Sub-10 fs pulse generation by post-compression for peak-power enhancement of a 100-TW Ti:Sapphire laser.

We demonstrated sub-10 fs pulse generation by the post-compression of a 100 TW Ti:Sapphire laser to enhance the peak-power. In the post-compression, the laser spectrum was widely broadened by self-phase modulation in thin fused silica plate(s), and the induced spectral phase was compensated with a set of chirped mirrors. A spatial filter stage, consisting of two cylindrical lenses and a spherical lens, was employed to reduce the intensity modulation existing in the laser beam, which effectively suppressed intensity spikes induced by self-focusing. The laser beam was post-compressed from 23 fs to 9.7 fs after propagating through a 1.5 mm fused silica plate, resulting in the peak-power enhancement by a factor of 2.1.

Wavefront-corrected post-compression of a 100-TW Ti:sapphire laser.

We analyzed and corrected the wavefront distortion induced during the post-compression of a 100-TW Ti:Sapphire laser and achieved the intensity enhancement. In the post-compression, the spectral broadening of the laser was obtained by propagating through three 0.5 mm-thick fused silica plates and the laser pulse duration was post-compressed from 24 fs to 11 fs using a set of chirped mirrors. We measured the wavefront aberrations due to the intensity-dependent nonlinear process during the post-compression of femtosecond high-power laser pulses. By compensating for the wavefront aberrations with an adaptive optics system, the Strehl ratio of the post-compressed beam was improved from 0.37 to 0.52 and the focused intensity of the post-compressed beam could be enhanced by a factor of 1.5, while the enhancement without the wavefront correction was only a factor of 1.1 in spite of the peak-power enhancement by a factor of 1.8.

Single-shot spatiotemporal characterization of a multi-PW laser using a multispectral wavefront sensing method.

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.

Analysis of angular dispersion induced by wavefront rotation in nanosecond optical parametric chirped pulse amplification.

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.

Generation of 0.7 mJ multicycle 15 THz radiation by phase-matched optical rectification in lithium niobate.

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 ∼0.04%. Our experimental setup is simple and easily upscalable to produce multi-millijoule, multicycle terahertz radiation with proper lithium niobate crystals.

Highly efficient double plasma mirror producing ultrahigh-contrast multi-petawatt laser pulses.

We present a highly efficient double plasma mirror (DPM) that provides ultrahigh-contrast multi-petawatt (PW) laser pulses with a temporal contrast ratio reaching 1017 up to 160 ps and 1012 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.

Achieving the laser intensity of 5.5×1022 W/cm2 with a wavefront-corrected multi-PW laser.

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.

Scalable terahertz generation by large-area optical rectification at 80 TW laser power.

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 ∼10-4, about 3 times better than ZnTe when pumped at 800 nm. The experimental setup is upscalable for multimillijoule terahertz generation with petawatt laser pumping.

Laser Acceleration of Highly Energetic Carbon Ions Using a Double-Layer Target Composed of Slightly Underdense Plasma and Ultrathin Foil.

We report the experimental generation of highly energetic carbon ions up to 48 MeV per nucleon by shooting double-layer targets composed of well-controlled slightly underdense plasma and ultrathin foils with ultraintense femtosecond laser pulses. Particle-in-cell simulations reveal that carbon ions are ejected from the ultrathin foils due to radiation pressure and then accelerated in an enhanced sheath field established by the superponderomotive electron flow. Such a cascaded acceleration is especially suited for heavy ion acceleration with femtosecond laser pulses. The breakthrough of heavy ion energy up to many tens of MeV/u at a high repetition rate would be able to trigger significant advances in nuclear physics, high energy density physics, and medical physics.

Spectral shaping of an OPCPA preamplifier for a sub-20-fs multi-PW 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.

4.2 PW, 20 fs Ti:sapphire laser at 0.1 Hz.

We demonstrated the generation of 4.2 PW laser pulses at 0.1 Hz from a chirped-pulse amplification Ti:sapphire laser. The cross-polarized wave generation and the optical parametric chirped-pulse amplification stages were installed for the prevention of the gain narrowing and for the compensation of the spectral narrowing in the amplifiers, obtaining the spectral width of amplified laser pulses of 84 nm (FWHM), and enhancing the temporal contrast. The amplified laser pulses of 112 J after the final booster amplifier were compressed to the pulses with 83 J at 19.4 fs with a shot-to-shot energy stability of 1.5% (RMS). This 4.2 PW laser will be a workhorse for exploring high field science.

Resolving multiple molecular orbitals using two-dimensional high-harmonic spectroscopy.

High-harmonic radiation emitted from molecules in a strong laser field contains information on molecular structure and dynamics. When multiple molecular orbitals participate in high-harmonic generation, resolving the contribution of each orbital is crucial for understanding molecular dynamics and for extending high-harmonic spectroscopy to more complicated molecules. We show that two-dimensional high-harmonic spectroscopy can resolve high-harmonic radiation emitted from the two highest-occupied molecular orbitals, HOMO and HOMO-1, of aligned molecules. By the application of an orthogonally polarized two-color laser field that consists of the fundamental and its second-harmonic fields to aligned CO2 molecules, the characteristics attributed to the two orbitals are found to be separately imprinted in odd and even harmonics. Two-dimensional high-harmonic spectroscopy may open a new route to investigate ultrafast molecular dynamics during chemical processes.

Ionizing terahertz waves with 260 MV/cm from scalable optical rectification.

Terahertz (THz) waves, known as non-ionizing radiation owing to their low photon energies, can actually ionize atoms and molecules when a sufficiently large number of THz photons are concentrated in time and space. Here, we demonstrate the generation of ionizing, multicycle, 15-THz waves emitted from large-area lithium niobate crystals via phase-matched optical rectification of 150-terawatt laser pulses. A complete characterization of the generated THz waves in energy, pulse duration, and focal spot size shows that the field strength can reach up to 260 megavolts per centimeter. In particular, a single-shot THz interferometer is employed to measure the THz pulse duration and spectrum with complementary numerical simulations. Such intense THz pulses are irradiated onto various solid targets to demonstrate THz-induced tunneling ionization and plasma formation. This study also discusses the potential of nonperturbative THz-driven ionization in gases, which will open up new opportunities, including nonlinear and relativistic THz physics in plasma.

Enhancement of electron energy to the multi-GeV regime by a dual-stage laser-wakefield accelerator pumped by petawatt laser pulses.

Laser-wakefield acceleration offers the promise of a compact electron accelerator for generating a multi-GeV electron beam using the huge field gradient induced by an intense laser pulse, compared to conventional rf accelerators. However, the energy and quality of the electron beam from the laser-wakefield accelerator have been limited by the power of the driving laser pulses and interaction properties in the target medium. Recent progress in laser technology has resulted in the realization of a petawatt (PW) femtosecond laser, which offers new capabilities for research on laser-wakefield acceleration. Here, we present a significant increase in laser-driven electron energy to the multi-GeV level by utilizing a 30-fs, 1-PW laser system. In particular, a dual-stage laser-wakefield acceleration scheme (injector and accelerator scheme) was applied to boost electron energies to over 3 GeV with a single PW laser pulse. Three-dimensional particle-in-cell simulations corroborate the multi-GeV electron generation from the dual-stage laser-wakefield accelerator driven by PW laser pulses.

Transition of proton energy scaling using an ultrathin target irradiated by linearly polarized femtosecond laser pulses.

Particle acceleration using ultraintense, ultrashort laser pulses is one of the most attractive topics in relativistic laser-plasma research. We report proton and/or ion acceleration in the intensity range of 5×10(19) to 3.3×10(20) W/cm2 by irradiating linearly polarized, 30-fs laser pulses on 10-to 100-nm-thick polymer targets. The proton energy scaling with respect to the intensity and target thickness is examined, and a maximum proton energy of 45 MeV is obtained when a 10-nm-thick target is irradiated by a laser intensity of 3.3×10(20) W/cm2. The proton acceleration is explained by a hybrid acceleration mechanism including target normal sheath acceleration, radiation pressure acceleration, and Coulomb explosion assisted-free expansion. The transition of proton energy scaling from I(1/2) to I is observed as a consequence of the hybrid acceleration mechanism. The experimental results are supported by two- and three-dimensional particle-in-cell simulations.

Multi-millijoule terahertz emission from laser-wakefield-accelerated electrons.

High-power terahertz radiation was observed to be emitted from a gas jet irradiated by 100-terawatt-class laser pulses in the laser-wakefield acceleration of electrons. The emitted terahertz radiation was characterized in terms of its spectrum, polarization, and energy dependence on the accompanying electron bunch energy and charge under various gas target conditions. With a nitrogen target, more than 4 mJ of energy was produced at <10 THz with a laser-to-terahertz conversion efficiency of ~0.15%. Such strong terahertz radiation is hypothesized to be produced from plasma electrons accelerated by the ponderomotive force of the laser and the plasma wakefields on the time scale of the laser pulse duration and plasma period. This model is examined with analytic calculations and particle-in-cell simulations to better understand the generation mechanism of high-energy terahertz radiation in laser-wakefield acceleration.

High-harmonic generation from a flat liquid-sheet plasma mirror.

High-harmonic radiation can be generated when an ultra-intense laser beam is reflected from an over-dense plasma, known as a plasma mirror. It is considered a promising technique for generating intense attosecond pulses in the extreme ultraviolet and X-ray wavelength ranges. However, a solid target used for the formation of the over-dense plasma is completely damaged by the interaction. Thus, it is challenging to use a solid target for applications such as time-resolved studies and attosecond streaking experiments that require a large amount of data. Here we demonstrate that high-harmonic radiation can be continuously generated from a liquid plasma mirror in both the coherent wake emission and relativistic oscillating mirror regimes. These results will pave the way for the development of bright, stable, and high-repetition-rate attosecond light sources, which can greatly benefit the study of ultrafast laser-matter interactions.

Generation of high-contrast, 30 fs, 1.5 PW laser pulses from chirped-pulse amplification Ti:sapphire laser.

High-contrast, 30 fs, 1.5 PW laser pulses are generated from a chirped-pulse amplification (CPA) Ti:sapphire laser system at 0.1 Hz repetition rate. The maximum output energy of 60.2 J is obtained, at a pump energy of 120 J, from a booster amplifier that is pumped by four frenquency-doubled Nd:glass laser systems. During amplification, parasitic lasing is suppressed by index matching fluid with absorption dye and the careful manipulation of the time delay between the seed and pump pulses. An amplified pulse passes through a pulse compressor consisting of four gold-coated gratings. After compression, the measured pulse duration is 30 fs, and the output energy is 44.5 J, yielding a peak power of about 1.5 PW. The output energy of 44.5 J and output power of 1.5-PW are the highest values ever achieved from the femtosecond CPA Ti:sapphire laser system. To maintain a sufficiently high temporal contrast, a saturable absorber is installed in the front-end system with two ultrafast Pockels cells in order to minimize the amplified spontaneous emission (ASE) and pre-pulse intensity. An adaptive optics system is implemented for PW laser pulses and a focused intensity of about 1 × 10(22) W/cm(2) can be obtained when an f/3 optic is used.

Single-pulse coherent diffraction imaging using soft x-ray laser.

We report a coherent diffraction imaging (CDI) using a single 8 ps soft x-ray laser pulse at a wavelength of 13.9 nm. The soft x-ray pulse was generated by a laboratory-scale intense pumping laser providing coherent x-ray pulses up to the level of 10(11) photons/pulse. A spatial resolution below 194 nm was achieved with a single pulse, and it was shown that a resolution below 55 nm is feasible with improved detector capability. The single-pulse CDI might provide a way to investigate dynamics of nanoscale molecules or particles.

Optical synchronization technique for all-optical Compton scattering.

In all-optical Compton scattering driven by a multi-petawatt laser, it is critical to have accurate spatiotemporal synchronization between the ultrarelativistic electron bunch and the ultrahigh-intensity laser beam. Such a synchronization was realized by using two complementary optical setups. The first setup, used for the initial synchronization, recorded the spatial interferogram between the two femtosecond lasers used for a GeV electron beam production and an ultrahigh scattering laser beam. The second one, consisting of spatial and spectral interferometers, measured the time delay between the two laser beams in the range of 0-200 fs in real time. These monitoring systems played an essential role in conducting Compton scattering experiments.