Pressure-induced Extra Resonances Research Articles

Field-induced resonances in four-wave mixing.

An experimental observation of field-induced resonances in four-wave mixing is reported. In experiments on Na vapor near the D lines, when the input fields are strong, new resonances appear, confirming recent theoretical predictions based on a nonperturbative approach to wave-mixing processes. The analogy to and differences from the pressure-induced extra resonances are discussed.

Pressure-induced extra resonances in nonlinear spectroscopy.

We present three examples of pressure-induced extra resonances that can be observed in nonlinear spectroscopy: fluorescence of a ``three-level atom'' driven by two laser fields, two-photon ionization of a ``three-level atom'' (plus continuum) driven by two laser fields, and excitation of a ``four-level atom'' driven by four laser fields. We show that all these extra resonances can be interpreted in terms of quantum pathways, each pathway involving a collisionally aided excitation. We also demonstrate that the two first extra resonances can be obtained with incoherent fields, while relatively coherent fields are required in the last example.

Pressure-induced effects in two-level atoms: New approach and simple physical interpretation

Pressure-induced effects in nonlinear optics and spectroscopy are studied using a nonstandard-dressed-atom formalism for a set of two-level atoms. We consider a two-level atom interacting with two radiation fields whose frequencies differ by \ensuremath{\delta} and undergoing dephasing collisions with a perturber bath. The atom-field detunings are large compared with all relaxation rates in the problem. The system is described by a Hamiltonian of an atom dressed by a single-mode field with a coupling constant slowly varying in time at frequency \ensuremath{\delta}. We show that the collisionally aided excitation of the initially unpopulated dressed states is modulated in time. All the characteristics of the pressure-induced effects are deduced from the properties of this time-modulated excitation. Apart from already known phenomena (pressure-induced extra resonances in four-wave mixing, pressure-induced phase shifts, etc.) that are reinterpreted using this approach, we present a new effect that can be described as fluorescence beats. Finally, we show that this approach gives a firm basis for use of a model involving collision-induced gratings and we interpret the pressure-induced effects in the context of this model.

Theory and interpretation of pressure-induced resonances.

Using both a bare-atom and dressed-atom picture of atom-field interactions, we present a theory of the pressure-induced resonances that can arise when four radiation fields interact with a three-level quantum system. The four fields drive two coupled atomic transitions which share a common level, the levels being arranged in a ``ladder'' scheme. As the field frequencies are varied, certain resonant structures in the excitation spectrum of the upper state population appear only in the presence of collisions. The resonant structures result from processes that have been referred to in the past as collisionally aided radiative excitation (CARE) and pressure-induced extra resonances in four-wave mixing (PIER4). We show that both CARE- and PIER4-type resonances have a common origin, which can be linked to conservation of energy. In contrast to conventional PIER4 signals, line shapes calculated in this work result from a ``single atom'' rather than a collective emission. Moreover, the signals vanish unless the applied fields are relatively coherent, again in contrast to conventional PIER4.

Phase conjugation by nondegenerate four-wave mixing with excited-state Zeeman coherences.

Nondegenerate four-wave mixing on the j=0\ensuremath{\rightarrow}j=1 transition, in a ``V '' system, using orthogonal, circularly polarized pump beams is used to study theoretically the generation of the phase-conjugate signals. The efficiency of generation of the conjugate wave is studied as a function of the various system parameters such as pump-probe detuning, Zeeman splitting, pump-atom detuning, collisional parameters, and the intensities of the pump beams. The origin of the various resonances in the reflectivity is explained. For weak pump fields, some resonances are shown to have subnatural linewidths. In the degenerate case, the reflectivity as a function of pump-atom detuning has, in contrast to a two-level system, a doublet structure for moderate absorption-length parameters and field strengths. The reflectivity is shown to have additional resonances due to pump imbalance. The phase-conjugate signal is also shown to exhibit the pressure-induced extra resonances.

Pressure-Induced Extra Resonances in Four-Wave Mixing: Evidence for a Breakdown of the Impact Approximation

Experiments on pressure-induced extra resonances in degenerate four-wave mixing involving Zeeman coherence have been performed on Ba atoms in an argon atmosphere under conditions of large detunings from the Ba resonance line. The dependence on the detuning gives evidence for a breakdown of the impact approximation, when the laser is detuned by more than 5\ifmmode\times\else\texttimes\fi{}${10}^{11}$ rad ${\mathrm{sec}}^{\ensuremath{-}1}$; the behavior can be interpreted semiquantitatively by a model based on optical collisions.

Theory of electronic-state coherences produced in combined laser-field—collisional reactions

A method for producing electronic-state coherences using either collisionally-aided radiative excitation (or "optical collision") or laser-induced collisional energy transfer (or "radiative collision") is proposed. Two atoms, $A$ and ${A}^{\ensuremath{'}}$, collide in the presence of two pulsed-laser fields having frequencies $\ensuremath{\Omega}$ and ${\ensuremath{\Omega}}_{1}$. It is shown that by choosing ${\ensuremath{\Omega}}_{1}\ifmmode\pm\else\textpm\fi{}\ensuremath{\Omega}$ such that an energy-conserving transition can occur in the composite $A{A}^{\ensuremath{'}}$ system, one can create an electronic-state coherence in the $A$ or ${A}^{\ensuremath{'}}$ atoms. The coherence can be produced between states of the same or of opposite parity; if it is between states of opposite parity, coherent emission at frequency ${\ensuremath{\Omega}}_{1}\ifmmode\pm\else\textpm\fi{}\ensuremath{\Omega}$ may be generated. The relationship of this work to theories of "pressure-induced extra resonances" in four-wave mixing is discussed.

Single-atom versus coherent pressure-induced extra resonances in four-photon processes

The spectrum of light emitted by a single atom in a thermal bath interacting with three electromagnetic fields is calculated using the dressed-atom picture. The emission spectrum consists of an elastic Raman-like component and dephasing-induced redistribution components that are inelastic in nature. In contrast to coherent four-wave mixing [as given by chi((3))], in which a pressure-induced elastic component (PIER 4) has been predicted and observed, the single-atom response does not contain any elastic pressure-induced components. We derive explicit expressions for the single atom and for the PIER 4 spectra that are not restricted to the impact limit.

Effective two-level description of pressure-induced extra resonances in four-wave mixing

The possibility of formulating a treatment of pressure-induced extra resonances (PIER) in four-wave mixing, in terms of effective two-level equations, is examined. With the use of the method of adiabatic elimination of fast variables, effective two-level equations, which are valid for arbitrary slowly varying field envelopes, are obtained. Such equations are found to have a structure that is very different from that of the Bloch equations. In the new structure important inhomogeneous terms appear that are crucial for PIER. The present description is then used to study the effect of laser fluctuations on PIER.

Pressure-Induced Extra Resonances in Four-Wave Mixing

The first observation of pressure-induced extra resonant enhancements of a coherent four-wave---mixing signal is reported. When the difference between two input frequencies, ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$, corresponds to the separation of two unpopulated upper levels, a resonant enhancement of the generated intensity at $2{\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$ is caused by collisions.