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Journal of Physics B Atomic Molecular and Optical Physics

Dipolar effects on propagation of ultrashort laser pulse in one-dimensional para-nitroaniline (pNA) molecules

Ke Zhao, Hong-Yu Li, Ji‐Cai Liu, Chuan‐Kui Wang, Yi Luo

Published Nov 9, 2005• 5 citations
Publisher

Abstract

The dynamic behaviour of ultrashort (femtosecond) laser pulses in a molecular medium is studied by solving the full Maxwell-Bloch equations beyond the limits of the slowly varying envelope approxim ...
DOI
10.1088/0953-4075/38/23/006

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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.
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.
The Maxwell–Schrödinger equations describing the ultrashort laser pulse propagation in multilevel molecular media are shown to be analytically solvable and to reduce to nonlinear multisine-Gordon equations for certain models of the field–molecule system. Special cases of level schemes are shown to lead to soliton propagation. A numerical study is presented in order to examine the effect of nonresonant (virtual) transitions on pulse propagation and amplification in a three-level molecular system as a function of intensity and detuning.
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.
Recent progress in ultrafast technology has led to the generation of optical pulses as short as 8 fs. The propagation of such short pulses in nonlinear optical media requires new computational techniques in which the standard approximations such as the slowly-varying envelope approximation (SVEA) and rotating-wave approximation (RWA) are not made while using the Maxwell-Bloch equations [1]. Previous calculations, which used the finite-difference time-domain (FDTD) method, found that for a a Kerr-like instantaneous nonlinearity [2] significant differences can occur when the full vector Maxwell equations are used instead of the nonlinear Schrodinger equation. Recently, the FDTD method has been extended to the case of a homogeneously broadened two-level atomic system [3].