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Journal of Tianjin Normal University

Propagation dynamics of double ultrashort laser pulses in a para-nitroaniline molecule medium

Zhu Yun

Published Jan 1, 2013• 0 citations

Abstract

By numerically solving the Maxwell-Bloch equations,the nonlinear propagation of two coherent ultrashort laser pulses in a para-nitroaniline molecule medium is investigated.The theoretical computations indicate that the molecule has only one charge-transfer state in lower energy region,and consequently the molecule can be regarded as a two-level sys-tem when studying the interaction of the molecule medium with laser pulses.When a 3π resonant pulse followed by a 4π nonresonant pulse enters into the molecule medium,the 3π pulse transfers its energy to the latter 4π pulse,and then the area of former pulse approaches 2π,only driving one complete Rabi flopping of population.The 4π pulse keeps a non-resonant propagation in the medium and only excites some incomplete Rabi floppings.The corresponding spectrum range broadens,and spectral components around the resonant frequency oscillate sharply.By introducing a pseudo-molecule without per-manent dipole moments as a comparison object,it is found that the propagation dynamics becomes more com-plex due to the permanent dipole moments of para-nitroaniline molecule.

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By numerically solving the Maxwell-Bloch equations,the nonlinear propagation of two coherent ultrashort laser pulses in a para-nitroaniline molecule medium is investigated.The theoretical computations indicate that the molecule has only one charge-transfer state in lower energy region,and consequently the molecule can be regarded as a two-level sys-tem when studying the interaction of the molecule medium with laser pulses.When a 3π resonant pulse followed by a 4π nonresonant pulse enters into the molecule medium,the 3π pulse transfers its energy to the latter 4π pulse,and then the area of former pulse approaches 2π,only driving one complete Rabi flopping of population.The 4π pulse keeps a non-resonant propagation in the medium and only excites some incomplete Rabi floppings.The corresponding spectrum range broadens,and spectral components around the resonant frequency oscillate sharply.By introducing a pseudo-molecule without per-manent dipole moments as a comparison object,it is found that the propagation dynamics becomes more com-plex due to the permanent dipole moments of para-nitroaniline molecule.

This paper investigates the effect of Lorentz local field correction (LFC) on the propagation of ultrashort laser pulses in a para—nitroaniline molecular medium under resonant and nonresonant conditions by solving numerically the full-wave Maxwell—Bloch equations beyond slowly-varying envelope approximation and rotating-wave approximation. The effect of the LFC is considerably obvious when pulses with large areas propagate in the dense molecular medium. In the case of resonance, the group velocity of the sub-pulses split from the incident pulse along propagation is severely decreased by the LFC, especially for the latest sub-pulse. However, in the case of nonresonance, the influence of the LFC on the temporal evolution of the pulse is less obvious and lacks homogeneity with an increase in incident pulse area, propagation distance and molecular density.

We have investigated the resonant propagation of femtosecond laser pulse in 4-trans-[p-(N, N-Di-n-butylamino)-p-stilbenyl vinyl] pyridine medium with permanent dipole moments. The electronic structures and parameters for the compound have been calculated by using density functional theory. In the optical regime, there is one charge-transfer state and the molecule can thus be simplified as a two-level system. Both the one- and two-photon transitions occur between the ground and charge-transfer states. The numerical results show that the permanent dipole moments have an obvious effect on the propagation of the ultrashort pulse laser. The ideal self-induced transparency disappears for 2π pulse and second harmonic spectral components occur significantly due to the two-photon absorption process. For the 6π pulse, continuum frequency generation is produced and a shorter duration pulse in time domain with 465 as is obtained.

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Propagation and amplification of an ultrashort laser pulse in a two-level medium is numerically investigated with the use of the finite-difference time-domain (FDTD) procedure simultaneously solving Maxwell and Schrodinger equations with no assumptions characteristic of slowly varying envelope approximation. Comparison of the results of FDTD numerical simulations with the predictions of the pulse area theorem has demonstrated that the developed numerical procedure provides an adequate description of pulse evolution in a two-level medium. Amplification of ultrashort light pulses in a two-level medium is analyzed. Two methods of improving the gain of ultrashort pulses in a two-level medium by modulating the spatial distribution of dipole moments of resonant transitions and using initially chirped frequency-detuned pulses are explored.

Using the finite-difference time domain technique(FDTD) for the fields and the predictor-corrector method for the material variables,we solve the Maxwell-Bloch equations beyond the slowly-varying envelope approximation and the rotating-wave approximation.4,4′-bis(dimethylamino) stilbene organic molecule is one type of one-dimensional symmetrical system and its electronic structure and parameters are calculated by density functional theory.The molecule can be simplified as a three-level system.By numerically simulating the propagation of two-color 2π ultrashort laser pulses in a cascade three-level organic molecule system,we analyze the temporal-spatial evolution properties of the ultrashort laser pulses and the population of each energy level under different resonant conditions.