We study here the high-order harmonic generation (HHG) from the hydrogen molecular ion H2+ aligned along the axis of a golden nanotip. The whole system is shined by a chirped laser pulse with its wavelength centered at 800nm. Due to the existence of the metallic nanotip, the light field experienced by the molecular ion is strongly enhanced owing to the generation of the local surface plasmon field. It is found that the cutoff position and the conversion efficiency of HHG depend strongly on the distance between the nanotip and the molecular ion, as well as on the chirp parameter of the input infrared laser field. The plateau of the HHG is largely extended with negative chirps, and the supercontinuum spectra that can support a single attosecond pulse are generated by the suppression of the long electron trajectory with an optimized field chirp parameter.
Propagation of dichromatic femtosecond laser pulses tuned respectively in single-photon resonances with the cascade three-level system is studied. The initial areas of the two pulses are both equal to 2π, which makes them respectively an optical soliton in ideal two-level systems. When the dichromatic solitons are synchronized into the medium, both the temporal shapes and the spectral distributions of the pulses are strongly distorted during propagation. The delay time between the dichromatic solitons plays an important role on the evolution of the fields.
Propagation of strong femtosecond hyper-Gaussian pulses in an organic molecular medium is studied by solving numerically the Maxwell-Bloch equations using an iterative predictor-corrector finite-difference time-domain technique. The carrier-wave frequency of the field is tuned in two-photon resonance with the molecular system simplified by a cascade three-level model. Strong two-photon absorption induced optical power limiting behavior is observed for the hyper-Gaussian pulses of different orders. For the ultrashort hyper-Gaussian pulses, it is found that the ”two-photon area” is no further the deciding quantity of the two-photon induced dynamics, and pulses with different temporal profiles will induce different two-photon absorption dynamics and optical limiting processes. With the same pulse duration and field amplitude, pulses of a lower order is found to have a larger input ”two-photon area” but a smaller output area, and therefore show a better optical limiting behavior. The population distribution and the generation of new fields during pulse propagation also depend on the shape of the incident field.
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