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Physical Review A

Rabi frequency and nonlinearity of a two-level atom for an ultrashort optical pulse

G. M. Genkin

Published Jul 1, 1998• 24 citations
Publisher

Abstract

We investigate the interaction of a two-level atom with a short femtosecond optical pulse without the rotating-wave approximation. We have derived the population inversion due to the counterrotating terms. It is shown that due to the counterrotating terms the population inversion oscillates with high frequency and there is the effective time-dependent addition to the Rabi frequency that varies inversely proportionally to the length of an optical pulse and this effect is actual for a short femtosecond optical pulse. It is shown that there is the transient cubic nonlinearity due to the counterrotating terms.

DOI
10.1103/physreva.58.758

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Journal of Physics B Atomic Molecular and Optical Physics•Apr 13, 2010•7 citations
We investigate the interaction of a two-level atom with a short femtosecond optical pulse without the rotating-wave approximation. We have derived the population inversion due to the counterrotating terms. It is shown that due to the counterrotating terms the population inversion oscillates with high frequency and there is the effective time-dependent addition to the Rabi frequency that varies inversely proportionally to the length of an optical pulse and this effect is actual for a short femtosecond optical pulse. It is shown that there is the transient cubic nonlinearity due to the counterrotating terms.
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.
We calculate analytically the error introduced by the rotating-wave approximation (RWA) to the description of a two-level-atom response to a resonant sufficiently intense femtosecond optical pulse of a few-cycle duration and any envelope. The analytically calculated non-RWA refinements to the RWA response are of the lowest order, i.e., linear in the small ratio of the Rabi to optical frequencies. One type of such refinements is small-amplitude rapid oscillations at nearly twice the optical frequency superimposed on the standard RWA population-inversion evolution. The other refinement is an extension of the spectrum of the induced atomic dipole moment in which the third harmonic of the transition frequency appears. The spectrum of this harmonic has an internal structure dependent on both strength and duration of the pulse. The analytically found non-RWA corrections are in qualitative agreement with the recent results of different numerical approaches by other authors and a recent experiment.
We considered the semiclassical Rabi model: a two-level atom fixed at a position and interacting with a classical, monochromatic electromagnetic field. In resonance and for an arbitrary initial state of the system, we obtained an approximate, analytic solution that takes into account the counterrotating terms and that is accurate as long as the Rabi frequency is smaller than or equal to 1/21/21/2 (1/41/41/4) of the angular frequency of the field and 111 (101010) Rabi oscillations are considered. In addition, the approximate solution has the same level of complexity as that obtained with the rotating-wave-approximation (RWA) and the approximate solution also leads to a simple criterion that allows one to determine the accuracy of the solution in the RWA. We found that the approximate solution is composed of the the solution in the RWA plus terms that oscillate at the angular frequency of the field and at the Rabi frequency. Moreover, these corrections are proportional to the Rabi frequency divided by two times the angular frequency of the field.
We consider the population inversion in a two-level system generated by a sub-one-cycle pulse excitation. Specifically, we explore the effect that the time derivative of the pulse envelope has on the Rabi dynamics. Our analysis is based on a combination of analytical, perturbative, and nonperturbative treatments and is complemented by numerical simulations. We find a shortening of the Rabi inversion period and show that complete inversion is unobtainable under resonant, ultrashort pulse conditions. The impact of nonresonant and carrier-envelope phase-dependent effects on the dynamics of two-level and multilevel systems is studied numerically, and conditions for complete population inversion are derived.
The influence of ultrashort low-frequency laser pulses of different intensities and pulse lengths on the dynamics of a resonantly driven atomic system is investigated theoretically. We demonstrate the periodical dependence of the total ionization rate on the appearance time of a strong few-cycle laser pulse. By depleting the excited state, the intense short pulse not only resets the Rabi oscillations, but also introduces a time shift that is proportional to the pulse length. This time shift can be employed to determine the ultrashort pulse length. In the case of ultrashort pulses with lower intensities, we observe side-bands in the two-photon ionization spectrum, whose intensities vary periodically with the appearance time of the pulse. This can be utilized to measure the period of Rabi oscillations. We have developed an S-matrix theory of two-colour ionization which can explain some of the observations of our numerical calculations. Furthermore, we have studied the high harmonic generation spectra as well as the attosecond pulses generated from the high harmonics. We show that the Rabi oscillation can be used to alter the ionization rate within laser sub-cycles and therefore select different quantum trajectories in the high harmonics generation.
The interactions between a two-level atom and a coherent field with a time-varying frequency in a Kerr-like medium have been investigated. It is found that the average of the atomic population inversion is deformed as the frequency changing with time in sine form. It is interesting that when the amplitude of field modulation and third-order nonlinear susceptibility are adjusted to certain values, the collapse–revival phenomena of population inversion in the standard Jaynes–Cumming model disappear and are replaced by a nearly Rabi-oscillation-like evolution. In other words, population inversion in the initial coherent field behaves like that in the initial Fock state. This provides a new way to manipulate the interaction between the atom and the field, and has potential applications in quantum information and quantum communication.