Laser-plasma particle accelerators (LPA) can make accessible, in a compact setup, the science and applications that are presently limited to large-scale accelerator facilities. Research efforts have been concentrated on stabilization of LPAs since the development of monoenergetic electron bunches. By adopting new techniques such as an artificial injection of electrons, plasma channel for guiding laser pulses, and magnetization of plasmas, the stability of LPAs can be improved considerably. Recently several approaches have been employed for opening up the applications of LPAs, for example, generation of femtosecond electron bunches, use of tunable ultrashort X-rays, and use of proton injectors for synchrotrons.
In laser-driven plasma acceleration, which is charged particle acceleration via the interaction of an intense laser pulse with a plasma, the accelerating electric field is a thousand times higher than that of conventional radio-frequency accelerators. Such high accelerating field enables a compact accelerator. Recently, the generation of quasi-monoenergetic electron beams has been demonstrated and the way toward a practical, compact electron accelerator is opened. Furthermore, the electron pulse duration is extremely short, of the order of femtosecond in laser-driven plasma acceleration. This set of unique characteristics opens the way toward a novel, compact, all-optical, ultrashort X-ray source based on such as laser Compton scattering scheme. The present status of the quasi-monoenergetic electron acceleration and prospect for all-optical ultrashort X-ray sources are reviewed.
The relativistic flying mirror concept uses nonlinear plasma waves formed by an ultra-short intense laser pulse in tenuous plasma to reflect incoming laser light. Because the nonlinear plasma wave is moving approximately at the speed of light, the reflected light is downshifted in wavelength and shortened in pulse length. This concept has been originally invented to intensify focused laser intensity towards extremely high electric field, in which a vacuum starts to break. This scheme is also useful to generate ultra-short, soft-X-ray to XUV light.
We have measured and evaluated the femtosecond bunch duration of electron bunches produced by a 12 TW 40 fs laser pulse focused into a He gas jet. Well reproducible electron bunches can be generated with a low-emittance and high-charge, which are very important characteristics for future time-resolved applications. The bunch duration of such electron bunches was determinded to be less than 2 ps (FWHM) at 340 mm downstream from the gas jet by spectral analysis of the coherent transition radiation. The effects of bunch elongation due to both the energy spread and the space charge of electron bunches were estimated, and this estimation is in good agreement with the experimental results. The feasibility for time-resolved applications is also discussed.
Approach to downsize an accelerator for particle-cancer therapy with the use of a very high acceleration-gradient by laser-plasma interaction has been performed combining laser-produced ions with a conventional RF acceleration technique. Protons produced by a high peak-power (∼a few tens TW) short-pulse (<50 fs) laser from a thin solid foil target several μm in thickness has been improved in its beam chracteristics by combination of a “Phase Rotation” and electron beam cooling in order to be accelerated with a short-rising-time (∼5 ms) synchrotron to the energy of ∼200 MeV needed for cancer-therapy. In the present paper, the scheme to improve the characteristics of the laser-produced protons in the longitudinal phase space by “Phase Rotation”, which has been demonstrated in its feasibility experimentally, is described together with the procedure of laser proton production.
Three-dimensional micro fabrication process is one of the most important processes for micro-electro-mechanical systems field, optical device and many advance applications. This paper describes reactive ion etching of silicon substrates using three-dimensional aluminum masks. Aluminum masks were fabricated by photolithography, anodization and chemical etching. A 150 nm thick aluminum film was deposited on titanium-coated silicon substrates. Subsequently, square masks were patterned on the aluminum film by photolithography. After anodizing the aluminum film in 2 vol% sulfuric acid, an anodic oxide film was formed at the photoresist/aluminum film interface in addition to the open surface regions. After the anodic oxide film was removed by chemical etching in 20 vol% phosphoric acid, the resulting aluminum film surface showed convex features. Silicon substrates were fabricated using these aluminum masks. By controlling the gas mass flow and pressure, an etching rate of 32-94 nm/min and selectivity of 8.4-218 were achieved. Thus, this process proved to be effecitve method for fabricated three-dimensional microstructures on silicon substrates.
Influence of the oxygen on island formation of titanium silicide on a Si(001) surface was studied by means of scanning tunneling microscopy to analyze the fundamental process of Metal-Oxide-Semiconductor (MOS) fabrication at the atomic scale. The shape of the nanoclusters differs from that formed without the oxide layer. The process is affected by desorption of SiO occurring at relatively lower temperature, while oxygen remains inside the silicide. With tunneling spectroscopy, the most of the clusters were identified to be crystallized in C49-TiSi2 even at 1073 K, indicating that transformation to C54-TiSi2 was disturbed by the presence of oxygen.
By irradiating ArF excimer laser with energies 40~70 mJ on the targets of ITO and AZO(Al-doped zinc oxide) by turns, the laminated transparent conducting films composed of ITO(50 nm)/AZO(250 nm) with a total films thickness of 300 nm were fabricated at substrate temperature of 220°C. At laser energy of 60 mJ, a sheet resistance of 6.12 Ω/□ was obtained under conditions of oxygen pressure of 0.5 Pa for ITO and 0 Pa for AZO. This value of 6.12 Ω/□ is superior to the value of 6.56 Ω/□ obtained from ITO thin films whose thickness is equal to the total thickness (300 nm) of the ITO/AZO films. As a result, 80 percent consumption of ITO was reduced at its maximum. After having examined environmental load, the sheet resistance of the laminated ITO/AZO transparent conductive oxide films did not change and therefore, the durability to the environmental conditions was maintained.