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Chinese Physics

Resonant propagation of femtosecond laser pulse in DBASVP molecule: one-dimensional asymmetric organic molecule

Ke Zhao, Ji‐Cai Liu, Chuan‐Kui Wang, Yi Luo

Published Sep 23, 2005• 3 citations
Publisher

Abstract

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.

DOI
10.1088/1009-1963/14/10/016

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Physical Review A•Jul 1, 1997•19 citations
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.
In this paper the phase-dependent features of ultrashort laser pulse resonant propagation in a two-level dipolar molecule are demonstrated by solving full Maxwell–Bloch equations. The electronic properties of dipolar molecule 4-trans-[p-(N, N-Di-n-butylamino)-p -stilbenylvinyl] pyridine (DBASVP) molecule, one-dimensional asymmetric organic molecule, is calculated by density functional theory at ab initio level. The numerical results show that the carrier propagation and the spectrum evolution of the pulse are sensitive to its initial phase and the phase sensitivity is more obvious for larger area pulse. The phase-dependent feature is more evident in dipolar molecule because the permanent dipole moment makes the nonlinear effects stronger.
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.
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.
Using a semiclassical, effective-state approach to laser pulse propagation in a dilute gas of polyatomic molecules, a numerical model of multiphoton excitation which includes quasi- continuum transitions is used to calculate the effects of coherent propagation on multi-photon absorption. The initial laser field is assumed to be a quasimonochromatic Gaussian envelope which is nearly resonant with a portion of the lower vibrational states of the molecule. The molecular model consists of a single ground state and several bands of discrete states which are successively connected via electric dipole transitions. This model correctly mimics some of the major characteristics of multiphoton absorption, such as power threshold, fluence dependent absorption, and complete population inversion at high fluences. The model also allows for the calculation of the polarization induced in the medium by the electric field, and the subsequent influence that the polarization has on the field. As the pulse propagates, the Rabi oscillations induced in the lower level states become encoded onto the pulse, creating a series of peaks and troughs similar to the behavior of pulse propagation in two-level systems. Another coherent propagation effect which can change the excitation process is coherent sideband generation. The temporal distortions and the generation of new frequencies results in changes in the population distribution among the states, as well as changes in the total population transferred to the quasi-continuum. The pulse reshaping occurs while the total fluence of the pulse is decreasing due to energy loss to the quasi-continuum. The effective increase in total population transfer due to propagation effects is offset by the decrease in fluence. The resulting transfer of energy from the pulse to the molecule depends strongly on the initial characteristic of the pulse. These processes play an important role in any system where the efficient transfer of laser energy to the medium is required, such as laser-induced chemistry and laser isotope separation.
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.
Two-photon resonant and near-resonant four-wave difference-frequency mixing in gases in the interaction regime when laser pulse durations are comparable to or shorter than the medium polarization relaxation time ${T}_{2}^{\ensuremath{'}}$ is investigated. The results of experimental studies of the process in Ar and Kr using pump pulses from the ArF-excimer laser are presented demonstrating a generation of tunable short pulse radiation in the range 102--124 nm. The results are discussed in terms of a theoretical model based on a self-consistent solution of the Bloch equations for the atomic transitions and the Maxwell equations for the fields. This enables one to interpret specific nonstationary resonant and quasiresonant phenomena involved in the frequency conversion process. It is shown that the femtosecond-pulse four-wave frequency-mixing technique with probe pulses significantly shorter than the pump pulses makes it possible to study the coherent dynamics of an atomic transition exposed to an intense field. Using atomic Kr as the nonlinear medium, coherent Rabi oscillations and the subsequent phase relaxation of excitation were observed under the condition of two-photon interaction of Kr with femtosecond 193-nm laser pulses. The obtained information is important for controlling and optimizing processes of two-photon resonant frequency conversion and for time-resolved studies of Rydberg states in atoms and molecules.