| High-order harmonics origin from high-order nonlinear effects of the interaction between light and matter.The photons generated by high-order harmonics are in the extreme ultraviolet range with high coherence,thus are widely used in the fields of atomic and molecular,chemical and laser spectroscopy.On the other hand,the physical process of high harmonic radiation is on the attosecond time scale,which is an ideal method for isolated attosecond pulse(IAP)generation.The pulse width of an IAP is as short as tens of attoseconds,which is comparable to the characteristic time of electron motion.Attosecond pulses based on high-order harmonics have ultra-high temporal resolution,providing the possibility to detect the ionization,transitions of electrons,and various ultrafast electron dynamics.Limited by the physical mechanism of high-order harmonic generation,the energy of high-order harmonics and attosecond pulses based on gas or solid is still limited,and it is difficult to induce nonlinear effects.How to obtain higher-energy high-order harmonics and attosecond pulses is an urgent issue.Based on these requirements,this thesis mainly studies the generation and optimization of high-order harmonics and attosecond pulses from theoretical and experimental aspects.The main contents and innovative results of this thesis are as follows:1.The optimized conditions for the generation of IAP is theoretically explored.The intensity of IAP was simulated in argon at different gas pressures and gas medium lengths.The simulation result shows that the IAP with highest intensity can be obtained by using a 6 mm length of gas medium with the gas pressure of 30 Torr under the laser parameters we used.Compared to the intensity of attosecond pulse generated by 1 mm gas medium,the maximum intensity of attosecond pulse is doubled.The simulation result shows that by increasing the length of the gas medium and optimizing the gas pressure,the intensity of IAP can be greatly enhanced,which provides a reference for the experiment.2.We design and build the high-energy high-order harmonic and attosecond pulse generation beamline.The driving laser is a home-made Ti:Sapphire femtosecond laser with an output energy of 20 m J,with the repetition rate of 1 k Hz,and the center wavelength of 800 nm.A loose focusing structure is used for the high harmonics generation in gas.The entire beamline includes the high harmonic generation,attosecond pulse width measurement and spectral measurement.Using this beamline,high-order harmonic generation was obtained in argon and neon,and the spectral range was measured and calibrated,down to the shortest wavelength of 12 nm.3.The intensity and divergence angle of high harmonics obtained in argon was systematically measured and optimized.Under the optimized gas pressure with the gas target of 2-30 mm,the divergence and intensity of the high-order harmonics were measured at different gas cell position relative to the laser focus.Under our laser parameters,the strongest high-order harmonic output per unit area can be obtained using a gas target with a length of 10 mm.The short gas cell can be used for the generation of harmonics with large divergence and high intensity.With the long gas cell,high harmonics with relatively low intensity and divergence of less than 1 mrad can be generated.Using a gas target with the length of 10 mm to 15 mm,the harmonic output with small divergence angle and strong intensity can be obtained in a wide spectral range,which provides a reference for the optimization and application of higher harmonics used in time-resolved ARPES.4.Based on the high harmonic spectrum obtained in argon,the time-domain characteristics of the attosecond pulse train were investigated.An attosecond pulse interference model is theoretically constructed,and the spectral distributions of short trajectory and long trajectory are obtained using this model,which are in good agreement with the results obtained by the traditional strong field approximation method.Based on this theory,the temporal envelope of the attosecond pulse corresponding to the high-order harmonic long orbit is inverted.The inversion result shows that with the increase of the driving laser intensity,the distribution of the attosecond pulse train moves from the back edge of the driving laser to the leading edge,and its envelope width first increases and then decreases.This scheme provides the possibility for the rapid measurement of the time-domain characteristics of attosecond pulses and the controlling of attosecond pulse generation.5.Using the scheme of two-color laser field,800 nm and 400 nm lasers were used together as driving lasers to generate high-order harmonics with photon energies of 9-18 e V in the silicon dioxide crystal.Compared to the harmonics generated by only the monochromatic field of 800 nm,the intensity of harmonics is enhanced by 2-3 orders of magnitude. |
