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Journal of Physics A Mathematical and General

Quantum theory of multiphoton lasers. II. Systems without detailed balance

K. J. McNeil, Daniel F. Walls

Published Jan 1, 1975• 28 citations
Publisher

Abstract

For pt.I see ibid., vol.8, 104 (1974). Investigations into possible multiphoton laser systems are extended to a less restrictive class than considered in the previous paper. The loss mechanism for the light field is now taken to be a single-photon loss. These systems no longer preserve the property of detailed balance. Explicit solutions are given for the stationary photon distribution for the two-photon laser. This exhibits similar photon statistics to the usual one-photon laser, with no significant increase in relative fluctuations at threshold. The results are equally applicable to multiboson lasers involving say the simultaneous stimulated emission of a photon and a phonon.

DOI
10.1088/0305-4470/8/1/017

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For pt.I see ibid., vol.8, 104 (1974). Investigations into possible multiphoton laser systems are extended to a less restrictive class than considered in the previous paper. The loss mechanism for the light field is now taken to be a single-photon loss. These systems no longer preserve the property of detailed balance. Explicit solutions are given for the stationary photon distribution for the two-photon laser. This exhibits similar photon statistics to the usual one-photon laser, with no significant increase in relative fluctuations at threshold. The results are equally applicable to multiboson lasers involving say the simultaneous stimulated emission of a photon and a phonon.
A quantum-mechanical model for a multiphoton laser is proposed by an obvious generalization of the usual one-photon laser model. The authors consider a class of lasers which preserve detailed balance. This condition is ensured by assuming a multiphoton loss mechanism for the laser light of the same order as the multiphoton gain from the stimulated emission process. Solutions for the stationary density operator of the light field are obtained by a straightforward generalization of the techniques of Scully and Lamb. The multiphoton laser around threshold exhibits behaviour analogous to a second-order phase transition. A non-thermal distribution of multiphoton laser light is achieved above threshold.
Laser actions and photon statistics for a nondegenerate two–photon laser in an effective three–level atomic system with arbitrary detuning are investigated both semiclassically and quantum mechanically, Due to the two–photon effect, the radiation field easily displays a sub–Foissonlan statistical distribution. The laser actions for a two–photon and a single–photon loss mechanisms are compared. It Is found that the detailed balance in the single–photon loss mechanism can still be preserved in the case that a first–order phase transtion takes place.
A quantum-statistical analysis of multi-photon lasers including the effect of detuning and atomic motion is presented. The equations are solved to fourth order in the atom-field coupling constant. Systems in detailed balance (with multi-photon loss mechanism) as well as systems without detailed balance (with single-photon loss mechanism) are considered separately. Treatments for the single-mode and multi-mode processes are given. Expressions for threshold conditions, peak photon number and the half width of photon distribution are obtained. The variation of mean output photon number, the effect of detuning, the coherence characteristics, etc. are studied and results are illustrated for two- and three-photon lasers as examples.
The fully quantum-mechanical model of a single-mode laser is examined. A previously introduced method of solution where both the pump and loss reservoirs are treated simultaneously as a weak perturbation of the atom-light system is employed. This method is similar to perturbation theory for degenerate systems in that the reservoirs remove the nonuniqueness (degeneracy) of the steady state of the unperturbed atom-light system. In the limit of weak reservoir coupling, the exact steady-state solution of the master equation is derived. This solution still contains the atomic variables explicitly and therefore remains valid even in situations where the conventional adiabatic elimination of the atomic variables is no longer possible. A compact discrete representation of the master equation in terms of atomic occupation numbers and photon numbers is developed which allows one to perform all calculations in an elementary manner, requiring only appropriate summations over photon and occupation numbers. General exact relations are obtained for the steady-state probability distribution function p(n,m)p(n,m)p(n,m) to find nnn photons in the light field and mmm atoms in the excited state. Our solution constitutes the first steady-state laser distribution function that is valid for an arbitrary number of laser atoms and in the complete region from far below to far above threshold. It provides a quantitative criterion for the range of validity of approximate methods in laser theory.
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.
We have solved a master equation to obtain the diagonal elements of the density matrix for laser light. The master equation represents a generalization of a master equation derived earlier for a single radiation mode interacting with NNN stationary two-level atoms. The generalization takes into account the pumping scheme which characterizes a three-level laser. For positive temperatures and a lossless cavity, the equilibrium solution of the master equation gives the correct statistical-mechanical description of the atoms and radiation. For negative temperatures, the distribution function for the number of photons in the mode is either exponential or peaked, depending on whether the laser is operating below or above threshold. The calculated intensity fluctuations are in good agreement with semiclassical results for lasers operating slightly above threshold.