Generation of Broadband Tunable Coherent X-rays by Using Optical Field Induced Ionization

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We have demonstrated vacuum ultraviolet emissions from pulsed rare gas jet excited by a high intensity ultrashort pulse laser. Various emission characteristics are investigated thoroughly. Ultrashort pulse Lasers are also used to control the dissociation process of alkali halide molecules in a femtosecond time scale. These knowledge should be used to develop a practical coherent x-ray source in the laboratory scale.As a result of the interaction of Ar and high intensity laser field, we have observed emissions from Ar_2^* at 126 nm for the first time together with some signals of high order harmonics. The focused laser intensity dependence of the Ar excimer intensity agrees well with that predicted by a tunnel ionization theory so called ADK theory. We, therefore, found out that the production of Ar excimers was initiated by high intensity laser produced electrons, which may become a substitution to those produced by an electron beam device.Experimental data show no difference in terms of the Ar excimer emission when linearly polarized and circularly polarized laser beams interacted with Ar. Considering about the energy transport of electrons in a high intensity laser produced plasma theoretically, we have found out a rapid conductive cooling of a plasma plays an important role for the excimer pioduction.The control of a molecule by using a high intensity laser has been demonstrated in the femtosecond time region as well. Alkali halide molecules have been excited and deexcited by using two different wavelength (ultraviolet and visible) lasers, which leads to the coherent control of the molecule with the minimum thermal effect This type of coherent molecular control means that the molecule can become a good optical amplifier even in a very short wavelength range, and can become a new coherent x-ray source.