We present the two experimental results of high-order harmonic generation (HHG) using mixed gases target, one is the dramatic enhancement effect, and the other is the destructive and constructive interference of HHG. The dramatic enhancement effect was realized by simultaneous irradiation of XUV attosecond pulse train (APT) to assist optical field ionization. The harmonic yield generated from He atoms increased by a factor of 4×10³with booster APT form Xe atoms. And also, the destructive and constructive interference of HHG in a mixed gas of He and Ne facilitate the coherent control of HHG. The observed interference modulation was attributed to the difference between the phases of the intrinsically chirped harmonic pulses from He and Ne, which leads to a novel method for broadband measurement of the harmonic phases. The mixture gas system is an attractive medium not only to control HHG but also to observe ultrafast dynamics of electrons and molecules.

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The pulse shape and phase of isolated attosecond extreme ultraviolet (XUV) pulses with a duration of 860 attosecond (asec) have been determined simultaneously by using frequency-resolved optical gating based on two-photon above-threshold ionization with 28-eV photons in He. From the detailed characterization, we succeeded in shaping isolated XUV pulses on an attosecond time scale either by precise dispersion control with generation gas density (Ar) or by changing the driving pulse width. These results offer a novel way to excite and observe an electron motion in atoms and molecules.

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Strong field approximation (SFA) is a theoretical technique to describe electron motion in intense laser fields. It is becoming a standard framework to understand ultrafast atomic and molecular processes in intense laser fields where the field-driven motion of electron wavepackets plays crucial roles. The most striking feature of SFA is the capability to describe non-perturbative strong-field processes in an intuitive as well as a quantitative way. In this paper, I will review the framework of SFA that naturally results in the classical-quantum duality, and its applications to attosecond sciences, especially to high harmonic generation, two-color photoionization for attosecond measurement, and tomographic imaging of molecular orbitals.

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Attosecond extreme ultraviolet and soft X-ray pulses are generated through high-order harmonic generation, a highly non-linear and non-perturbative phenomenon. Nonlinear optical effects and electron-electron interaction play an essential role in various applications of attosecond pulses. The best way to study atomic dynamics under such circumstances theoretically is direct numerical simulation of the time-dependent Schrödinger equation (TDSE). The present article introduces how to model the high-field and ultrafast phenomena on the basis of TDSE, and reviews recent theoretical findings on attosecond pulse generation and attosecond phenomena, peculiar to the time scale of electronic motion inside an atom.

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The interference is an intrinsic feature of waves. In the quantum world, a variety of interferometric measurements of matter waves have so far been performed to demonstrate the wave-particle duality. In this short note, we review such quantum interferometric measurements of wave packets (WP's) in atoms and molecules such as Rydberg WP's in atoms and vibrational WP's in molecules. We also show the results of our recent WP interferometry with the iodine molecules, which have been performed by phase-locking a pair of femtosecond laser pulses on the attosecond time scale.

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Diamond-like carbon (DLC) films have excellent properties: high hardness and low friction. However, their adhesion to most materials is generally very low. The method that we developed to enhance the adhesion in the cace of Si substrates involves the deposition of a Si-C buffer layer between the DLC film and the Si substrate by pulsed laser deposition (PLD) using a Si-C target. This method allows us to easily control the fraction of sp3 in the DLC film over the range of 50 to 80 % by changing the Si content of the target.

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