Koji Hatanaka

Ultrafast x-ray pulse generation by focusing femtosecond infrared laser pulses onto aqueous solutions of alkali metal chloride

X-ray pulses were generated by focusing femtosecond infrared laser pulses (775 nm, 130 fs, 0.58 mJ/pulse) onto an alkali metal (Cs or Rb) chloride aqueous solution jet. The mechanism of x-ray pulse generation was studied by measuring x-ray emission spectra and transmitted laser pulse spectra. Addition of CsCl to distilled water was found to enhance markedly x-ray intensity and x-ray cutoff energy. The frequency-up-shift observed in transmitted laser pulses was larger in CsCl solution than in distilled water. The results suggested that a high-density plasma-like medium was produced in CsCl solution due to secondary electrons.

Picosecond and nanosecond photo-dynamics of a naphthopyran merocyanine

For the first time the photochemistry of the trans, trans n(TT) to trans, cis n(TC) isomerisation for merocyanines (MCs) of a chromene (CHR) compound has been investigated using steady-state nanosecond and time-resolved femtosecond irradiation. The dominant photo-transformation of the TT isomer by these irradiations is conversion to TC. The yield of the TT to TC photoreaction is inherently low in all solvents studied due to efficient ground state recovery in competition with inter-system crossing, but it is lowest of all in polar solvents. Several intermediate states have been identified in the first 6 ns of the reaction that we tentatively assigned, including a possible twisted MC T1 state. We assign other distinct states to TT S1, TT T1 and a further species to TC T1. The full photochemical conversion from TT to TC is a slower process than 6 ns. The presence nof oxygen marginally increases the overall bleaching of the TT isomer and slightly enhances TC formation.

Switching from photochemical to photothermal mechanism in laser ablation of benzene solutions

Nanosecond KrF excimer laser ablation of benzyl chloride, benzyl alcohol, toluene, ethylbenzene, and n-propylbenzene diluted in n-hexane, n-heptane, dichloromethane, and 1,2-dichloroethane was investigated by time-resolved photographic, photoacoustic, and absorbance measurements. Ablation threshold values, Fth, for high concentration solutions (α=250u2009cm−1, 0.6–1 M) were confirmed to be correlated to photochemical reactivity (β-bond cleavage) of the solute molecules, whereas no distinct relation between Fth and boiling point of solvents was observed. Time-resolved absorbance at the laser wavelength was almost constant during the excitation pulse, which means that the main light-absorbing molecules were fixed to the ground-state solutes. It is considered that this type of ablation is initiated by the photochemical fragmentation. On the contrary, Fth observed in relatively low concentration solutions (α=25u2009cm−1, 0.06–0.1 M) were about twice higher than those for the high concentration solutions, and had no d...

Luminescence of undoped β-Ga2O3 single crystals excited by picosecond X-ray and sub-picosecond UV pulses

Abstract Undoped β-Ga 2 O 3 single crystals were grown using the floating zone technique under a pressure of 2xa0atm oxygen. Luminescence spectra of the crystals were measured with steady-state X-ray (

Chirp effect in hard X-ray generation from liquid target when irradiated by femtosecond pulses

The chirp effect on a X-ray emission intensity from a CsCl aqueous solution jet irradiated by femtosecond pulses was systematically studied. The p-polarized chirped pulses were more efficient as compared with the shortest pulses determined by the spectral bandwidth. The negatively-chirped pulses of approximately 240 fs duration produced up to 10 times larger X-ray intensity as compared with the transform-limited 160 fs pulses. The angular dependence of X-ray generation can be explained by the resonant absorption. Numerical simulations of electron density evolution due to the avalanche and multi-photon absorption supports qualitatively well the experimental observations.

X-ray pulse emission from cesium chloride aqueous solutions when irradiated by double-pulsed femtosecond laser pulses

The intensity of x-ray emission from the aqueous solutions irradiated with focused femtosecond laser pulses (main pulses and relatively low-intensity prepulses) was measured as a function of the delay time between the main pulses and prepulses. Four different x-ray intensity peaks were observed in the picosecond and nanosecond ranges. The prepulse irradiation caused an increase in the x-ray intensity by a factor of 103–104, as confirmed by x-ray emission spectroscopy. Time-resolved reflectance measurements and imaging revealed that the increase was caused by the solution surface dynamics such as the picosecond plasma formation and decay and the transient (nanoseconds) surface roughness.

Photothermal Ablation of Polystyrene Film by 248 NM Excimer Laser Irradiation:a Mechanistic Study by Time-Resolved Measurements

Laser ablation mechanism at 248 nm irradiation of polystyrene film was investigated and discussed. An ablation threshold was determined by etch depth measurement and nanosecond photographic observation. Temperature at the threshold was evaluated to be 370℃ by using an effective absorption coefficient which was confirmed by transmission measurement of the excitation pulse. The temperature was in good agreement with that of thermal degradation of the polymer, showing that 248 nm ablation of polystyrene was brought about by a photothermal process. Nanosecond photography upon the ablation could be well interpreted from the photothermal viewpoint. Moreover, dynamics of excited species revealed by nanosecond time-resolved luminescence measurements was inferred in relation to the ablation.

Explosive boiling of water after pulsed IR laser heating

By focussing 1 J of 1064 nm Ndu2006∶u2006YAG beam into 30 atmospheres of hydrogen we could Raman shift to produce a 10 ns, 300 mJ, 1.9 µm laser pulse. This pulse can directly heat water by more than 100 K (average), during the laser pulse, inducing vaporisation. Vaporisation was studied using time-resolved shadowgraphy and Raman spectroscopy to obtain macro and molecular level information. The O–H stretching Raman bands of water are sensitive to temperature allowing us to measure the average temperatures during the boiling process. After the T-jump, explosive boiling occurred within 100 ns during which time the bulk temperature decreased, indicating that the vaporising water molecules deprived heat from their surroundings. Shadowgraphs confirmed the timescale for this phenomenon visually. After 10 µs, vaporised gas molecules condensed and formed droplets, which were observed by a morphology-dependent resonance (MDR) Raman.

Fabrication and luminescence properties of single-crystalline, homoepitaxial zinc oxide films doped with tri- and tetravalent cations prepared by liquid phase epitaxy

Single-crystalline, homoepitaxial ZnO films doped with tri- and tetravalent cations were fabricated by liquid phase epitaxy (LPE) from low-temperature lithium chloride solution. The LPE is applied as a fast-screening tool to obtain mechanically undisturbed (0001) surfaces of the undoped and In3+ and Ge4+-doped ZnO films. Time-resolved photoluminescence characteristics upon the excitation by a femtosecond laser pulses are studied. Characteristics of the films are discussed and particularly the effect of In3+ and Ge4+ on the luminescence decay kinetics is examined. We find the double-exponential decay at room temperature consisting of an ultra fast and a slower component with decay times τ around 30–60 and 250–800 ps, respectively. High-intensity emission due to donor–acceptor pair recombination peaking around 420 nm at room temperature is obtained from the Li+, In3+ co-doped ZnO film, which shows the inverse power law decay at longer times.

Time-resolved surface scattering imaging of organic liquids under femtosecond KrF laser pulse excitation

Time-resolved surface scattering imaging was performed for liquid benzyl chloride and liquid toluene under femtosecond KrF laser ablation conditions. No scattering image was obtained until 1 ns, while scattering started from 2 ns when the laser fluence exceeded 25u2009mJ/cm2, and its intensity increased with the passage of time. The higher the laser fluence was, the steeper the increasing slope was. The scattering is due to surface roughness, which is the initial stage of macroscopic morphological changes. Root-mean-square surface roughness was estimated from the scattering intensity by using frosted fused-silica plates as reference samples. The induced surface roughness increases to a few hundred nm in 10 ns.