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Proceedings of the IRE

Cross-Relaxation Masers

G S Bogle

Published Mar 1, 1961• 9 citations
Publisher

Abstract

A survey is made of the available ways of using ruby, spinel, rutile and emerald in masers which amplify at frequencies small compared to their zero-field splitting frequencies. Particular attention is paid to means of taking advantage of cross relaxation which, it is shown, can give typically a two-fold improvement in maser performance. Analytic expressions for the energies are given which, to a large extent, remove the need for machine computations in the design of masers of this type.

DOI
10.1109/jrproc.1961.287737

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Soviet Journal of Quantum Electronics•Dec 31, 1975•1 citation
A survey is made of the available ways of using ruby, spinel, rutile and emerald in masers which amplify at frequencies small compared to their zero-field splitting frequencies. Particular attention is paid to means of taking advantage of cross relaxation which, it is shown, can give typically a two-fold improvement in maser performance. Analytic expressions for the energies are given which, to a large extent, remove the need for machine computations in the design of masers of this type.
Cross relaxation effects in ruby maser crystals are treated by introduction of a cross relaxation probability in the rate equations. Detailed solutions have been obtained for several specific processes and compared to recent experiments. It is shown that cross relaxation can improve maser performance even in the absence of impurity doping. Pulse experiments at 0.06 and 0.14% chromium ion concentrations in a ruby traveling wave maser are interpreted in terms of a five-spin process in addition to a four-spin process.
Cross relaxation between adjacent intervals may be used to secure an advantage in maser operation comparable to that obtained by ``push-pull'' pumping. Equations for the rate of change of populations in a scheme of four levels (A,B,C,D) when AC is pumped and cross relaxation takes place between BC, CD have been formulated, and the steady-state condition corresponding to various assumptions regarding lattice relaxation times is discussed. In an experimental investigation of maser operation based on this scheme and using ruby as the active material, an inversion of 0.9:1 was obtained at a signal frequency of 14.5 kMc when pumping at 24.5 kMc.
Summary This article is intended to give the reader some understanding of why masers have suddenly become of such great interest. It begins with an account of the background to the problem of microwave amplification and then passes to a description of the properties of the populations of energy levels in thermal and other kinds of equilibrium. Particular attention is paid to the way in which a population reversal can be used for amplification, the basis of the maser. The next considerations are related to the performance characteristics to be anticipated and the article ends with descriptions of the ammonia maser, the ruby 3-level maser and a brief mention of the 2-level maser work. In having to choose between describing the technological and physical problems which are involved the basic physics has seemed of more importance and has been stressed.
The reversals of maser behaviour which occur in ruby at low fields on cooling from 80° to 4 oK are explained in terms of cross-relaxation transitions (resonant interchanges of energy between neighbouring paramagnetic ions). For an angle of 29° between the magnetic field and the crystal axis a cross-relaxation process has been demonstrated which involves groups of three ions and has a transition probability of 700± 100 per second in 0·013% ruby. The profile of the cross-relaxation resonance has a half-width of 190±30 Mcts at half-intensity, and has an approximately Gaussian shape. It should be possible to obtain useful low-field L-band maser action at 4 oK by pumping V41 with the magnetic field nearly perpendicular to the crystal axis, and also at 80 oK using a concentration ten times higher, i.e. 0·13%