Fumihiko Kannari

An advanced kinetic model of electron‐beam‐excited KrF lasers including the vibrational relaxation in KrF*(B) and collisional mixing of KrF*(B,C)

Computer models developed so far on electron‐beam‐excited KrF(B–X, 248 nm) lasers that include the vibrational relaxation process in the upper lasing B level at the finite rate could not predict the high intrinsic laser efficiency which was experimentally reported. This is attributed to the reduction of the laser extraction efficiency. We have developed a four‐level KrF laser model that includes the vibrational relaxation process and also the collisional mixing of the KrF*(B) and the KrF*(C) levels. The collisional quenching rates for KrF*(B,C) that we used and the vibrational relaxation rate were carefully estimated by using the effective spontaneous lifetimes for KrF*(B,C). As a result, the model prediction was in quite good agreement with many experimental results for a saturation behavior of KrF*(B–X) fluorescence, for small‐signal gains, for small‐signal absorptions, and for intrinsic efficiencies. Estimated rate constants in this model for the vibrational relaxation and the KrF*(B,C) mixing are 4×10...

Extreme ultraviolet free electron laser seeded with high-order harmonic of Ti:sapphire laser

The 13th harmonic of a Ti:sapphire (Ti:S) laser in the plateau region was injected as a seeding source to a 250-MeV free-electron-laser (FEL) amplifier. When the amplification conditions were fulfilled, strong enhancement of the radiation intensity by a factor of 650 was observed. The random and uncontrollable spikes, which appeared in the spectra of the Self-Amplified Spontaneous Emission (SASE) based FEL radiation without the seeding source, were found to be suppressed drastically to form to a narrow-band, single peak profile at 61.2 nm. The properties of the seeded FEL radiation were well reproduced by numerical simulations. We discuss the future precept of the seeded FEL scheme to the shorter wavelength region.

Dissociative ionization of ethanol in chirped intense laser fields

The dissociative ionization of ethanol C2H5OH in an intense laser field is investigated with a chirped laser pulse. From the sensitive dependence of the relative yields of the fragment ions on the absolute values of the linear chirp rate, it is shown that the light-dressed potential-energy surface (LDPES) at the singly charged stage governs the nuclear dynamics, and that the nuclear wave packet flow into the breaking of either of the C–C and C–O chemical bonds could be characterized by the holding time thold during which the LDPESs are maintained. It is also understood in term of the holding time that the enhanced ionization into the doubly charged stage followed by the Coulomb explosion at C–C or C–O proceeds when the nuclear wave packet at the singly charged stage reaches the critical distance for the further ionization.

Deposition of fluoropolymer thin films by vacuum‐ultraviolet laser ablation

Crystalline thin films of polytetrafluoroethylene were deposited on Si(100) wafers by F2 laser (157 nm) ablation in 200 mTorr Ar gas atmosphere. X‐ray photoemission spectra indicated that the composition of the deposited films was similar to the source material. The surface morphology of films deposited at room temperature contained numerous fibrous structures in size of 100–400 nm, but they were smoothed out at elevated wafer temperature of ∼370 K, while the crystalline feature was still maintained. The refractive index was ∼1.35 at 633 nm.

Theoretical simulation of electron-beam-excited xenon-chloride (XeCl) lasers

By developing a comprehensive computer code for e -beam excited XeCl lasers, we studied mainly the effect of Ar and Ne diluents on the performance characteristics of XeCl lasers. According to the analysis of the XeCl* formation process, the XeCl* relaxation process, and the 308 nm absorption process, it is found that the XeCl* formation efficiency is determined mainly by the rate of the charge transfer process (from Ar+ and Ne+ diluent ions to Xe+); in other words, by the difference between ionic potentials of Xe and the diluent gas used. The extraction efficiency is found to be decided mainly by the quenching rate of a three-body reaction for a short-pulse (55 ns) and a high-excitation-rate (∼ 3 MW/cm3) pumping, and by the absorption process for a long-pulse (500 ns) and a low-excitation-rate (∼ 0.2 MW/cm3) pumping. However, note that no appreciable difference in the intrinsic efficiency is found between the Ar/Xe/HCl and Ne/Xe/HCl mixtures. We also analyzed the dependence of the intrinsic XeCl laser efficiency on the pumping pulse width and excitation rate for Ar/Xe/HCl and Ne/Xe/HCl mixtures. As a result, the same intrinsic efficiencies are obtainable for both Ar- and Ne-based mixtures although the optimum operating conditions are slightly different. The maximum intrinsic efficiency of 5 percent is obtainable both for the Ar/Xe/HCl mixture at 3 atm and with 1.5 MW/cm3, 200 ns (FWHM) pumping and for the Ne/Xe/HCl mixture at 4 atm and with 2 MW/cm3, 200 ns (FWHM) pumping.

Adaptive pulse shaping of phase and amplitude of an amplified femtosecond pulse laser by direct reference to frequency-resolved optical gating traces

By adaptive control of both spectral phase and amplitude of seed pulses with an acousto-optic programmable dispersion filter located before a pulse stretcher, accurate pulse shaping was achieved for ultrashort laser pulses delivered through a regenerative amplifier. Two-dimensional patterns obtained from frequency-resolved optical gating were used for calculating cost functions in an adaptive control algorithm without pulse shape reconstruction during the control of pulse shape.

A coherent all-solid-state laser array using the Talbot effect in a three-mirror cavity

Lateral mode coupling in a diode-pumped Nd:YAG microchip laser array is demonstrated with a Talbot cavity for the first time. The relatively low laser gains of solid-state lasers compared with diode lasers and CO/sub 2/ lasers, to which the Talbot cavity has already been applied successfully, are solved by employing a novel auxiliary Talbot cavity configuration. A brighter twin-peak far-field pattern indicating an out-of-phase array mode, whose spot is 9.3 times smaller than that obtained by incoherent superposition of the individual microchip laser outputs, is obtained from the phase-locked microchip laser array with a mode-selecting slit. Without the mode-selecting slit, a far-field pattern with a single narrow peak is obtained, showing that the array is locked in an in-phase mode, presumably because of multiple reflections in the auxiliary Talbot cavity.

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.

7dB quadrature squeezing at 860nm with periodically poled KTiOPO4

We observed 7.2 dB squeezing of continuous-wave light at 860 nm with a periodically-poled KTiOPO4 crystal in an optical parametric oscillator. The squeezing level was obtained as the LO phase was locked.

Spatiotemporal measurements based on spatial spectral interferometry for ultrashort optical pulses shaped by a Fourier pulse shaper

We measured the space–time profile of ultrashort optical pulses shaped with a Fourier pulse shaper. Spatial chirp, which originated in the space–time coupling at the pulse shaper, was observed directly with two-dimensional spatial spectral interferometry. By analyzing the two-dimensional fringe pattern, we successfully obtained a spatial and a temporal distribution in both amplitude and phase for shaped pulses. Numerically predicted spatiotemporal patterns were compared with experimental results.