Shin-ichi Zaitsu

Optical thin films consisting of nanoscale laminated layers

The control of the refractive index of laminated coatings consisting of alternating stacks of nanoscale Al2O3 and TiO2 sublayers grown by atomic layer deposition has been achieved. The refractive index of the coating linearly changed from 1.870 to 2.318 as the thickness of the single TiO2 sublayer was varied from 2.0 to 39 A while that of the single Al2O3 sublayer was kept constant at 5.5 A. The refractive index could be varied by adjusting only the number of growth cycles of each material. This approach will have potential applications to optical multilayer coatings consisting of well-controlled extremely thin layers.

Large-Area Optical Coatings with Uniform Thickness Grown by Surface Chemical Reactions for High-Power Laser Applications

We prepared optical thin films using an atomic layer deposition (ALD) procedure in order to apply this coating method to optical components for high-power and large-scale lasers. Film thickness shows a proportional relationship to the number of operation cycles even in the case of room-temperature growth, and the distribution is uniform with a thickness error of less than 1% over an area of 240 mm diameter. We examined the laser damage thresholds of the films with 1 ns laser pulses at 1.064 µm. The highest thresholds (TiO2: 5 J/cm2, Al2O3: 5.2 J/cm2) are obtained in the amorphous films grown at low growth temperatures (25–50°C). Results from the analysis of film structure and composition, and measurement of optical absorption reveal that the decrease in laser damage threshold as the growth temperature rises is caused by the crystallization of films.

Generation of a continuous-wave pulse train at a repetition rate of 17.6 THz

We demonstrated that a 17.6THz pulselike intensity modulation resulted from the coherent superposition of multifrequency continuous-wave emissions generated from a hydrogen-filled high-finesse cavity through a cascade-stimulated Raman scattering process. We pointed out that the complete phase-locked operation was hindered by the intracavity dispersion that caused nonequal separations between adjacent emission lines.

Generation of intense 11-fs ultraviolet pulses using phase modulation by two types of coherent molecular motions

The use of two types of phase modulations arising from the coherent rotations of ortho-hydrogen and para-hydrogen to generate an intense ultrashort ultraviolet pulse without substantial generation of sub-pulses was demonstrated. This technique allows use of a high-energy long-probe pulse in the pump-probe regime for generating a high-energy compressed pulse. A 100-fs ultraviolet pulse was compressed to 11-fs by the phase modulation followed by dispersion compensation with chirped mirrors.

Enhancement of molecular ions in mass spectrometry using an ultrashort optical pulse in multiphoton ionization

The spectral domain of an ultraviolet femtosecond laser was expanded by stimulated Raman scattering/four-wave Raman mixing, and the resulting laser pulse was compressed using a pair of gratings. The pulse width was then measured using an autocorrelator comprised of a Michelson interferometer equipped with a multiphoton ionization/mass spectrometer which was used as a two-photon detector. A gas chromatograph/mass spectrometer was employed to analyze triacetone triperoxide (TATP), and the molecular ion induced by multiphoton ionization was substantially enhanced by decreasing the laser pulse width.

Molecular-optic modulator

An ultrafast light-intensity modulator, based on stimulated Raman scattering, is described. The intensity of a continuous wave laser is fully modulated at 17THz using hydrogen in a high-finesse cavity. The modulation frequency is determined by the molecular constant of the Raman medium, i.e., the Raman shift frequency. The modulation frequency can be changed in the tetrahertz range by replacing the Raman medium. Due to the accurate modulation frequency and the high beam coherence, this device is amenable to a variety of applications such as in basic science and also in advanced industrial technology.

Laser Damage Properties of Optical Coatings with Nanoscale Layers Grown by Atomic Layer Deposition

The notable change in the structure and laser damage properties of coatings using periodically incorporated Al2O3 nanoscale layers grown by atomic layer deposition is demonstrated. Coatings with a nanoscale laminated structure had a higher transparency than pure TiO2 films grown at 400°C. The laser damage resistance of nanolaminate coatings is three times larger than that of pure TiO2 films. The morphology of laser-induced damage shows a significant reduction in the number of damage sites.

Near-ultraviolet femtosecond laser ionization of dioxins in gas chromatography/time-of-flight mass spectrometry

Gas chromatography/multiphoton ionization/time-of-flight mass spectrometry (GC/MPI/TOF-MS) was applied to the trace analysis of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). To determine the optimum wavelength for analysis of PCDD/Fs, the wavelength of the femtosecond laser utilized for multiphoton ionization was converted to near-ultraviolet status using stimulated Raman scattering. A femtosecond laser emitting at 300 nm completely eliminated the background signal arising from the bleeding compounds generated from a stationary phase of the capillary column in GC.

Interferometric characterization of ultrashort deep ultraviolet pulses using a multiphoton ionization mass spectrometer

The temporal characterization of a femtosecond laser pulse in the deep ultraviolet region using an interferometric autocorrelation scheme is demonstrated. Two-photon ionization of a molecule in a time-of-flight mass spectrometer was used as a nonlinear detector to obtain an autocorrelation trace. This setup proved useful in not only providing a temporal characterization of a pulse but also investigating the ultrafast dynamics of photochemical processes.

Pulse compression based on coherent molecular motion induced by transient stimulated Raman scattering

A novel method for compressing a laser pulse, using a combination of transient stimulated Raman scattering and a pump-probe technique, is proposed. The approach does not require a short laser pulse, in contrast to a reported method based on impulsive stimulated Raman scattering. The observed spectrum was sufficiently broad to generate a sub-10 fs pulse. In fact, a 100-fs pulse in the near-ultraviolet region was compressed to the sub-30 fs. Further compression of the laser pulse would be achieved by compensating for phase distortion, as suggested from the observed data of the spectral phase.