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Physical Review A

Model microfield theory of redistribution of laser radiation

B. Talin, Lewis S. Klein

Published Nov 1, 1982• 13 citations
Publisher

Abstract

A theoretical study is presented which permits the analysis of multiphoton processes in the presence of homogeneous, inhomogeneous, and power-broadening mechanisms. A method derived from the Markov model microfield theory is used. This is based on the statistics of the fluctuations of the interaction between the radiator and a perturbing environment. Application is made to velocity-changing collisions in Doppler-free spectroscopy and to homogeneous and inhomogeneous pressure broadening in two-photon absorption or resonance fluorescence.

DOI
10.1103/physreva.26.2717

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A theoretical study is presented which permits the analysis of multiphoton processes in the presence of homogeneous, inhomogeneous, and power-broadening mechanisms. A method derived from the Markov model microfield theory is used. This is based on the statistics of the fluctuations of the interaction between the radiator and a perturbing environment. Application is made to velocity-changing collisions in Doppler-free spectroscopy and to homogeneous and inhomogeneous pressure broadening in two-photon absorption or resonance fluorescence.
Up to this point we have always assumed that all the atoms which interact with the radiation field have the same atomic transition frequency, with an atomic linewidth given by γ ⊥ . This configuration is called homogeneous broadening. However, in the great majority of lasers the linewidth is inhomogeneously broadened , because the atomic frequency follows a distribution. For example, in solid-state lasers such as the ruby laser and fiber lasers the inhomogeneity is due to the imperfections in the crystalline structure of the material. In gas lasers, where the atoms or molecules are free to move, the inhomogeneous broadening is due to the Doppler effect, for which a molecule moving with velocity v in a field of frequency ω and wave vector k experiences an effective frequency ω ′ = ω − k · v [8]. Since what matters is the difference between the atomic transition frequency and the field frequency, we can keep the field frequency fixed and imagine that the field interacts with different groups of atoms that move with different velocities and therefore have different transition frequencies. The realistic velocity distribution is a Gaussian, for which the corresponding distribution for the modulus of the velocity is Maxwellian. However, in the following we will mainly analyze the case of a Lorentzian distribution for the atomic transition frequency, because this allows an analytical treatment. Again for the sake of simplicity, we will limit our analysis to the case of a ring cavity. In Section 15.1 we will generalize to the inhomogeneous-broadening configuration the modal equations for the case of homogeneous broadening, which were formulated in Chapter 12. In Section 15.2 we will specialize to the single-mode model and discuss its stationary solutions for the laser case. Multimode dynamical equations The atomic transition frequency of the generic group of atoms is given by ω a + δ D , where ω a is the central atomic frequency and δ D takes positive and negative values. We call ω c the frequency of the cavity mode nearest to ω a and denote by δ ac the difference ω a − ω c .
Two-photon absorption is theoretically analyzed within the semiclassical formalism of radiation-matter interaction. We consider an ensemble of inhomogeneously broadened three-level atoms subjected to the action of two counterpropagating fields of the same frequency. By concentrating in the limit of large detuning in one-photon transitions, we solve perturbatively the Bloch equations in a non-usual way. In this way we derive an analytical expression for the width of the two-photon resonance that makes evident sub-Doppler two--photon spectroscopy. We also derive an analytical expression for the Stark shift of the two-photon resonance.
A previously developed diagrammatic method for calculating the nonlinear response of a dilute gas at near resonance with strong classical radiation fields is extended to include velocity-dependent homogeneous and inhomogeneous broadening phenomena. The method is applied to two-level saturated-absorption (Lamb-dip) spectroscopy. Expressions are obtained for the collision-broadened Lamb dip probed by a weak field in the presence of an arbitrarily strong saturating field and are compared to the usual infinite-Doppler-width, weak-saturation expression. We study the effects of velocity and momentum transfer on the line-shape parameters and discuss the effect of phase-changing and strong inelastic collisions on the intensity.
A quantum statistical analysis of a two-photon laser is presented in a generalization of the one-photon laser. A two-photon loss mechanism is assumed by taking detailed-balance considerations into account. The expressions for the photon statistical distribution exhibit the exact influence of the Doppler parameter and other relaxation parameters. In addition, the photon statistics can be studied as a function of the detuning. Threshold conditions for the laser action are noticed. The peak photon number and half-width of the photon distributions are obtained. It is found that the photon distribution can be written as a ratio of the one-photon laser-photon distribution to the Poisson distribution for photons.
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 ).