Sub-Femtosecond Pulse Generation And Its Control

Observation and control of ultrafast events, such as chemical reactions, biological interactions, and electronic processes, is the core of ultrafast physics and enables us an insight of the underlying physics. Over the last two decays, femtosecond technologies have renovated the fields of chemistry, biology, physics, and optical communications. Furthermore, the generation of sub-femtosecond pulses will extend the horizon of ultrafast measurements to the time scale of electronic motions and allow us a stirring new view on the nature.Breaking the femtosecond barrier needs new approaches. Generation of sub-femtosecond pulses in the soft x-ray region of the spectrum has been demonstrated by using high harmonic generation. In the optical region of spectrum, we can produce sub-femtosecond pulses based on Raman approach through molecular modulation with near unit conversion efficiency. On the other hand, sub-femtosecond pulses can be obtained by directly Fourier synthesizing the coherent optical fields in a wide spectral range.This dissertation documents our main results on generation and control of sub-femtosecond pulses based on the approaches mentioned above.Firstly, based on the molecular modulation, the following studies have been carried out. For the first time, we show that the pulse spacing of sub-femtosecond pulse trains generated by molecular modulation can be increased in a controllable way, rather than a constant restrained by the oscillating frequency of the molecules. We also demonstrated that sub-femtosecond pulse train with a large pulse spacing can be generated by using three correlated Raman generators between three Raman levels with strong molecular modulation to control the generation and propagation of the cascade stimulated Raman sidebands. Using the modulated molecular modulation, we show that an oscillatory Raman coherence with periodically changed phase is established. It leads to periodic compression of the generated broadband sidebands in the same medium. Compared to the standard scheme using non-modulated Raman excitations where sub-femtosecond pulses can be generated only with negative Raman detunings, the modulated molecular modulation scheme supports sub-femtosecond pulse generation with positive, negative, or even opposite Raman detunings around the Raman resonance. Additionally, we further extended the ideal of modulated molecular modulation to control the modulation features of the Raman sidebands. We demonstrated that the modulation features can be transferred from a modulated laser field to a non-modulated one with different wavelengths and/or intensities. This provides an efficient way to overcome the intrinsic limitations of commercial modulators, including the limited transparent ranges and damage thresholds. In order to obtain pump fields for molecular modulation with narrow line-width, we carried out a study of cavity stabilization by means of dither-locked method based on phase sensitive detection. On the other hand, we show that the pulses form seeded amplification of colored conical emission and 2D multicolored transverse arrays can be used as pump for impulsive molecular modulation and near-resonant Raman scattering to support sub-femtosecond pulse generation.Secondly, based on the multicolored coherent laser fields resulted from cascaded non-collinearly frequency mixing processes, we demonstrated that it was possible to produce sub-femtosecond pulses by directly Fourier synthesizing the coherent optical fields in a wide spectral range.Thirdly, concentrated on high harmonic generation, the following studies have been performed. With the rapid development of cavity enhanced femtosecond laser technique, it is possible now to investigate high harmonic generation in an oscillator based cavity. We demonstrated that non-collinear high harmonic generation scheme with a small cross angle is a good idea to couple the harmonics out of the cavity without any additional influence. A Ti:Sapphire femtosecond oscillator with pulse width down to 10-fs was built. It can be used as the driving field for high harmonic generation. In order to get a better understanding on the underlying physics of high harmonic generation, we carried out a systematic investigation on the ionization and dissociation dynamics of atoms and molecules in ultrashort laser pulses. A considerable amount of new phenomena and physics on the ionization behavior of atoms and molecules in a high non-perturbative strong field regime were explored. This allows us a better way to understand and control the generation of high harmonics and sub-femtosecond pulses. It will also stimulate further investigations along this direction.