By applying intense 30-fs Ti:sapphire laser pulses to Ar gas well above the saturation intensity for optical-field ionization, a strong blueshift of high harmonics 2 times as large as the laser frequency was generated. A semiclassical calculation showed that the observed large blueshift resulted from a rapidly increasing electric field, existing much earlier in time than the peak of the laser pulse, i.e., a nonadiabatic effect.
A strong blueshift of high-order harmonics was observed when intense femtosecond Ti:sapphire laser pulses were applied to Ar and Ne. The blueshift was systematically investigated for various experimental parameters, such as laser intensity, laser-pulse duration, and gas density. We observed the harmonic blueshifts of two times the laser frequency in the 39th harmonic from Ar and four times the laser frequency in the 85th harmonic from Ne. A semiclassical calculation predicted the inverse proportionality of the blueshift to the laser-pulse duration, which was confirmed in the experiment done with 30 fs and 80 fs pulses. The density dependence of blueshift showed that the self-phase modulation in an ionizing medium was not a major factor of the observed blueshift. The experimental results along with theoretical calculations confirmed that the observed blueshift resulted mainly from the nonadiabatic effect due to the rapid increase in the femtosecond laser electric field.
We have developed a laser isotope separation technology for the production of the 168 Yb and 176 Yb isotopes. 168 Yb is very useful for the generation of a non destructive testing source, 169 Yb. 176 Yb can be used to produce 177 Lu which is known to be a promising radioisotope for a medical application. For these applications, the abundances of 168 Yb and 176 Yb isotopes should be enriched to more than 15% and 97%, respectively. Our developed system consists of three dye lasers pumped by a diode-pumped solid-state laser, a Yb evaporator, and a photo-ion extractor. Up to now, we could enrich 168 Yb to more than 31% with a productivity of 0.5 mg/h. Also, we succeeded in enriching 176 Yb to more than 97% with a productivity of 27 mg/h.
By applying an intense 28-fs laser pulse to a helium gas jet, spectrally resolved high-order harmonics originating from different quantum paths were obtained. The selection of high-order harmonics from a single quantum path could be achieved by controlling the phase-matching condition. Comparison of the observed harmonic spectrum with a calculation indicated that an attosecond pulse train with a single attosecond pulse per half-optical cycle was produced.
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