Femtosecond Pulse Filamentation In Temperature Controlled Noble Gas

Intense few to monocycle pulses are the mJ or sub-mJ ultrashort pulses which have one to two optical periods (800 nm corresponding to 2.67 fs). They play more and more important roles today in many scientific and technological research fields, such as time-resolved measurements of electron dynamics in atoms and molecules, high-order harmonics and isolate attosecond pulse generation. To date, the major way of achieving intense few to monocycle pulses has been the compression of the spectra broadened through hollow fiber filled with noble gases or filamentation, or chirped pulse parametric amplifiers. All of which the filamentation is one of the simplest way to obtain a high energy few-cycle pulses.In the filamentation scheme, the competition of the multi-filament when the energy is high introduces instability filaments and restricts the generation of high quality ultrashort pulses. In this thesis, we propose and demonstrate a new technique to control the filamentation ? gradient temperature to broaden the pulse spectrum.When the pulse propagates in noble gas, the multi-filament is most possible to happen near the focus, for the peak power there is highest. In the place distant from the focus, the filamentation does not happen because the peak power is low. In our scheme, we heat the gas near the focus to make the gas density there lower, so that the nonlinear index becomes lower and the self-focusing threshold increases. In this way, the multi-filament can be suppressed. In the meantime, single-filament can happen at higher gas density. By doing this, we can make the nonlinear interaction length longer and expect a better spectrum-broadening than in the case of uniform temperature when pulse can break down.We designed and made a cylindrical furnace and its temperature controller by ourselves. We tested the performance of the heater and proved its temperature is linearly distributed along the tube. Then we conducted the experiment and verified that heating the tube near the focal point can indeed suppress the multi-filament. We studied the dependence of single filament on gas pressure, temperature, heating position and pulse energy. Finally we obtained the high-energy (pulse energy larger than 1mJ), broadened spectrum (>100 nm) and <10fs pulses if dechirped. Future work involves the dechirping the pulse and sending the pulse through the tube again for the second broadening. A shorter pulse in the order of less than two cycles should be expected to achieve.