From resonance equations using density matrix formalism, we obtain several features of rf or mw optical double resonance experiments at zero field. Resonance signal intensity formulas are derived. The question of the spatial and polarization dependence is then reduced to a minimum number of parameters through the use of multipole tensors; the effect of a resonance within the excited state is formulated as a redistribution of excited state multipole moments. In the case of transitions between hyperfine sublevels which are separated on the order of the natural linewidth, coherence is produced by interference between different rf-optical transition pathways. Depending on the method of detection, such interference may lead to a dispersion-shaped contribution to the ODR line shape, as observed in the experiments on the NO A state (Bergeman and Zare, accompanying paper). Second order radiation field corrections for multiplet transitions (generalizations of the Bloch-Siegert shift) are also derived.
From resonance equations using density matrix formalism, we obtain several features of rf or mw optical double resonance experiments at zero field. Resonance signal intensity formulas are derived. The question of the spatial and polarization dependence is then reduced to a minimum number of parameters through the use of multipole tensors; the effect of a resonance within the excited state is formulated as a redistribution of excited state multipole moments. In the case of transitions between hyperfine sublevels which are separated on the order of the natural linewidth, coherence is produced by interference between different rf-optical transition pathways. Depending on the method of detection, such interference may lead to a dispersion-shaped contribution to the ODR line shape, as observed in the experiments on the NO A state (Bergeman and Zare, accompanying paper). Second order radiation field corrections for multiplet transitions (generalizations of the Bloch-Siegert shift) are also derived.
Recent experiments (Dodd, Fox, Series & Taylor, I959; Colegrove, Franken, Lewis & Sands, i959) have shown that coherence effects can be observed between the Zeeman components of an optical line provided some coherence exists in the excited state. This coherence may be due to a radio-frequency field inducing transitions between the Zeeman sublevels of this excited state, or to the excitation process itself or to both. These effects are actually more or less a generalization of the well-known Hanle's experiments on the polarization of resonance radiation.: In the case of 'double resonance' experiments (Dodd et al. I959), they appear as a modulation of the intensity of the fluorescence radiation at one of the harmonics of the r.f. field; they can be interpreted as interference between the radiations of different frequencies emitted by the atoms which are submitted to the 'double resonance' ('light beat'). These coherence effects are related to those observed in the trapping of resonance radiation. In this paper, the methods used in former papers (Barrat I959 a, b and c, hereafter referred to as I, II, III) for the problem of light trapping are applied to the light beats experiments, using a density matrix formalism. The expression for the density matrix representing the states of the atoms just after the excitation will first be derived; then the intensity of the re-emitted light will be calculated. It will be shown finally how this method can be applied to practical problems. Notation unexplained in this paper is the same as in papers on coherence in light trapping (papers I, II and III).
In the domain of semiclassical formulation and for the Doppler-free atom–field interaction, we construct the optical Bloch equations involving an M-type six-level system coupled to two pump fields and a probe field. The response of the system is probed for different pump-induced transitions in double and triple-resonance situations. In order to obtain the coherent lineshapes (absorptive and dispersive), we use the usual perturbation method for obtaining the approximate analytical solutions to these coupled optical Bloch equations for the density matrix elements. The interferences between the probability amplitudes for different energy levels (dipole allowed and dipole forbidden) are taken care of. For off-resonance pump positions, the linewidths of the three probe transitions are insensitive to the pump Rabi frequencies. On the other hand, the shifts of the three resonance peaks are extremely sensitive to the pump Rabi frequencies. However, for on-resonance pump conditions, the sensitivities of pump Rabi frequencies on the linewidths of the resonance peaks and on the shifts of the resonance peak positions are opposite to those of their off-resonance counterparts. In particular, we have shown the asymmetric and symmetric Rabi splittings under different physical conditions, for non-zero and near-zero probe detuning, respectively. The Rabi splitting under triple-resonance conditions, significantly, modifies the dispersive lineshape at the centre of the absorption line. The two- and three-photon absorptions are also reported for different off-resonant pump positions.
The theory of optical-microwave double resonance is extended to the case of the off-resonant optical field. Line shapes for fixed optical and variable microwave frequencies are calculated under various conditions by the density matrix method. Analytical expressions are given for small microwave or small optical saturation. When the off-resonance is much greater than the Doppler width of the optical transition, optical Stark effect and optical-microwave two-photon transition are the principal effects. When the signal is observed as a variation of microwave absorption, the former effect is much stronger than the latter. When the signal is detected optically, the two-photon and the stepwise transitions for small microwave saturation are the effects with the same order of magnitude, while the optical Stark effect is a secondary effect. The line shapes are also calculated numerically for arbitrary microwave and optical saturation.
The interaction of a four-level atomic system with two electromagnetic radiations has been studied. The two intermediate levels are closely spaced such that a single radiation frequency can induce transitions from the ground state to both of them. In a cascade-type double resonance, the pump electric-field amplitude is much stronger than the amplitude of the signal field, which can induce transitions from both of the intermediate levels to the upper state. The 15 optical Bloch equations obtained from the Liouville equations are solved analytically to obtain the Doppler-free signal line shape. The computed signals, exhibiting two Lorentzian line profiles, are presented for different pump frequencies. In addition to these one-photon transitions, we obtain a weak two-photon resonance peak. The two-photon resonance becomes weaker when the detuning of the pump frequency from a resonance frequency is increased. When the pump frequency ${\mathrm{\ensuremath{\Omega}}}_{\mathit{p}}$ is intermediate between the pump resonance frequencies ${\mathrm{\ensuremath{\omega}}}_{1}$ and ${\mathrm{\ensuremath{\omega}}}_{2}$, the two-photon resonance is very weak and almost vanishes when ${\mathrm{\ensuremath{\Omega}}}_{\mathit{p}}$=(${\mathrm{\ensuremath{\omega}}}_{1}$+${\mathrm{\ensuremath{\omega}}}_{2}$)/2. When the pump frequency is on resonance with one of the transition frequencies, the Rabi splitting of the signal shows asymmetry because of the presence of a close-lying transition which is pumped off resonance. The intensity-dependent shifts of the transitions are such that they repel each other more and more as the intensity of the pump field is increased. These shifts arise from the nonresonant interaction between the atom and the field.
A general formalism is presented to describe resonance line polarization for a two-level atom in an optically thick, three-dimensional medium embedded in an arbitrary varying magnetic field and irradiated by an arbitrary radiation field. The magnetic field is supposed sufficiently small to induce a Zeeman splitting much smaller than the typical line width. By neglecting atomic polarization in the lower level and stimulated emission, an integral equation is derived for the multipole moments of the density matrix of the upper level. This equation shows how the multipole moments at any assigned point of the medium are coupled to the multipole moments relative at a different point as a consequence of the propagation of polarized radiation between the two points. The equation also accounts for the effect of the magnetic field, described by a kernel locally connecting multipole moments of the same rank, and for the role of inelastic and elastic (or depolarizing) collisions. After having given its formal derivation for the general case, the integral equation is particularized to the one-dimensional and two-dimensional cases. For the one-dimensional case of a plane parallel atmosphere, neglecting both the magnetic field and depolarizing collisions, the equation here derived reduces to a previous one given by Rees (1978).
By means of the recursive perturbation method developed by Hioe and Eberly, we present an analytic expression of general fourth-order effects in the frequency shifts of new coherence resonances and multiphoton transitions for the general case that a spin-1/2 atom interacts with an oscillating rf field in optical pumping. Moreover, the validity of the theoretical results is checked by an optical-rf double resonance experiments with cesium vapor, and agreement between observed and calculated results becomes close.