Abstract

A strong pump-power dependence of the four-wave mixing (FWM) signal for an aqueous solution of Malachite green is reported. The characteristics of the pump-power dependence of the nonlinear signal are reproduced by a theoretical model based on the coupling between pump-probe, considering signal fields and propagation effects. The effect of the intramolecular coupling on the nonlinear intensity of the FWM signal is studied using a model molecule consisting of two-coupled harmonic curves of electronic energies with minima displaced in energy and nuclear positions. Two-vibrational states are considered while including non-adiabatic effects for the two-state model. Moreover, the coupling among the field components, as well as the propagation effects, are studied by considering a constant pump-intensity. Our calculation scheme, considering both the intramolecular coupling effects in the description of the molecular structure and the effects produced by the propagation of the FWM signal along the optical length, allows the exponential dependence of the latter, as the intensity of the pumping beam increases. Our treatments do not require the inclusion of other non-resonant processes outside the RWA approximation, due to the consideration of an adiabatic basis.

Highlights

  • Multiphoton processes of intense electromagnetic fields interacting with molecular systems are phenomena of interest with a great variety of applications [1–6] in non-linear optics

  • Our model considers a marked influence of the magnitudes of both the permanent and transition dipole moments in the four-wave mixing (FWM) signal response

  • The calculation was performed at the center of the nd-FWM spectrum, considering an inhomogeneous Gaussian distribution of linewidth 800 cm−1 half width at half maximum (HWHM), with T1 = 2.5 × 10−12 s and T2 = 0.5 × 10−12 s [49]

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Summary

Introduction

Multiphoton processes of intense electromagnetic fields interacting with molecular systems are phenomena of interest with a great variety of applications [1–6] in non-linear optics. Our analysis in the present paper aims to examine the propagation of the nondegenerate four-wave mixing signal along a defined optical length, which is subject to the saturative effects of the intense pump field and the details of the molecular structure in terms of vibronic coupling.

Models
Molecular Model of a Two-Level System with Intramolecular Coupling
Radiation-Matter Interaction Model
Nonlinear Induced Polarization and Propagation Effects
Results and Discussions
Conclusions
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