Hidetoshi Nakano

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.

Generation of 50-fsec pulses from a pulse-compressed, cw, passively mode-locked Ti:sapphire laser

Stable pulses of less than 150 fsec are generated directly from a tunable cw passively mode-locked Ti:sapphire laser, through a balance of self-phase modulation in the Ti:sapphire rod and negative group-velocity dispersion produced by a prism pair. After external fiber compression, 50-fsec pulses are obtained at approximately 750 nm.

All solid-state cw passively mode-locked Ti:sapphire laser using a colored glass filter

All solid‐state cw passive mode locking of a Ti:sapphire laser is accomplished using a colored glass filter, instead of an organic dye, as a saturable absorber. The tuning range is remarkably wide (785–855 nm), and 2.7 ps pulses are obtained directly from the cavity.

Laser generation of antisymmetric Lamb waves in thin plates

Abstract A convenient method is described for optically exciting and monitoring Lamb waves in thin plates. The interference pattern of two high power laser beams on a sample surface produces periodic heating and thus coherent antisymmetric Lamb waves are excited. By varying the interface fringe spacing, the acoustic frequency is easily and continuously tunable from 2.5 to 23 MHz with the present experimental apparatus. The ultrasound velocities as a function of frequency are measured to determine a dispersion curve, from which the Youngs modulus is deduced. Furthermore, the temperature dependence is estimated from the velocity changes. Experimental results are presented for several kinds of brittle plate and thin film materials.

cw passive mode locking of a Ti:sapphire laser

cw passive mode locking of a Ti:sapphire laser is achieved with 1,1’‐dietyl‐2,2’‐dicarbocyanine iodide as the saturable absorber dye, using a 5 μm thin dye jet flow. The pulse width is 4.0 ps, which is almost the transform‐limited pulse for the observed spectrum width. The output power is ∼50 mW, when it is pumped by a 5 W cw Ar laser, while the tuning range is 745–755 nm.

X-ray generation enhancement from a laser-produced plasma with a porous silicon target

X-ray generation enhancement from a laser-produced plasma with a porous Si target is reported. For a porous surface formed on a Si wafer, the self-reflectivity of a femtosecond pulse becomes considerably small. The observed energy penetration depth is 25–30 μm, which is much larger than the skin depth of solid density matter. Using a porous Si target, the threshold of the pre-pulse intensity required for soft x-ray emission enhancement can be reduced. It also contributes to enhance the pre-pulse effect, and soft x-ray generation enhancement ranging from 1.6 to 6.5 times is observed depending on the pre-pulse intensity.

Time-resolved soft x-ray absorption spectroscopy of silicon using femtosecond laser plasma x rays

We measured time-resolved soft x-ray absorption of photoexcited silicon by means of pump-probe spectroscopy, using a picosecond soft x-ray pulse from femtosecond laser-produced plasma as a probe. We observed a 5% increase in the absorption caused by 1010 W/cm2 intensity laser pulse irradiation near the LII,III edge at 100 eV. The change was observed only when the laser and the soft x-ray pulses overlapped on the sample both in time and space.

Synthesis of GaAs nanowires with very small diameters and their optical properties with the radial quantum-confinement effect

We report the synthesis and optical properties of GaAs nanowires with very small diameters. We grew the GaAs nanowires by using size-selective gold particles with nominal diameters of 5, 10, 20, 40, and 60 nm. The diameter-controlled nanowires enable us to observe blueshifts of the free exitononic emission peak from individual nanowires with decreasing gold-particle size due to the two-dimensional radial quantum-confinement effect. We also analyze the absorption and emission polarization anisotropies of these bare GaAs quantum nanowires.

Two-body Coulomb explosion and hydrogen migration in methanol induced by intense 7 and 21fs laser pulses

Two-body Coulomb explosion with the C-O bond breaking of methanol induced by intense laser pulses with the duration of Delta t=7 and 21 fs is investigated by the coincidence momentum imaging method. When Delta t=7 fs, the angular distribution of recoil vectors of the fragment ions for the direct C-O bond breaking pathway, CH(3)OH(2+)-->CH(3) (+)+OH(+), exhibits a peak deflected from the laser polarization direction by 30 degrees -45 degrees , and the corresponding angular distribution for the migration pathway, CH(2)OH(2) (+)-->CH(2) (+)+H(2)O(+), in which one hydrogen migrates from the carbon site to the oxygen site prior to the C-O bond breaking, exhibits almost the same profile. When the laser pulse duration is stretched to Delta t=21 fs, the angular distributions for the direct and migration pathways exhibit a broad peak along the laser polarization direction probably due to the dynamical alignment and/or the change in the double ionization mechanism; that is, from the nonsequential double ionization to the sequential double ionization. However, the extent of the anisotropy in the migration pathway is smaller than that in the direct pathway, exhibiting a substantial effect of hydrogen atom migration in the dissociative ionization of methanol interacting with the linearly polarized intense laser field.

Greatly enhanced soft x-ray generation from femtosecond-laser-produced plasma by using a nanohole-alumina target

By making an array of nanoholes on an alumina target, x-ray emission from laser-produced plasma can be greatly enhanced even in soft x-ray energy regions (<0.25 keV). X-ray fluence enhancement around 30 times was achieved in the 5–25 nm wavelength range. The enhancement increases as the ionization level of Al becomes higher and the x-ray wavelength becomes shorter. Over 50-fold enhancement was obtained at a soft x-ray wavelength around 6 nm, which corresponds to the emission from Al8+,9+ ions. X-ray pulse duration was 17 ps, which is much shorter than that obtained by using the prepulse technique.