Frequencies-Selected Enhancement Of The High-Order Harmonic From An Initial Atom With Superposition States

The interaction of intense laser pulses with atoms, molecules,cluster, and solids can lead to high-order harmonics generation(HHG) of the laser frequency, as a consequence of highly nonlineardynamics. A general characteristic of atomic HHG spectra has beendemonstrated by a variety of experiments: a sharp decline of theintensity for the first few harmonics, followed by a plateauconsisting of many harmonics with the roughly same intensity andthen an abrupt cutoff at I p + 3.2Up ( I p is the ionization potentialof the atom and U p is the ponderomotive energy). The plateaustructure of HHG power spectra makes HHG a promising way togenerate coherent light in the extreme ultraviolet and x-ray regionand to produce attosecond x-ray pulses. Due to the applicationprospects of HHG, researchers are interested in HHG since it is firstobserved in laboratory at 1987. In order to realize these applicationsthe enhancement of the power of the harmonic plateau becomesurgent. In this thesis, we use a Ti:sapphire laser to irradiate on anatom initially prepared on a superposition of the ground state and anexcited state equally, and higher powers for certain harmonics areobtained than powers of the atom prepared on the ground state.If an atom initially prepared on the ground state is irradiated byan intense laser pulse, we can define an optimal laser intensityunder which the highest power of harmonic emission in certainregion of the plateau can be obtained. When a laser with optimalintensity interacts with an atom initially prepared in an equalsuperposition of the ground and the 10th-excited, it is found that thepowers in several regions are heightened in excess of one or twoorders. By using the wavelet transformation and the evolvement ofthe wavepacket of ionization driven by the laser pulse, weillustrated the mechanism of the enhancement of the power insuperposition-state case.Due to the smaller energy of ionization, the electron on the10th-excited ionizes more rapidly than the electron on the groundstate, so the wavepacket of ionization from the 10th-excited statehas high peaks somewhere while the wavepacket of ionization fromthe ground state has no high peaks owing to its long duration ofionization.It is known that the 10th-excited state covers a much largerregion than the ground state, so according to the uncertaintyprinciple, its wavepacket of momentum covers a smaller region thanthe ground-state case. Thanks to the fast ionization of the10th-excited state, the wavepacket of ionization from it inherits itslimited region of momentum. As a result, it disperses much moreslowly than the wavepacket of ionization from the ground state.Because of the above-mentioned reasons, the wavepacket ofionization from the 10-excited state near the core is higher than thewavepacket of ionization from the ground state in excess of oneorder when harmonics are generated. It is known that the higher theprobability density near the core, the higher conversion efficiencyobtained. So the peaks of the superposition-state case havepredominant conversion efficiency compared with the wavepacketof ionization from the ground state when they are both passing thecore.Frequencies-selected enhancement can be achieved by usingsuperposition states of different excited states. Excited states of theatom have a remarkable characteristic that the peak probabilitydensities are far away from the core and differ from each other. So ifthe excited state alters, the peaks position of the ionizationwavepacket will change. These peaks will get different kineticenergy when they pass the core. As a result, photons of differentorders will be emitted (we pay our most attention to the peakprobability densities of the wavepacket of ionization considering itshigh conversion efficiency when passing the core).By preparing the initial state on the superposition of the groundstate and the 6th-excited state equally, we get HHG spectra that thepowers are promoted for different harmonics. However, the powersof the harmonics near the cutoff ( I p + 2.9Up) can't be heightened inboth cases because the electron ionizes rapidly in the first cycle, Solarge kinetic energy of recombination can't be gained consideringthe SMT. So ionization must be put off to get better powers forhigher harmonics in the future.