A. Iwasaki

Full-coherent free electron laser seeded by 13th- and 15th-order harmonics of near-infrared femtosecond laser pulses

By introducing 13th- (61.7?nm) and 15th-order harmonics (53.4?nm) of femtosecond laser pulses at 800?nm into an undulator of SCSS (SPring-8 Compact SASE Source) test accelerator at RIKEN, these harmonic pulses were amplified by a factor of more than 102?with a high contrast ratio through the interaction between accelerated electron bunches and the harmonic pulses. From numerical simulations of the amplification processes of high-order harmonic pulses in the undulator, optimum conditions of the electron bunch duration interacting with the high-order harmonic pulses were investigated for generating full-coherent and intense pulses in the extreme ultraviolet wavelength region.

Selective inner-valence ionization of aligned polyatomic molecules for controlling molecular fragmentation

We show experimentally and theoretically, using acetylene as an example, that the strong preponderance of ionization from specific molecular orbitals to the alignment of the molecular axis with respect to the laser polarization direction allows implementing a method for controlling fragmentation reactions of polyatomic molecules.

Control of fragmentation reactions in impulsively aligned polyatomic molecules by selective removal of inner-valence electrons

Recently, selective fragmentation of polyatomic molecules using the carrier-envelope phase (CEP) of few-cycle ionizing laser pulses as the control parameter has been demonstrated. It was shown that the underlying mechanism is selective removal of specific (inner-valence shell) electrons, which puts the molecule into a dedicated dissociative electronic state from which the molecule then fragments along a desired pathway.

Temporal overlapping for HHG seeded EUV-FEL operation by using EOS-based timing-drift controlling system

Free-electron lasers (FELs), which are based on a self-amplified spontaneous emission (SASE) scheme, have attractive characters as the intense light source in short-wavelength region. However, their temporal profile and frequency-domain spectra have shot-to-shot fluctuating spikes originated from the SASE process. One of the promising approaches to these problems is a seeded FEL scheme, which is possible to realize full-coherent light pulses without shot-noise structures. We already reported the higher-order harmonic (HH) seeded FEL operation at the fundamental wavelength of 61.2 nm using a seeding source of a HH pulses from Ti:sapphire laser at the SPring-8 Compact SASE Source (SCSS) test accelerator. However, the seeded FEL pulses (intensity >4σ of SASE) were only obtained 0.3% in 3000 shots.

Full-coherent HHG-seeded EUV-FEL locked by EOS timing feedback

We are developing seeded XFEL pulse with HH for coherent XFEL pulse. Using the monitor of temporal overlap with EO sampling technique, seeded pulse was obtained long time against timing drift.

A few tens of microjoule full-coherent EUV-FEL seeded by high-order harmonic beam

Output energy exceeding 20 μJ has been achieved in the EUV region at 61 nm by the harmonic seeded FEL scheme. Event ratio of the seeded FEL operation has been dramatically improved by a feedback system with an electro-optic sampling technique.

Femtosecond Laser-Induced X-Ray Emission from Gold Nano-Colloidal Solutions

By irradiating droplets of chloroauric acid aqueous solution (\(\sim 1{0}^{-3}\,\mathrm{mol/l}\)) and those of gold nano-colloidal solution with near-IR (∼800 nm) femtosecond (35 fs) laser light, the X-ray intensity and the X-ray emission spectra were recorded. The laser pulses were focused onto the surface of droplets (∼90μm in diameter) ejected from an ink-jet nozzle by an off-axis parabolic mirror. The X-ray intensity was about 660 times higher with the gold nano-colloidal solution than with the chloroauric acid aqueous solution. Electron temperatures calculated from the X-ray emission spectra on the basis of the assumption of the Boltzmann distribution were 0.074 keV for the chloroauric acid aqueous solution and 2.9 keV for the gold nano-colloidal solution. The significantly high X-ray intensity and electron temperature obtained with the gold nano-colloidal solution can be ascribed to the enhancement of the laser field intensity by surface plasmon resonance at the gold nano-particles.

High energy proton ejection from hydrocarbon molecules driven by highly efficient field ionization

We report on the observation of an ionization-fragmentation process that puts a polyatomic molecule into a very high charge state of +12 and beyond at laser intensities as low as a few 1014 W/cm2 and subsequently leads to complete molecular decomposition. Our surprising experimental evidence shows that all of the molecules protons are ejected with high kinetic energies, which is inconsistent with the explanation based on dynamic charge localization used in [1] to explain the proton energy. Furthermore, the energies of all the protons are very similar, which indicates that at the instance of decomposition the protons interact with the same molecular charge state, suggesting an all-at-once (concerted) fragmentation scenario.

EUV-FEL seeded by high-order harmonic

We first demonstrated a seeded FEL radiation in the EUV region at 61.2 nm with the 13th high-order harmonic beam. We observed single-peak spectra with drastic enhancements of intensity by nearly three-orders of magnitude.

Observation of High Energy Protons Ejected from Small Polyatomic Molecules in Laser Induced Fragmentation

Remarkably high energies of protons ejected from three different species of small polyatomic molecules during laser-induced fragmentation are observed using coincidence momentum spectroscopy. The results imply that the responsible field-driven dynamics are a general phenomenon.