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Nonresonant interaction of ultrashort electromagnetic pulses with multilevel quantum systems

É. M. Belenov, Isakov Va, A. Nazarkin

Published May 1, 1994• 0 citations
OpenAlex

Abstract

Some features of the excitation of multilevel quantum systems under the action of electromagnetic pulses which are shorter than the inverse frequency of interlevel transitions are considered. It is shown that the interaction is characterized by a specific type of selectivity which is not connected with the resonant absorption of radiation. The simplest three-level model displays the inverse population of upper levels. The effect of an ultrashort laser pulse on a multilevel molecule was regarded as an instant reception of the oscillation velocity by the oscillator and this approach showed an effective excitation and dissociation of the molecule. The estimations testify to the fact that these effects can be observed using modern femtosecond lasers.

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Some features of the excitation of multilevel quantum systems under the action of electromagnetic pulses which are shorter than the inverse frequency of interlevel transitions are considered. It is shown that the interaction is characterized by a specific type of selectivity which is not connected with the resonant absorption of radiation. The simplest three-level model displays the inverse population of upper levels. The effect of an ultrashort laser pulse on a multilevel molecule was regarded as an instant reception of the oscillation velocity by the oscillator and this approach showed an effective excitation and dissociation of the molecule. The estimations testify to the fact that these effects can be observed using modern femtosecond lasers.
A theory is derived for the excitation of a multilevel quantum system by an electromagnetic pulse whose length is much shorter than the reciprocal of the frequency of transitions between levels. This interaction is characterized by a selectivity of a new type, which does not involve resonant properties of the substance. It instead stems from selection rules for transitions between levels and from the vanishing of the "area" defined by the integral of the electromagnetic field intensity. This selectivity mechanism could be exploited to create a deviation from equilibrium in a multilevel quantum system. In particular, there is the possibility of a nonresonant excitation and dissociation of molecules by femtosecond electromagnetic pulses.
We investigate the interaction of a multilevel atom with a short femtosecond optical pulse beyond the scope of the rotating-wave approximation. We consider the behavior of a three-level atom under the action of two optical pulses resonant with two allowed transitions and a two-level atom under the action of a resonant optical pulse. It is shown that the imaginary part of the off-diagonal elements of the density matrix oscillates with high frequency due to the counter-rotating terms. It is shown also that an effective time-dependent addition to the Rabi frequency exists, which is of importance for ultrashort optical pulses.
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.
A review is given of recent theoretical investigations on coherent propagation and nonlinear interaction of multifrequency ultrashort radiation pulses in resonantly absorbing media. An analysis is made of frequency conversion by parametric interaction and stimulated Raman scattering of laser pulses in resonant three-level media. New ways of controlling the parameters of coherent pulses and the use of multifrequency pulses in the spectroscopy of multilevel media are discussed. The experimental capabilities of the investigated effects are considered.
Excitation of multilevel Rydberg states (atoms and diatomic molecules) is studied under an intense time-periodic perturbation that is instantaneously turned on and off. For a one-quantum excitation from a lower lying state, general expressions for the Laplace images of the population amplitudes of Rydberg states are obtained with regard to their decay characteristics. It is shown that the problem considered is reduced to the determination of the positions and widths of the energy levels of a quantum system in the field of monochromatic laser radiation of the same frequency and intensity as that in a pulse mode. To determine these quantities, an integral formulation is proposed for the eigenvalue problem for energies that is relatively simply solved with regard to the effect of the ionic core and its complex vibration-rotation structure on diatomic molecules. The specific features of the excitation of Rydberg states and the behavior of Rydberg wave packets are studied depending on the intensity and duration of laser radiation. A quantum phenomenon of rotational orientation of electron-excited diatomic molecules is considered.
Here, I discuss the interaction of two matched ultrashort pulses with a \ensuremath{\Lambda}-like system in which the upper intermediate state is replaced by a manifold of discrete states. Many of these states are simultaneously accessed by the broad bandwidth of the two excitation pulses. The equations of motion for the probability amplitudes of the levels involved in the excitation process are solved analytically in the quasi-impulsive regime. In this regime, the excitation pulses are made shorter than the characteristic time of the excited manifold; population is then transferred to the manifold without Rabi cycling. Under the appropriate conditions, this ``multi-\ensuremath{\Lambda}'' system can exhibit coherence effects such as coherent population trapping and electromagnetically induced transparency of one of the excitation pulses.