GENERATION OF MULTIFREQUENCY/ULTRASHORT OPTICAL PULSES BASED ON TWO COLOR STIMULATED RAMAN EFFECT AND ITS APPLICATION TO ANALYTICAL CHEMISTRY

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In this study, we investigated multifrequency/ultrashort laser emission generated based on the two-color stimulated Raman effect that we originally found. In order to achieve this aim, high-order rotational Raman lines should be generated by using a transform-limited femtosecond laser pulse.We first confirmed that we could generate many rotational and vibrational lines by using a transform-limited excimer laser (248nm, 120fs). We also investigated the effects of laser polarization and hydrogen pressure in detail. On the other hand, more than 40 high-order pure rotational lines could be exclusively generated when we used a Ti : sapphire laser (800nm, 200-1000fs) as a pump beam. This result implies that a 0.4-fs laser pulse can, in theory, be generated. However, strong self phase modulation and self focusing occurred, when the pulse width was reduced to 200fs. Thus we had to increase the pulse width to 800fs to obtain the above result. In order to avoid this problem, we may have to use a seed beam and/or a low-temperature para-hydrogen to enhance the rotational lines in the future. We also discussed various effects affecting the generation of the rotational lines and tried to understand the nonlinear phenomena in the femtosecond regime systematically.In this study, we also develop a method for measurements of the pulse width of the ultrashort laser emission, which is based on self phase modulation. The decrease of the pulse width, hence the increase of the peak power, increased the spectral which and then this effect was concluded to be used for the measurements of the laser pulse width in the femtosecond regime.We also developed a multi-color laser for illumination works ; an etalon was inserted in the dye laser cavity and also a Nd : YAG laser was used as a pump beam for the generation of high-power multi-color laser emission for practical use.