Michiaki Mori

Quartz micromachining using laser plasma soft x rays and ultraviolet laser light

We have investigated a technique for micromachining inorganic transparent materials. In the technique, patterning and coloration are performed by the direct irradiation of materials with pulsed laser soft x rays and the patterned areas are ablated using ultraviolet laser light. The technique utilizes the high precision of the soft x rays and the high energy density of conventional laser light. For demonstration, we irradiated quartz plates with Ta laser plasma soft x rays. This results in generation of transient surface opaque layers that absorb more than 40% of the 266nm Nd:YAG laser light. Applying the technique, quartz plates are found to be ablated smoothly at 85nm∕shots.

Phase rotation scheme of laser-produced ions for reduction of the energy spread

In order to widely spread out particle beams utilized in cancer therapy, laser-produced ions are developed as the injection beam for an ion synchrotron dedicated for cancer therapy. Such a laser ion source is expected to contribute largely to the realization of compactness of the size and low cost of the cancer therapy accelerator. The energy spectrum of the laser-produced ions, however, has no peak, but their intensities decrease exponentially according to the increase of the energy. For the purpose of modifying such a situation, we have proposed a scheme to rotate the beam in the longitudinal phase space with the use of the RF electric field, which is phase-adjusted with the pulse laser. We aim for a reduction of the energy spread of ± 5% selected by an energy analyzer and slits to ±1% by such phase rotation. For this purpose, a quarter wavelength resonator with two gaps with the same resonant frequency as the source laser has already been fabricated, together with its RF power source. The above phase rotation system and its recent experimental realization are presented.

Electron Energy Spectrometer for Laser-Driven Energetic Electron Generation

An electron energy spectrometer for studying the energy spectrum of electrons emitted from solid foils irradiated by a femtosecond intense laser pulse and its calibration with a β source is described. The intensity distribution of the magnetic field induced by a dipole magnet and the time decay of the photostimulated luminescence of an imaging plate were determined. Both the energy scale and the electron intensity conversion ratio of the electron energy spectrometer were calibrated with a 90Sr–90Y β source. The energy spectrum of hot electrons emitted from a 3-µm-thick Tantalum foil target irradiated by laser pulses with a pulse duration of 50 fs and a peak intensity of 2×1018 W/cm2 was determined with the calibrated electron energy spectrometer.

Behaviour of the peripheral electron temperature at the H-transition observed on the JFT-2m tokamak

Abstract The behaviour of the peripheral electron temperature at the H-mode-like transition which occurs in limiter discharges is found to be quite similar to that at the H-transition in divertor discharges. The treshold temperature for the divertor H-transition has certain relations with the threshold power and density.

Single-Shot Creation of Nanometer-Sized Silicon Tadpoles by Ultrahigh-Intensity Laser

The single-shot creation of tadpolelike silicon nanoparticles constructed with multi-crystalline heads and amorphous tails by a high brightness fs-pulse laser was demonstrated. This is also the first demonstration of the creation of a nanosized connection of multicrystalline silicon with amorphous silicon. This result should expand the creation of new materials by a laser ablation using a high-intensity fs laser, and the created silicon nanoparticles can be applied to scientific and industrial fields.

Nanomachining of inorganic transparent materials using an x-ray exciton method

We have investigated microfabrication of inorganic transparent materials using laser plasma soft X-rays with the diffraction limit of 10 nm. As a soft X-ray source, we used Ta laser plasma soft X-rays, whch has a light emission band at around 10 nm. A SiO2 film was confirmed to have absorption band in the soft X-ray region. Micromachining a quartz plate was demonstrated by irradiation with focused soft X-rays and pulsed 266 nm Nd:YAG laser light. The quartz plates are ablated smoothly at 85 nm/shots. It is found that more than 40% of 266 nm laser light is absorbed by the quartz plates just after soft X-ray irradiation at room temperature. Thus, it is shown that a transient state such as an X-ray generated exciton (X-ray exciton), which have absorption band in UV region, are generated by soft X-ray irradiation. The micromachining technique (X-ray exciton method) can utilize the high space resolution of soft X-ray and the high energy density of conventional UV/visible laser light. Further, we found that a variety of materials such as SiO2, LiNO3, Si, CaF2, LiF and Al2O3 are ablated smoothly by irradiation of the focused laser plasma soft X-rays without 266 nm laser light.

High-Intense, High-Contrast J-KAREN Laser at Advanced Photon Research Center

We demonstrate a compact high-intense, high contrast OPCPA/Ti:sapphire hybrid laser. The 80 TW peak power at 10 Hz repetition rate with ~9 orders temporal contrast in a few picoseconds region was obtained.

Observation of Thin Foil Preformed Plasmas with a Relativistic-intensity Ultra-short Pulse Laser by Means of Two-color Interferometer

Intense laser driven thin foil target acts as a double layered coulomb system that produces collimated ions as well as high-order harmonics, hard X-ray, and so on. From this point of view, we observed the electron density distribution of preformed plasmas of a tantalum foil target with two-color probe beams as an interferometer. The preformed plasma at the front side of the target was generated by a prepulse with a high-intensity Ti:sapphire laser pulse. On the other hand, the preformed plasma at the rear side was not observed. In our experimental condition, high-energy electrons and protons were observed. The characterization of preformed plasmas will contribute to describe the high-intensity laser-thin foil interactions.

Hot Electrons Emitted from a Thin Foil Target Irradiated by Ultrashort Intense Laser Pulses

The experimental study on hot electrons emitted from a 3‐μm thick tantalum foil irradiated by s‐ and p‐polarized intense femtosecond laser pulses with a peak intensity of ∼1018 W/cm2 are reported here. Three hot electron flows are found along the rear target normal, specular and target surface directions. The characters of hot electrons are discussed and applied to explain the experimental results of bremsstrahlung X‐rays and generation of protons.

Attosecond pulse generation in the framework of the sliding mirror model

We present the theory of the attosecond pulse generation by a relativistically intense few-cycle laser pulse interacting with a thin plasma slab. In the limit of high plasma density or in the case of two identical pulses irradiating plasma from opposite sides, the electron displacement in the direction perpendicular to the plasma slab is negligible. Electrons moving along the plasma slab form the sliding mirror. The relativistic dynamics of the electrons result in the generation of the phase-locked high order harmonics. After spectral filtering, isolated attosecond pulses with the duration less than 200 as can be obtained. We also find a very efficient regime of the attosecond pulse train generation without any spectral filtering with the energy conversion efficiency into the main peak of about 3%.