This paper shows that a single laser pulse with a chirped frequency can generate trapped superposition of the ground states in the /spl Lambda/-atom when the width of the transform-limited laser pulse envelope frequency spectrum (without chirp) is smaller but the peak Rabi frequency of the pulse is larger than frequency interval between the two ground states of the /spl Lambda/-atom. This study also analyzes the propagation of the frequency-chirped laser pulse in a medium consisting of /spl Lambda/-atoms analytically and by exact numerical solution of coupled Bloch-Maxwell equations. The results show that there is nearly absorptionless propagation of the single frequency-chirped laser pulse on distances more than 40 times the pulse duration, when the formulated above conditions of negligible excitation of the /spl Lambda/-atom are fulfilled. The described effect of absorptionless propagation of the single frequency-chirped pulse is referred to as electromagnetically self-induced transparency.
This paper shows that a single laser pulse with a chirped frequency can generate trapped superposition of the ground states in the /spl Lambda/-atom when the width of the transform-limited laser pulse envelope frequency spectrum (without chirp) is smaller but the peak Rabi frequency of the pulse is larger than frequency interval between the two ground states of the /spl Lambda/-atom. This study also analyzes the propagation of the frequency-chirped laser pulse in a medium consisting of /spl Lambda/-atoms analytically and by exact numerical solution of coupled Bloch-Maxwell equations. The results show that there is nearly absorptionless propagation of the single frequency-chirped laser pulse on distances more than 40 times the pulse duration, when the formulated above conditions of negligible excitation of the /spl Lambda/-atom are fulfilled. The described effect of absorptionless propagation of the single frequency-chirped pulse is referred to as electromagnetically self-induced transparency.
We present a scheme of population transfer between two metastable (ground) states of the $\ensuremath{\Lambda}$ atom without considerable excitation of the atom using single frequency-chirped laser pulses. The physics of the process is generation of the ``trapped'' superposition of the ground states by the laser pulse at sufficiently high laser peak intensity. The main conditions for realization of this regime are the following: The width of the transform-limited laser pulse envelope frequency spectrum (without chirp) must be smaller and the peak Rabi frequency of the pulse must be larger than the frequency interval between the two ground states of the $\ensuremath{\Lambda}$ atom. During the frequency chirp, the laser pulse must first come into resonance with the transition from the initially occupied ground state to the excited state and after that with the transition between the excited and second initially empty ground states. In the case when the envelope frequency spectrum width (without chirp) of the pulse exceeds the frequency interval between the two ground states, we show a possibility of controllable generation of superposition of the ground states with a controllable excitation of the $\ensuremath{\Lambda}$ atom.
A quasi-transparent propagation phenomenon is presented for a pair of frequency-modulated laser pulses in an optically thick medium of cold atomic gas. The noninteracting, identical lambda-structured atoms are driven by two classical laser fields, which are frequency-chirped in the same way around the corresponding atomic transition frequencies, maintaining two-photon (Raman) resonance. It is shown by numerical analysis that after propagating over a relatively short distance (determined by the absorption length), the frequency-chirped pulse pair is affected by the atoms in such a way that instead of exciting the atoms (as it would happen in an optically dilute medium), they create a certain coherent superposition of the ground states (which can be varied by the parameters of the incoming fields), and they propagate in the remainder of the medium without significant further losses. This quasi-lossless propagation effect of the Raman-resonant frequency-chirped pulses, described above, is not only interesting in the point of view of the laser fields, but the on-demand creation of coherent superpositions among atomic states along the optically thick medium may also find applications in quantum optical experiments and quantum informatics.
Quantum coherence and interference in atomic systems can lead to interesting optical phenomena such as nonabsorbing resonances, lasing without inversion, generating an ultrahigh index of refraction without absorption, propagation of light pulses without losses. In the last-named is given the title transparency. Up to now it is known some types of transparency in multi-level resonant media. One of them is electromagnetically induced transparency (EIT). The phenomenon of EIT in the propagation of laser radiation through an absorbing medium has been recently investigated and explained in terms of quantum coherence and interference for multi-level atomic system. For instance in the (Lambda) -system, if the atomic medium is prepared in a quantum superposition of states, matched pulses, i.e., a pair of optical pulses whose amplitude and phase have a well defined relation, propagate without absorption. The mechanism of coherent population trapping is responsible for EIT in a (Lambda) -system. There is another, well known mechanism for creating transparency in a two-state atom: self-induced transparency, or SIT. Here, a single pulse whose area (the integral of its Rabi frequency over time) is 2(pi) is applied to the medium. Such a pulse causes the atoms to cycle smoothly from the ground to the excited state and then back again. In principle, such a pulse can propagate over long distances. SIT can be generalized to multi-level systems and multiple quasi-monochromatic fields. Generalization of SIT in a (Lambda) -system was obtained in reference 2. In this report we present a new type of transparency (combined transparency). This type describes propagation of laser pulse through a (Lambda) -type medium where the initial atomic state is prepared in the form of spatially varying coherent superposition of low-energetic states (initial coherency gradient). The common envelope behaves as usual 2(pi) hyperbolic secant pulse, but the ratio between the components of frequencies transitions 1> yields 3> and 2> yields 3> changes due to coherent Raman scattering so (phi) equals atan(E1/E2) equals (pi) /2-Gz, where z is the coordinate, G is the constant determined by both the incident pulse parameters and the initial coherence between the atomic states. Our solution demonstrates a direct connection of EIT in adiabatic regime to the SIT effects. This type of a transparency is given the title screw-type transparency, because for the given form of incident pulse (pitch of screw) medium is transparent, if medium's the initial coherence gradient is correspondingly prepared (thread on a engine nut). Besides a transparency, this phenomenon contains a transformation of the incident pulse frequency during the process of the propagation. This property of the phenomena can be result in a complete conversion of the frequency.
In the present paper, we examine the propagation of frequency-chirped laser pulses through a medium of Lambda atoms, pulses that are capable of producing adiabatic population transfer between the stable states. We compare the propagation of these pulses to their propagation in a medium of two-level atoms, and show that the distortion of the pulses is greatly reduced in the three-level case. In effect, there exists a transparency phenomenon for frequency-chirped laser pulses propagating in the three-level medium that is somewhat similar to electromagnetically induced transparency. It is concluded that the population transfer induced by chirped pulses does populate the excited state during the process to a slight extent, even if the transfer is perfectly adiabatic. Thus the transparency of the medium is also imperfect, the pulse is eventually distorted and its capability to produce the population transfer is lost, even in the perfectly adiabatic limit.
We study the propagation of frequency-chirped laser pulses in optically thick media. We consider a medium of atoms with a $\ensuremath{\Lambda}$ level-scheme (Lambda atoms) and also, for comparison, a medium of two-level atoms. Frequency-chirped laser pulses that induce adiabatic population transfer between the atomic levels are considered. They induce transitions between the two lower (metastable) levels of the $\ensuremath{\Lambda}$-atoms and between the ground and excited states of the two-level atoms. We show that associated with this adiabatic population transfer in $\ensuremath{\Lambda}$-atoms, there is a regime of enhanced transparency of the medium---the pulses are distorted much less than in the medium of two-level atoms and retain their ability to transfer the atomic population much longer during propagation.
Traditional schemes for coherent population transfer or generation of coherent superposition states in multilevel atoms or molecules usually utilize two or more laser beams with radiation bandwidth smaller than the frequency interval between the working levels. We show the possibility of creation of the coherent superposition of three metastable states of a four-level atom with tripodlike level structure using a single short frequency-chirped laser pulse. The bandwidth of the pulse envelope (without chirp) must be comparable to or exceed the frequency distance between the two metastable levels. No appreciable excitation of the atom takes place during the creation of the coherent superposition state, thus diminishing significantly the effect of decoherence due to the spontaneous decay of the excited state. The proposed method of creation of superposition states is robust against variations in the laser pulse parameters. Since this method does not require maintaining steady resonance with the atomic transitions (owing to the frequency chirp of the laser pulse), it is effective both in homogeneously and inhomogeneously broadened media.