Masanori Kaku

Generation of extreme ultraviolet continuum radiation driven by a sub-10-fs two-color field

We have proposed and demonstrated a novel approach for generating high-energy extreme-ultraviolet (XUV) continuum radiation. When a two-color laser field consisting of a sub-10-fs fundamental and its parallel-polarized second harmonic was applied to high-order harmonic generation in argon, a continuum spectrum centered at 30 nm was successfully obtained with an energy as high as 10 nJ. This broadband emission indicates the possibility of generating intense single attosecond pulses in the XUV region.

Generation of 5 fs, 0.5 TW pulses focusable to relativistic intensities at 1 kHz

We have demonstrated the generation of 5 fs, 0.5 TW pulses at 1 kHz repetition rate using a pulse compression technique in a hollow fiber with a pressure gradient. Owing to the excellent beam quality by passing through the hollow fiber, the beam after pulse compression could be focused to a nearly diffraction-limited spot size. We obtained for the first time a peak intensity as high as 5x10(18) W/cm(2) in the 2-cycle regime.

Observation of revival structure in femtosecond-laser-induced alignment of N2 with high-order harmonic generation

We report a new sensitive method using high-order harmonic generation to observe revival structure in fs-laser induced alignment of a rotational wave packet of molecules. Pump and probe fs-laser pulses with a time delay were focused collinearly into a pulsed N2 gas jet, so that the pump pulse induces alignment of a ground-state rotational wave packet, and the delayed probe pulse produces harmonic radiation from the aligning molecules. The harmonic signal observed as a function of time delay has clearly demonstrated a typical time-dependent revival structure in the field-free alignment of molecules.

Pointing stabilization of a high-repetition-rate high-power femtosecond laser for intense few-cycle pulse generation

We have developed a beam stabilization system for high-power femtosecond lasers operating at a repetition rate of 1kHz. The beam pointing at a hollow fiber input was stabilized to within 1μm rms and the fluctuations of the broadened spectra and power were significantly improved. This technique enables us to increase the input power to a hollow fiber for intense few-cycle pulse generation.

Observation of vacuum-ultraviolet Ar2* radiation gain at 126 nm produced by an ultrashort high-intensity laser pulse propagating in a hollow fiber.

We observed a small-signal gain of Ar2* emission at 126 nm by use of a hollow fiber to guide the high-intensity laser propagation in high-pressure Ar. The small-signal gain coefficient was measured to be 0.05 cm(-1) at 126 nm. Kinetic analysis revealed that the electrons produced by the high-intensity laser through an optical-field-induced ionization process initiated the Ar2* production processes. The increase in the emission intensity was measured to be exp(2.5), with an increase in the fiber length.

Vacuum ultraviolet spectroscopic system using a laser-produced plasma

We have developed a vacuum ultraviolet (VUV) spectroscopic system using a laser-produced plasma. Laser-produced rare gas plasma was initiated by a 10 Hz Q-switched YAG laser at the focused intensity of 1011 W/cm2. Among three different rare gas plasma emissions (He, Ne, and Ar), argon plasma was characterized by broad continuum emission with an integrated emission power of 8.4 kW in the VUV spectral region. The pulse width of the VUV emission of 62 ns (Full width at half maximum), which was governed by recombination processes in the plasma, determined the temporal resolution. Using this emission source, we have measured VUV transmission spectra of Nd3+:LaF3 crystal, which was a potential laser medium in the VUV. The transmission peak of the crystal coincided with the reported laser oscillation wavelength at 174 nm. The absorption coefficient at the wavelength was measured to be 0.7 cm-1.

Suppression of suprathermal ions from a colloidal microjet target containing SnO2 nanoparticles by using double laser pulses

We have demonstrated suppression of suprathermal ions from a colloidal microjet target plasma containing tin-dioxide (SnO2) nanoparticles irradiated by double laser pulses. We observed a significant decrease of the tin and oxygen ion signals in the charged-state-separated energy spectra when double laser pulses were irradiated. The peak energy of the singly ionized tin ions decreased from 9to3keV when a preplasma was produced. The decrease in the ion energy, considered as debris suppression, is attributed to the interaction between an expanding low-density preplasma and a main laser pulse.

Deposited debris characteristics and its reduction of a laser-produced plasma extreme ultraviolet source using a colloidal tin dioxide jet target

Debris characteristics and their reduction have been investigated for a laser-produced plasma extreme ultraviolet source by using a colloidal jet target containing tin dioxide nanoparticles. The amounts of deposited debris on a witness plate were determined by total laser energy irradiated onto a target. In situ low-temperature (100°C) heating of a plate was effective to reduce the amounts of deposited debris, since colloidal debris was easily vaporized by the heat.

Amplification properties of vacuum ultraviolet Ar*2 produced by infrared high-intensity laser.

We report optical amplification of Ar(2)* at 126 nm, pumped by optical-field-induced ionization (OFI) created by an infrared high-intensity laser. A gain-length product of 0.84 was obtained by using multipass amplification with a vacuum ultraviolet (VUV) cavity. The gain-length product was increased up to 4.3 through the use of single-pass amplification with a VUV reflector and a hollow 5.0 cm-long fiber. Similar small signal gain coefficients of 0.84 and 0.86 cm(-1) were obtained in two different experiments, in which OFI Ar plasma gain media were produced in free space filled with Ar and inside an Ar-filled hollow fiber.

EUV and debris characteristics of a laser-plasma tin dioxide nano-particle colloidal jet target

Debris characteristics and its reduction have been investigated for a laser-produced plasma (LPP) extreme ultraviolet (EUV) source using a colloidal jet target containing tin dioxide nano-particles. Dominant deposited debris on a witness plate was found to have a form of oxidized tin (SnOx) originated from nano-particles. Quantitative debris amounts were determined by total laser energy irradiated onto a target, not by laser irradiation modes, such as single or double pulse irradiation. In-situ low-temperature (100°C) heating of a plate was effective to reduce the deposited debris amount, since colloidal debris was easily vaporized by the heat. Another approach to remove the deposited debris was roomtemperature photon processing using incoherent vacuum ultraviolet (VUV) emission at 126 nm. X-ray photoelectron spectroscopy (XPS) analysis has shown that the deposited SnOx debris layer was deoxidized by the 126 nm VUV photon energy.