We report soft x-ray spectra produced by the interaction of 25-100 fs laser pulses at an intensity of up to 7 by 1016 W/cm2 with cryogenically cooled Ar gas jet. New spectral lines from Ar8+, Ar9+ and Ar10+ charge states appeared with decreasing pre- expansion gas temperature. Nonlinear increase of X-ray line emission from Ar7+, Ar8+ and Ar9+ was observed with cooling, which saturated below certain temperature. The change in x-ray spectrum is attributed to efficient collisional heating and collisional ionization of growing small-to-medium sized Ar clusters from the cooled jet. When the laser pulse was extended to 100 fs we have observed considerably stronger emission on lines from high charge states, such as Ar8+, Ar9+ and Ar10+ which suggests that resonance absorption condition could be reached for the 100-fs pulse.
The cascaded second-order nonlinear optical process has recently received considerable attention because the effective third-order nonlinear susceptibility X(3)eff in this process becomes drastically enhanced in the materials of very large second-order nonlinear susceptibility, such as KTP and KNbO3. Since the process is off-resonant, the optical nonlinear response is very fast due to the spontaneous characteristics of optical parametric processes [1].
A real time interferometric autocorrelator was fabricated for characterizing the pulse width and the phase distribution of ultrashort laser pulses. A data acquisition system based on the direct memory access (DMA) was devised in order to monitor the real time autocorelation trace. The system is capable of measuring not only the intensity distribution but also the phase distribution of optical pulses. The output pulses of a femtosecond Ti:sapphire laser were analyzed utilizing the system. Applications were made to demonstrate the characterization of the nonlinear optical processes in KDP and (beta) - BBO crystals. In a (beta) -BBO crystal, the phase mismatch effect was successfully identified and measured in the second harmonic generation (SHG) processes.
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