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Low Temperature Physics

Effect of the bichromatic electromagnetic field on the random molecular aggregate or the quantum spin chain

Andrei A. Zvyagin, Iryna Yu. Ropakova

Published Jul 1, 2020• 5 citations
Publisher

Abstract

The linear response of the molecular aggregate chain with the random distribution of the strength of the neighboring intermolecular interactions to the bichromatic electromagnetic field has been calculated. The presence of the linearly polarized field produces additional resonance absorption at the combined frequencies of the circularly polarized and linear polarized field. The strength of the absorption oscillates with the amplitude and the frequency of the linearly polarized electromagnetic field. Additional resonance absorption is weaker than the main absorption at the frequency of the circularly polarized field. Similar behavior has been predicted for the interaction of the bichromatic magnetic field with the quantum spin-1/2 chain with the random distribution of the exchange constants for nearest neighbor spins.

DOI
10.1063/10.0001363

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The linear response of the molecular aggregate chain with the random distribution of the strength of the neighboring intermolecular interactions to the bichromatic electromagnetic field has been calculated. The presence of the linearly polarized field produces additional resonance absorption at the combined frequencies of the circularly polarized and linear polarized field. The strength of the absorption oscillates with the amplitude and the frequency of the linearly polarized electromagnetic field. Additional resonance absorption is weaker than the main absorption at the frequency of the circularly polarized field. Similar behavior has been predicted for the interaction of the bichromatic magnetic field with the quantum spin-1/2 chain with the random distribution of the exchange constants for nearest neighbor spins.
The linear response of the quantum spin chain with the impurity to the bi-chromatic ac magnetic field has been calculated. The linearly polarized component of the ac field produces additional resonance absorption with respect to the one caused by the circularly polarized field. The strength of the absorption oscillates with the amplitude and the frequency of the linearly polarized magnetic field. Additional resonance absorption is weaker than the main absorption at the frequency of the circularly polarized field. The absorption due to the impurity can be seen in the absorptions at local levels and the delocalized states.
The resonance absorption and fluorescence of a bi-chromatic electromagnetic field by a single molecule, coupled to a one-dimensional quantum molecular aggregate is studied. Depending on the values of the coupling of the single molecule and its own frequency, with respect to the own frequency of the aggregated molecules, and the strength of the aggregation, the absorption and the fluorescence of the molecule show the contribution from the exciton band, or/and the ones from the local level(s) caused by the single molecule. The linear polarized component of the electromagnetic field produces additional resonances, compared to the only circular polarized component, due to the coherent interference. The strength of the absorption and the fluorescence, caused by those additional resonances, depends on the ratio of the magnitude and the frequency of the linear polarized component. For additional resonances the magnitude of the absorption and the frequency is the decaying oscillating function of the mentioned ratio. Copyright (C) EPLA, 2017
The performance of approximate theories for the collective nonlinear optical response of assemblies of interacting molecules is investigated by comparing their predictions to exact calculations. To this end, the nonlinear absorption coefficient of linear molecular aggregates with energetic disorder is studied with the aid of Monte Carlo simulations. It is shown that the well-known local-field approximation (LFA) cannot describe the near-resonance nonlinear absorption spectrum for disorder values up to the nearest-neighbor intermolecular interactions; surprisingly, it is found that the validity of the LFA does not at all improve with growing disorder. By contrast, the ``excitonic two-level system'' approximation, in which each collective one-photon transition (Frenkel exciton) of the assembly is treated as an independent two-level system, turns out to describe the collective nonlinear response of the aggregates in a much better way and, moreover, rapidly improves for growing disorder values. It is shown that both models correctly describe the linear optical response and that, within the framework of the LFA, a close connection exists between the rotating-wave approximation and the Heitler-London approximation.
The effect of the circularly polarized laser field on quantum spin systems in the steady-state regime, in which relaxation plays the central role, has been studied. The dynamical mean-field-like theory predicts several general results for the behavior of the time-average magnetization caused by the laser field. The induced magnetization oscillates with the frequency of the laser field (while Rabi-like oscillations, which modulate the latter in the dynamical regime, are damped by the relaxation in the steady-state regime). At high frequencies, that magnetization is determined by the value to which the relaxation process is directed. At low frequencies the slope of that magnetization as a function of the frequency is determined by the strength of the laser field. The anisotropy determines the resonance behavior of the time-averaged magnetization in both the ferromagnetic and antiferromagnetic cases with nonzero magnetic anisotropy. Nonlinear effects (in the magnitude of the laser field) have been considered. The effect of the laser field on quantum spin systems is maximal in resonance, where the time-averaged magnetization, caused by the laser field, is changed essentially. Out of resonance the changes in the magnetization are relatively small. The resonance effect is caused by the nonzero magnetic anisotropy. The resonance frequency is small (proportional to the anisotropy value) for spin systems with ferromagnetic interactions and enhanced by exchange interactions in the spin systems with antiferromagnetic couplings. We show that it is worthwhile to study the laser-field-induced magnetization of quantum spin systems caused by the high-frequency laser field in the steady-state regime in ``easy-axis'' antiferromagnetic spin systems (e.g., in Ising-like antiferromagnetic spin-chain materials). The effects of the Dzyaloshinskii-Moriya interaction and the spin-frustration couplings (in the case of the zigzag spin chain) have been analyzed.
The dynamical mean-field-like theory of the electron spin resonance (ESR) in spin-1/2 chains with nearest and next-nearest neighbor interactions (or zigzag spin ladders) and alternation is developed. It is explained that either the magnetic anisotropy of spin-spin couplings or the Dzyaloshinskii-Moriya interaction causes shifts of the resonance position in spin chains, comparing to only magnetically isotropic interactions. The power of the ac ESR field, absorbed by the spin chain, is calculated. It is shown that the temperature dependence of the shift of the resonance position is related to the temperature dependence of the ESR absorption. Depending on the strength of next-nearest neighbor interactions, the shift of the ESR position can become either exponentially small or can be increased near quantum critical points. The absorbed by the alternating spin chain power of the ac magnetic field is calculated, manifesting two resonances. The influence of mentioned spin-spin interactions on the linewidth of the ESR is analyzed.
The elimination of the absorption of a weak electromagnetic wave in a three-level atomic system in the presence of a strong resonant electromagnetic wave is well known in optics. This effect is termed electromagnetically induced transparency. We have theoretically investigated this effect in a crystal of molecular magnets. The crystal is subject to two (weak and strong) resonant magnetic fields. We discuss the choice of polarization of the magnetic fields. The influence of the distribution of the longitudinal anisotropy constant and the Land\'e factor on the manifestation of electromagnetically induced transparency has been studied. The amplitude of the strong field required for observation of the effect has been estimated.