determined by generation of fast electrons resulting from efficient conversion of the absorbed laser radiation. Composite materials offer the possibility to control the absorption by choice of the host material and dopants. Reported here strong absorption of ultrashort laser pulse in a composite carbon-based nanomaterial including single-walled carbon nanotubes (SWCNTs) or multilayer graphene was measured in the intensity range between 1012 and 1016 W cm−2. A protein (lysozyme) was used as the host. The maximum absorption of femtosecond laser pulse has reached 92–96 %. The optical damage thresholds of the coatings were registered at an intensity of (1.1 ± 0.5) × 1013 W cm−2 for the embedded SWCNTs and at (3.4 ± 0.3) × 1013 W cm−2 for the embedded graphene. Encapsulated variant of the dispersed nanomaterial was investigated as well. It was found that supernatant protein in the coating material tends to dominate the absorption process, independently of the embedded nanomaterial. The opposite was observed for the encapsulated material.

The advantages of a laser-driven proton acceleration have prompted studies of laser-induced proton radiography. As the CR-39 solid-state nuclear-track detector is suitable for measuring charged particles and can be used in proton radiography, we studied laser-induced proton radiography with the CR-39 for several years, and we were able to obtain a spatial resolution of about 10 μm. For obtaining an image with a spatial resolution of a few μm, we investigated the effect of the CR-39 etching conditions on the spatial resolution and carried out imaging experiments using fine phantoms. Experiments were performed using the 100-TW titanium-sapphire laser system at the Advanced Photonics Research Institute of the Gwangju Institute of Science Technology. We have demonstrated that images with a spatial resolution of about several μm can be achieved using laser-induced proton radiography.

We demonstrate high-energy terahertz generation from a large-aperture (75 mm diameter) lithium niobate wafer by using a femtosecond laser with energy up to 2 J. This scheme utilizes optical rectification in a bulk lithium niobate crystal, where most terahertz energy is emitted from a thin layer of the rear surface. Despite its simple setup, this scheme can yield 0.19 mJ of terahertz energy with laser-to-terahertz conversion efficiencies of ~0.01%, about 3 times better than ZnTe when pumped at 800 nm. The experimental setup is up-scalable for multi-mJ terahertz generation with petawatt laser pumping., 4 pages, 4 figures

The generation of ultrahigh intensity laser pulses was investigated by tightly focusing a wavefront-corrected multi-petawatt Ti:sapphire laser. For the wavefront correction of the PW laser, two stages of deformable mirrors were employed. The multi-PW laser beam was tightly focused by an f/1.6 off-axis parabolic mirror and the focal spot profile was measured. After the wavefront correction, the Strehl ratio was about 0.4, and the spot size in full width at half maximum was 1.5×1.8 μm2, close to the diffraction-limited value. The measured peak intensity was 5.5×1022 W/cm2, achieving the highest laser intensity ever reached.

We characterized the transmission properties of a color-glass-filter (RG850) saturable absorber (SA) in a wide range of pulse durations (from 25 fs to 5 ps). The transmission properties were strongly related to the energy fluence, pulse duration, and chirp parameter. On the basis of these properties, the input pulse duration, chirp parameter, and energy fluence were optimized to maintain the width of the transmitted laser spectrum as much as possible with minimal energy loss. We demonstrated that, by transmitting a positively chirped 2.8-ps laser pulse to two identical SAs at an energy fluence of 15 mJ/cm2, the temporal contrast ratio of the main pulse to the amplified spontaneous emission was enhanced by 4 orders of magnitude without any significant energy loss or strong spectral narrowing in a 10-Hz, 100-TW femtosecond laser system.

International audience; The achievable energy and the stability of accelerated electron beams have been the most critical issues in laser wakefield acceleration. As laser propagation, plasma wave formation and electron acceleration are highly nonlinear processes, the laser wakefield acceleration (LWFA) is extremely sensitive to initial experimental conditions. We propose a simple and elegant waveform control method for the LWFA process to enhance the performance of a laser electron accelerator by applying a fully optical and programmable technique to control the chirp of PW laser pulses. We found sensitive dependence of energy and stability of electron beams on the spectral phase of laser pulses and obtained stable 2-GeV electron beams from a 1-cm gas cell of helium. The waveform control technique for LWFA would prompt practical applications of centimeter-scale GeV-electron accelerators to a compact radiation sources in the x-ray and γ-ray regions. Laser wakefield electron accelerators 1 have strong potential for a next generation accelerator due to its huge acceleration field. Laser wakefield acceleration (LWFA) has been intensively investigated to produce quasi-mono-energetic collimated GeV electron beams in centimeter-scale acceleration length 2–4. Recent progresses of LWFA demonstrated quasi-mono-energetic electron beams in the hundreds MeV range that are produced in a stable way by utilizing methods such as colliding laser pulses 5, 6 , ionization injection 7–10 and density gradient injection 11–14 with an improvement of the stability that results from explored injection schemes. Here we propose a simple method to enhance the energy and the stability of multi-GeV electron beams 15–17 by controlling the waveform of PW laser pulses using a fully optical, programmable method 18–20. LWFA is based on the interaction between an underdense plasma and an intense laser pulse. The interaction between a driving laser pulse and a plasma in LWFA is a complex process occurring in the relativistic regime, because the formation of plasma waves and electron acceleration processes are highly nonlinear while the propagating laser pulse is strongly modified by the medium 21, 22. The LWFA is, thus, very sensitive to the initial properties of driving laser pulse, and the waveform of the laser pulse can significantly affect the LWFA process. The waveform control technique brought significant advancement of light-matter interactions by realizing coherent control 23, 24 of a chemical reaction or atomic excitation/ionization. In the coherent control of a quantum system, the probability of a certain reaction or excitation/ionization pathway can be selectively enhanced by controlling the waveform of a driving laser pulse. Here, we propose and demonstrate that the LWFA can be optimized by controlling the waveform of the driving laser pulse so as to enhance the energy and the stability of accelerated electron beams; the laser-plasma interaction in the LWFA is strongly affected by the coherent property of the laser pulse.

We developed an OPCPA pre-amplifier for the 4-PW laser at CoReLS. The output spectrum was shaped by controlling the pump laser temporal profile. Spectrally shaped laser pulse was generated with an energy of 240 mJ.

A 0.1-Hz 4-PW Ti:sapphire laser has been developed. The amplified energy reached 112 J by adding another booster amplifier to the current PW laser. The broad spectrum and the high temporal contrast were obtained by employing the cross-polarized wave generation and the OPCPA techniques.

We have demonstrated the generation of 20-fs, 0.15-TW laser pulses with high temporal and spatial qualities from a kHz femtosecond Ti:sapphire laser system with a single-stage amplifier. A maximum output energy of 4.1 mJ could be achieved from the amplifier by suppressing the thermal lensing effect. A spectral bandwidth of 57 nm was obtained by maximizing the reflection bandwidth that the amplifier could support and by compensating for gain narrowing. A temporal profile close to the Fourier-transform-limited pulse profile was obtained by minimizing the spectral phase error, and the pulse duration was 20 fs at full width at half maximum. The measured beam quality factors were 1.2 and 1.4 in the horizontal and the vertical directions, respectively.

We theoretically investigated the temporal profile of a background pedestal in a single-stage optical parametric chirped pulse amplification (OPCPA) system, and showed that the amplified spontaneous emission (ASE) prepulse in the saturation regime degrades the contrast ratio. To improve the contrast ratio in a saturated OPCPA system, a method using pump pulse shaping and the time delay control between pump and seed pulses was utilized. Through numerical simulations, the effect of the rising time and the time delay of a pump pulse on the background pedestal was subsequently investigated in terms of the contrast ratio. From the simulation results, it was determined that an optimal rising time and the time delay of the pump pulse are critical for obtaining a high-contrast broadband laser pulse from a saturated OPCPA system.

We design a two-stage non-collinear optical parametric chirped pulse amplifier using BBO crystals for a high-power Ti:sapphire laser system, and numerically analyze the output characteristics of the OPCPA system. We find optimal phase-matching conditions and calculate the parametric gains as a function of pump intensity and interaction length. By constructing a two-stage OPCPA system using two BBO crystals, we obtain 12 mJ output energy corresponding to a high gain of 10 7 , a pulse duration of 37 fs, and a contrast ratio of 5×10 −8 .

An ultrahigh contrast laser pulse of over 1011 for 6 ps before the main pulse was achieved by employing a double plasma mirror installed at the end of a 100 TW Ti:sapphire laser system. Spatial beam qualities such as focusability and stability were found to be extremely sensitive in the range of 14–360 J/cm2 on the double plasma mirror, while ultrahigh contrast was maintained. At the fluence of 90 J/cm2 the focusability of the ultrahigh contrast laser was not degraded, and the stability was very close to that obtained without the double plasma mirror when the 2-dimensional normalized standard deviation and the correlation function for several laser beam profiles were analyzed. These results are requisites for carrying out relativistic laser-plasma interactions with ultrahigh contrast laser pulses, enabling the use of ultrathin solid targets.

We investigate the temporal profile of a prepulse via amplified spontaneous emission (ASE) in the saturation regime of the amplifier of an ultrashort high power Ti:sapphire laser, which plays an important role in applications of a high peak power laser. We experimentally show that the temporal profile of the ASE prepulse is strongly related to the gain of the amplifier due to the saturation effect. We demonstrate that the contrast ratio can be improved by using gain control of the amplifiers in a 100-TW Ti:sapphire laser.

We developed an OPCPA preamplifier with an actively shaped output spectrum to obtain a sub-20-fs-duration pulse for a 4-PW laser. The active spectral shaping was facilitated by controlling the temporal profile of a pump pulse in the OPCPA preamplifier. By optimizing the output spectrum of the OPCPA to compensate for the gain-depletion effect in the 4-PW laser, a final laser pulse with a broad spectrum of 101-nm in width (FWHM), resulting in a short pulse duration of 17 fs, was achieved.

We have designed and constructed various experimental systems to perform interaction experiments with a 100-TW Ti:sapphire laser and a solid or a gas target, and we have used them for proton, electron, and X-ray generations. They consist of a laser beam delivery system, a target chamber, a laser focusing and monitoring system, a target manipulation system, and target diagnostic systems, such as a visible emission imager and an X-ray pinhole camera. The laser beam can be focused with a peak intensity as large as ∼10 W/cm and a focal spot size as small as ∼4 μm. The solid target is precisely positioned at the laser focus with an accuracy of ∼20 μm, which is much smaller than the Rayleigh range of the focused laser beam. The target diagnostic systems allow us to investigate the status of laser-target interactions with real-time and online operation when the laser and the target conditions are optimized to obtain the desired characteristics for the proton, electron, and X-ray sources. Strong correlations between the images in the diagnostic systems and monoenergetic electron and higher-energy proton generations are presented.

A chirped-pulse amplification Ti:sapphire laser system has been designed using a 10-Hz 100-TW Ti:sapphire laser to generate 0.1-Hz 0.5-PW laser pulses and optimize their temporal qualities such as temporal contrast and pulse duration. A high-energy booster amplifier to be added is expected to produce an energy above 30 J through the parasitic lasing suppression and the efficient amplification. To improve the temporal contrast of the laser pulses, a high-contrast 1-kHz amplifier system is used as a front-end. A grating stretcher which makes the laser pulse have 1-ns duration is used to prevent optical damages due to high pulse energy during amplification. A grating compressor has been designed with group delay analysis to obtain the recompressed pulse duration close to the transform-limited pulse duration. The final laser pulses are expected to have energy above 20 J and duration below 40 fs.

In order to obtain high quality images of thin objects, we performed an experiment of proton radiography by using low energy protons generated from the interaction of an ultrashort ultraintense laser with solid targets. The protons were produced from a thin polyimide target irradiated by the laser pulse, and their maximum energy was estimated at up to 1.8 MeV. A CR-39 nuclear track detector was used as a proton radiography screen. The proton images were obtained by using an optical microscope and the spatial resolution was evaluated by a Modulation Transfer Function (MTF). We have achieved about $10\;{\mu}m$ spatial resolution of images. The obtained spatial resolution shows about $4{\sim}5$ times better value than the conventional X-ray radiography for inspection or non-destructive test (NDT) purpose.

A significant level of back reflected laser energy was measured during the interaction of ultra-short, high contrast PW laser pulses with solid targets at 30° incidence. 2D PIC simulations carried out for the experimental conditions show that at the laser-target interface a dynamic regular structure is generated during the interaction, which acts as a grating (quasi-grating) and reflects back a significant amount of incident laser energy. With increasing laser intensity above 1018 W/cm2 the back reflected fraction increases due to the growth of the surface modulation to larger amplitudes. Above 1020 W/cm2 this increase results in the partial destruction of the quasi-grating structure and, hence, in the saturation of the back reflection efficiency. The PIC simulation results are in good agreement with the experimental findings, and, additionally, demonstrate that in presence of a small amount of pre-plasma this regular structure will be smeared out and the back reflection reduced.

The measurement and correction of wavefront aberrations of the 100 TW Ti:sapphire laser beam are presented. Several monochromatic aberrations, such as astigmatism (Z2-2, Z22), coma (Z3-1), trefoil (Z3-3, Z33), and spherical aberration (Z40) are observed in the 100 TW Ti:sapphire laser beam as well as defocus (Z20), which is responsible for the converging or diverging of a laser pulse. An adaptive optics system using a bimorph deformable mirror successfully corrected a monochromatic wavefront in the 100 TW Ti:sapphire laser beam. The customization of the focal spot is also investigated using the adaptive optics system for research on the efficient generation of a high-energy electron beam. The increase in the depth of focus is demonstrated and the temporal broadening is calculated when the spherical aberration is introduced using the adaptive optics system.

A laser-driven proton beam with a maximum energy of a few MeV is stably obtained using an ultra-short and high-intensity Ti:sapphire laser. At the same time, keV X-ray is also generated at almost the same place where protons are emitted. Here, we show the successful demonstration of simultaneous proton and X-ray projection images of a test sample placed close to the source with a resolution of ~10 µm, which is determined from the source sizes. Although the experimental configuration is very simple, the simultaneity is better than a few hundreds of ps. A CR-39 track detector and imaging plate, which are placed as close as possible to the CR-39, are used as detectors of protons and X-ray. The technique is applicable to the precise observation of microstructures.

We developed a new variant of Fourier transform holography (FTH) to overcome the separation condition of FTH by subtracting the autocorrelation signal, named as autocorrelation-subtracted FTH (AS-FHT). AS-FTH is advantageous in imaging large specimen using light sources of limited coherent photons because AS-FTH requires a much smaller coherent illumination area than conventional FTH. We experimentally demonstrated the AS-FTH using a Ni-like Ag x-ray laser at 13.9 nm. The hidden part of an image under its autocorrelation was successfully recovered by subtracting an independently measured autocorrelation signal.

We report the development of x-ray sources and their applications to x-ray microscopy. We applied a Ni-like Ag x-ray laser at 13.9 nm to Fourier transform holography (FTH) and obtained a reconstructed image with a resolution of 87 nm using a single-shot x-ray laser pulse. As the achievable spatial resolution of the single-shot FTH is limited by the strength of the reference wave, we enhanced the resolution of the x-ray imaging using the holography with extended reference by autocorrelation linear differential operation (HERALDO). Clear holograms could be recorded over the full size of an x-ray CCD, and the hologram reconstruction provided an image with a minimum resolution of 24 nm, close to the diffraction limited resolution of the imaging system.

0.1-Hz 4.0-PW laser is being developed for research on relativistic laser-matter interactions. The upgrade will be achieved by increasing the laser energy of the current 1.5-PW laser to 80 J and decreasing the pulse duration to 20 fs.

A two-stage non-collinear optical parametric chirped pulse amplifier (OPCPA), using type-I BBO nonlinear crystal, has been developed as a pre-amplifier of the 4 PW laser at CoReLS.

We fabricated micro-lens array on fused silica glass using femtosecond and CO2 laser. We micro-machined periodic micro-grooves on the glass surface with femtosecond laser, and polished the patterned surface with CO2 laser. We confirmed that curvature was formed on the surface by heat from the CO2 laser beam as the surface roughness was removed. Depending on the fabricated pattern size, we could fabricate micro-lens array with controlling sizes. Using such this laser fabrication technique, we have demonstrated micro-lens array with various dimension.

A two-color interferometer for preformed plasma characterization is developed. We observe the electron density distribution of preformed plasmas on a 5 μm-thick copper target irradiated with a high-intensity Ti:sapphire laser. The two-color interferometer extended the observable electron density region using a fundamental (800 nm) probe beam to cover the lower density region and a second harmonic (400 nm) probe beam to cover the higher density region, simultaneously. This characterization of the electron density distribution of preformed plasmas with femtosecond time resolution significantly contributes to the understanding of high-intensity laser–thin-foil interactions during high-energy electron, ion, and X-ray generation.

Optical pulses with 1.1-mJ energy and 5.5-fs duration have been generated at 1-kHz repetition rate from a chirped pulse amplification Ti:Sapphire laser incorporating a differentially pumped hollow-fiber chirped-mirror compressor. The effects of self-focusing and multi-photon ionization during the beam propagation were minimized by differentially pumping the hollow fiber filled with neon. The spectral broadening at the hollow-fiber compressor was optimized by adjusting gas pressure, laser intensity, and laser chirp, covering from 540 nm to 950 nm.