Theorectical And Experimental Study On Femtosecond Pulse Propagation 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.Due to the gas breakdown and self-focusing, the energy is limited toμJ level in hollow fibers. Recently, the using of gradient temperature has been proposed for increasing the input energy. In spite of the advantage of this method, it has its intrinsic limitations and disadvantages such as complex system, large consumption of noble gases and instability introduced by the gas flow. In the filamentation scheme, the competition of the multi-filament when the energy is high introduces instability between the filaments and restricts the achievement of high quality ultrashort pulses.In this dissertation, we proposed a novel technique to achieve intense few-cycle pulses which is to compress the broadened spectra of femtosecond pulse through the hollow fiber filled with noble gas in gradient temperature. It opens a novel and feasible way for intense few-cycle pulse generation.The optical characteristics of gases such as refractive index and nonlinear coefficient are functions of temperature. We can control the optical characteristics of gases and the evolution of femtosecond pulses in time and frequency domain by controlling the temperature. The optical phenomena including self-focusing and filamentaion can also be controlled by temperature. In the gradient temperature or pressure scheme, the input end has the lower nonlinear refractive index n2 which allows more energy to be coupled into the fiber or tube due to the critical self-focusing power is inversely proportional to n2. During the propagation of the pulses, the broadening of spectrum will continue to the end because the increasing n2 provided by the increasing pressure or decreasing temperature, which compensates for the decreasing peak power due to dispersion and loss. It is obvious that increasing the temperature can enhance the critical power of self-focusing and restrain or retard the multi-filaments, even the single filament formation. In this dissertation, we studied the phenomena and essences of femtosecond pulses propagation through hollow fiber filled with gradient temperature controlled noble gas both theoretically and experimentally. The discussions implied a few guidelines for achieving intense few-cycle pulses.In the theoretical point, we proposed the concept of using gradient temperature to control filaments such as restraining multi-filament and controlling filament. By this method, we can broaden the spectrum of ultrafast pulses and prevent from the multi-filament formation to achieve high-quality and high-energy ultrashort pulses while avoiding the disadvantages of gradient pressure. We built a model to demonstrate the advantages of this scheme comparing with the gradient pressure. The concept of ideal gradient temperature curve had also been presented.The experiments were performed for various input pulse, gas pressure and temperature conditions and demonstrated that the gradient temperature can control the filament, including suppression of the multi-filament formation, while keep the spectrum broadening. Experiments verified that the gradient temperature is a new effective way to achieve intense few-cycle pulses.