Probe field spectroscopy in three-level Λ systems under arbitrary collisional relaxation of low-frequency coherence

We investigate theoretically the spectrum of weak probe field absorption by three-level atoms with the Λ configuration of levels in the field of a strong electromagnetic wave acting on an adjacent transition and colliding with buffer gas atoms. Analysis is carried out for the general case of arbitrary collisional relaxation of low-frequency coherence at a transition between two lower levels. It is shown that, in the absence of collisional relaxation of low-frequency coherence, the probe field spectrum always exhibits clearly manifested anisotropy with respect to mutual orientation of wavevectors of the strong and probe radiation (even under small Doppler broadening). It is found that the probe field spectrum may acquire under certain conditions supernarrow resonances with a width proportional to the diffusion coefficient for atoms interacting with radiation. This fact may form the basis for a spectroscopic method for measuring transport frequencies of collisions between absorbing and buffer particles. A large-amplitude supernarrow resonance (with an amplitude much larger than the amplitude of the resonance near the line center), which is observed in the far wing of the absorption line, exhibits collisional narrowing (a nonlinear spectroscopic analog of the Dicke effect) at collision frequencies several orders of magnitude lower that the Doppler linewidth. Simple working equations proposed for describing the probe field spectrum are convenient for experimental data processing.     

Phase memory effects in probe-field spectroscopy of two-level systems at low collision frequencies

The absorption (amplification) spectrum of a weak probe field by two-level atoms located in a strong resonant laser field and colliding with buffer-gas atoms is analyzed theoretically. The analysis is performed for low collision frequencies compared to the Doppler absorption linewidth (low gas pressure) and with allowance made for an arbitrary change in the phase of the radiation-induced dipole moment at elastic collisions between gas particles. The phase memory effects have been found to lead to a strong qualitative and quantitative transformation of the probe-field spectrum even at rare collisions, when the well-known Dicke manifestation mechanism of the phase memory effects (the removal of Doppler broadening due to the restriction of the spatial particle motion by collisions) is inoperative. The strong influence of the phase memory effects on the spectral resonances at low gas pressures stems from the fact that the phase-conserving collisions change the velocity dependence of the partial refractive index n(v) (the refractive index for particles moving with velocity v).     

Probe-field spectroscopy in two-level systems with low Doppler broadening

The spectrum of absorption (amplification) of a weak probe field by two-level atoms colliding with atoms of a buffer gas in the field of a strong electromagnetic wave is studied theoretically. A universal, free of any collision model, solution is obtained for systems with small (compared to collision rates) Doppler broadening with no restrictions on the strong-field intensity and with the possible absence of dephasing of the light-induced dipole moment in elastic collisions of the gas particles. The relation of the collisional parameters of the problem with the characteristics of an elementary scattering event and with the macroscopic characteristics connected with the transport phenomena is established. It is found that the nontrivial features of the probe-field spectrum discovered by us previously [Zh. Éksp. Teor. Fiz. 127, 320 (2005)] in the framework of the strong-collision model are not connected with a particular model of collisions and have a universal nature.     

Ground-state pump-probe spectroscopy

The absorption/gain spectrum of a weak probe field is theoretically analyzed for an atomic three-level Λ system driven by a strong pump field and perturbed by collisions with buffer-gas atoms. It is assumed that both pump and probe drive the same transition. When the dark state is sufficiently long lived, the probe spectrum exhibits an extremely narrow (possibly subnatural-width) resonance. It is shown that coherence-preserving collisions drastically (both qualitatively and quantitatively) change the probe absorption spectrum even at a low collision frequency, when Dicke narrowing is negligible. Strong coherence effects on the resonance structure at low gas pressures is explained by a change in refractive index due to coherence-preserving collisions for particles in the resonant velocity group.     

Effect of collisions on the resonance fluorescence spectrum

The effect of collisions on the resonance fluorescence spectrum of two-level atoms excited by monochromatic resonance radiation is studied. Analysis is performed for systems where the Doppler broadening is small compared to the collision frequency (high buffer gas pressures) and for the general case where collisions arbitrarily change the phase of the radiation-induced dipole moment (from completely interrupted to completely unaffected phase of the dipole moment). Both at a relatively low and at a high excitation radiation intensity, the resonance fluorescence spectrum is shown to depend on whether the two-level system is closed or open. This is especially true for the narrow unshifted Rayleigh scattering line. It is shown that, although the absorption line is homogeneously broadened, the resonance fluorescence spectrum exhibits a clearly pronounced anisotropy. In a direction close to the direction of propagation of the excitation radiation, the Rayleigh scattering line is maximally narrowed. Under certain conditions (that can easily be created in experiments), the width of this line is proportional to the diffusion coefficient of atoms interacting with the radiation.     

Phase transition in a radiation-matter interaction with recoil and collisions

The standard model introduced to describe the collective atomic recoil of an ensemble of atoms interacting with a strong electromagnetic field has been here extended by the inclusion of collisions with a buffer gas. As a result, we find that in the thermodynamic limit the coherent emission of radiation exhibits a continuous phase transition upon increasing the pump intensity. The output laser field is strictly larger than 0 only above a critical value. We find that the transition is not associated with the onset of spatial ordering but rather with the onset of a synchronization between the polarization phase and spatial position. A coherence parameter is introduced to characterize the phase transition.     

Effect of diffraction of molecules on collisional line narrowing

A new model of line profile is proposed, which simultaneously takes into account hard and soft (in velocity) collisions leading to collisional line narrowing. It is shown that parameters of collisional narrowing and broadening obtained by processing experimental profiles on the basis of the standard model of hard collisions contain almost no contribution of diffraction scattering of molecules by buffer gas particles. The model proposed for the line profile makes possible a correct comparison of frequencies of the collision integral measured by methods of linear and nonlinear spectroscopy.     

The Boundary Condition in the Classical Derivation of the BBR

The classical derivation of the black body radiation (BBR) spectrum by Boyer was based on an equilibrium mechanism such that in the absence of thermal radiation particles in a container can gain kinetic energy from the random electromagnetic zero point field (ZPF) radiation. Their loss of that energy was to be by means of their collisions with the walls of the container. Theoretically, energy dissipation through collisions with the walls might lead to a divergent kinetic energy value for the particles. This is because the box can be taken large enough to minimize the collisions probability, and that can lead to a particle’s indefinite growth in energy. Furthermore, a derivation of a general phenomenon such as the BBR should be possible without relying on the walls boundary of a box. Therefore, a new boundary condition is proposed here which is related to relativistic effects. It is shown that with the new boundary condition one may still recover the BBR spectrum. A discussion is presented that shows how the new boundary condition is indeed responsible for energy dissipations.     

Optical resonant processes in thin excited films

It is shown that coherent processes of elastic scattering of resonant radiation form a “buffer” electromagnetic field near the boundary of excited media. This part of radiation is not governed by the standard refractive index, but precisely this part of radiation forms the beams reflected and refracted by the excited medium. The presence of the “buffer” field causes the suppression of stimulated emission near the boundary and leads to the appearance of a frequency angular broadening of the beam transmitted through a thin film of excited atoms at an oblique angle.     

Nonlinear resonances free of field and doppler broadenings

For the case where the Rabi frequencies of the guiding fields are much larger than the relaxation constants but much smaller than the Doppler broadening, it is shown that resonances which are neither field nor Doppler broadened can appear in the absorption (or gain) spectrum of the probe field. A classification of four-level systems according to the number of resonances is made for cases where two strong fields interact either with opposite or adjoining transitions. The conditions under which the number of resonances reaches eight, while for stationary atoms the maximum number is four, are found. A method is proposed for calculating the number of resonances in a multilevel system with several strong fields using analysis of the extremum points of the frequency branches in the velocity-frequency plane.     

Ultracold inelastic collisions in two dimensions

We show that cross sections for inelastic collisions of ultracold atoms or molecules confined by a harmonic potential have the same energy dependence as in pure 2D geometry. This indicates that chemical reactions and inelastic collisions may be suppressed in an ultracold gas under strong confinement in one dimension. We present a numerical proof of the threshold collision laws in 2D. Our results indicate that inelastic collisions in weak electromagnetic fields may be controlled by varying the orientation of the external field axis with respect to the plane of confinement.     

Manifestation of the gas translational nonequilibrium in a resonance electromagnetic field

The translational nonequilibrium of the resonance impure gas in a field of a traveling electromagnetic wave produced by both the radiation pressure and the difference in cross-sections of elastic collisions between the excited- and ground-state atoms and a buffer gas has been studied.     

Change in polarization of a weak signal induced by the optical pumping of atoms at an adjacent transition

A change in polarization of a weak wave under optical pumping of atoms at an adjacent atomic transition is studied. It is shown that an elliptically polarized probe signal undergoes substantial polarization changes in the field of a circularly polarized pump wave. For the probe wave component with the same circular polarization as that of a strong signal, the medium becomes transparent.     

Electromagnetically induced transparency and electromagnetically induced absorption in Y-type system

The propagation of a probe field through a four-level Y-type atomic system is described in the presence of two additional coherent radiation fields, namely, the control field and the coupling field. An expression for the probe response is derived analytically from the optical Bloch equations under steady state condition to study the absorptive properties of the system under probe field propagation through an ensemble of stationary atoms as well as in a Doppler broadened atomic vapor medium. The most striking result is the conversion of electromagnetically induced transparency(EIT) into electromagnetically induced absorption(EIA) as we start switching from weak probe regime to strong probe regime. The dependence of this conversion on residual Doppler averaging due to wavelength mismatch is also shown by choosing the coupling transition as a Rydberg transition.     

Determination of polarization relaxation times of atomic levels

Rotation of polarization plane of a probe signal under influence of a time-advanced intense pulse is investigated taking into account optical pumping of atoms in a resonant medium. A formula is derived for the angle of rotation of polarization plane of the probe signal. It follows from this formula that modification of the polarization plane is caused by two processes: radiative decay of atoms and collision-induced redistribution of atoms over magnetic sublevels. It is shown that this method enables direct measuring long times of atomic collisions on the background of short times of the radiation decay.     

Dynamic effect of collision phase lockout in a gas of cold “dark” atoms

In a gas of slow atoms exhibiting the effect of coherent population trapping (CPT) on the sublevels of the ground state in a spatially nonuniform light field, rare collisions destroying the CPT state initiate the irreversible exchange of momentum between radiation and atoms. This exchange is manifested as an additional force that acts on the particles and is of geometric origin; i.e., it is determined only by the structure of the field of local CPT states. When this force is not masked by the standard collision change in atomic momentum, the observation of the kinetics of the particles may provide information on the physics of the collisions.     

Biexciton resonance linewidth in CuCl: Collision broadening or not?

The width of the biexciton two-photon resonance in CuCl, as measured with weak probe beams, shows a marked broadening if a high density of biexcitons is injected with an intense pump beam. These results are attributed to collisions between excitonic particles.     

Competition of Short-Range and Long-Range Mechanisms of Spectral Line Broadening in Emission of Atomic and Molecular Gases

An analogy is shown for broadening the spectral line of atoms in the case of competition between the short-range Doppler broadening mechanism and the long-range impact mechanism in atomic gases, as well as for the absorption band of a molecular gas in the infrared (IR) region of the spectrum, where the longrange part of the spectrum is also determined by the impact mechanism of the broadening. The long-range part in the pedestal region of the absorption band is associated with the distribution of molecules over the rotational states, and, in the range of wings of the absorption band, it is due to a finite time of collisions between emitting and perturbed molecules. The flux of resonant radiation produced by an excited atomic gas is estimated using the concepts of the spectral absorption band and a large optical thickness of the gas. These concepts are used for a molecular gas where, in the framework of the regular model (the Elsasser model), expressions are given for the absorption coefficient related to a specific spectral absorption band, separately in the pedestal region and for wings of the absorption band. As a demonstration of the possibilities of these concepts, the IR flux is calculated on the surface of Venus from its atmosphere, which includes seven vibrational transitions of the carbon dioxide molecule and amounts to 26% of the total flux of IR radiation emitted by the Venus surface. An analysis of the Venus energy balance leads to the conclusion that the main part of IR radiation of the Venus atmosphere falling on its surface is formed by a microscopic dust in the Venus atmosphere. This channel of the Venus energy balance is realized if the mass of a microscopic dust in the Venus atmosphere is seven orders of magnitude less than the mass of atmospheric carbon dioxide.     

Quantum noise property in coherent atomic system

The coherent superposition of atomic states leads to the characteristic change of interacting lights because of the coupling between the lights and atoms. In this paper, the noise spectrum of the quantified light interacting with the atoms is studied under the condition of electromagnetically induced transparency (EIT). It is shown that the noise spectrum displays a double M-shape noise profile resulted from the conversion of phase noise of probe beam. A squeezing of 0.3 dB can be observed at the detuning of probe light at the proper parameters of atoms and coupling beam.