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Physical Review A

Coherent population trapping in ultrashort pulsed excitation of multilevel systems

Luı́s E. E. de Araujo

Published Jan 21, 2004• 5 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.69.013408

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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.
We present an analysis of coherent population trapping in a three-level Λ system when excited by an ultrashort pulse train near the one- and two-photon resonances. Numerical results using Bloch equations give the transient response of the system and clearly reveal a comb of electromagnetically induced transparency (EIT) lines. An analytical solution to describe the EIT lines is derived in the steady-state regime and in the weak field limit. Moreover, numerical results indicate that selective velocity spectroscopy makes possible the observation of the comb of EIT lines in Doppler broadening media.
We consider the coherent population trapping effect with a train of ultrashort pulses in a $\ensuremath{\Lambda}$-type system with two bound states and a continuum replacing the upper state. Population transfer to the continuum can be totally suppressed provided that the splitting between the bound states is a multiple of the pulse repetition rate. Compared to the traditional case with only two laser fields coupling the bound states to the continuum, CPT with a pulse train allows one to avoid incoherent population losses due to interaction of the fields with ``wrong'' transitions, which results in only partial suppression of ionization. A method of suppression of excited-state absorption of pumping radiation in laser crystals, with pumping in the form of the pulse train, is proposed. As a particular example pumping dynamics of a Ti3+:YAlO3{\mathrm{Ti}}^{3+}:{\mathrm{YAlO}}_{3}Ti3+:YAlO3​ crystal using this technique is numerically analyzed.
We analyze the effects of partial coherence of ground-state preparation on two-pulse propagation in a three-level $\ensuremath{\Lambda}$ medium, in contrast to previous treatments that have considered the cases of media whose ground states are characterized by probabilities (level populations) or by probability amplitudes (coherent pure states). We present analytic solutions of the Maxwell-Bloch equations, and we extend our analysis with numerical solutions to the same equations. We interpret these solutions in the bright and dark dressed-state basis, and show that they describe a population transfer between the bright and dark state. For mixed-state $\ensuremath{\Lambda}$ media with partial ground-state phase coherence the dark state can never be fully populated. This has implications for phase-coherent effects such as pulse matching, coherent population trapping, and electromagnetically induced transparency (EIT). We show that for partially phase-coherent three-level media, self-induced transparency dominates EIT and our results suggest a corresponding three-level area theorem.
We study the dynamics of three-level $\ensuremath{\Lambda}$-type atoms driven by a coherent train of short, nonoverlapping laser pulses. We derive analytical nonperturbative expressions for the density matrix by approximating the pulses by a $\ensuremath{\delta}$ function. We demonstrate that, depending on the train parameters, several scenarios of system dynamics can be realized. We show the possibility of driving Raman transitions between the two ground states of the $\ensuremath{\Lambda}$ system, avoiding populating excited states by using pulses with effective area equal to $2\ensuremath{\pi}$. The number of $2\ensuremath{\pi}$ pulses needed to transfer the entire population from one ground state to another depends on the ratio between the Rabi frequencies of two allowed transitions. In the case of equal Rabi frequencies, the system can be transferred from one ground state to another with a single $2\ensuremath{\pi}$ pulse. When the total pulse area differs from $2\ensuremath{\pi}$ and the two-photon resonance condition is fulfilled, the system evolves into a ``dark'' state and becomes transparent to subsequent pulses. The third possible scenario is the quasi-steady-state regime when neither the total single-pulse area is equal to $2\ensuremath{\pi}$ nor is the two-photon resonance condition fulfilled. In this regime the radiative-decay-induced drop in the population following a given pulse is fully restored by the subsequent pulse. We derive an analytical expression for the density matrix in the quasi-steady-state regime. We analyze the dependence of the postpulse excited-state population in the quasi-steady-state regime on the train parameters. We find the optimal values for the train parameters corresponding to the maximum of the excited-state population. The maximum of the excited-state population in the steady-state regime is reached at an effective single-pulse area equal to $\ensuremath{\pi}$ and is equal to 2/32/32/3 in the limiting case when its radiative lifetime is much shorter than the pulse repetition period.
We report an ultrafast and efficient way to create the maximum coherence between the two lower states in a -like atomic systems, driven by two nonlinearly chirped few-cycle pulses. The phenomenon of coherent population trapping and electromagnetically induced population transfer are investigated by solving the appropriate density matrix equations without invoking the rotating wave approximation. The robustness of the scheme for maximum coherence against the variation of the laser parameters are tested numerically. We also demonstrate that the proposed scheme could be used to obtain complete population transfer to an initially empty ground state.