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Bell System Technical Journal

Use of Active Material in Three-Level Solid State Masers*

E. O. Schulz-DuBois, H. E. D. Scovil, R. W. DeGrasse

Published Mar 1, 1959• 25 citations
Publisher

Abstract

The three-level excitation method for solid state masers, including some background material on paramagnetic resonance, is reviewed. With respect to the experimental application of the maser material, two cases can be distinguished, In the first, maser action is based mainly on a favorable relaxation time ratio in signal and idler transitions. It is shown that the relaxation time ratio can be changed artificially by a doping technique. Experimental evidence is presented for two such doping techniques, one self-doping, the other impurity doping. In the second case, maser action is based primarily on a favorable frequency ratio of signal and idler transitions. Maser experiments using this approach are described. In addition, excitation of unidirectional gain and attenuation by circular polarization is discussed. Properties of practical isolator materials are surveyed; they include high-concentration paramagnetic and polycryslolline ferrimagnetic materials.

DOI
10.1002/j.1538-7305.1959.tb03891.x

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Bell System Technical Journal•Jan 1, 1959•120 citations
The three-level excitation method for solid state masers, including some background material on paramagnetic resonance, is reviewed. With respect to the experimental application of the maser material, two cases can be distinguished, In the first, maser action is based mainly on a favorable relaxation time ratio in signal and idler transitions. It is shown that the relaxation time ratio can be changed artificially by a doping technique. Experimental evidence is presented for two such doping techniques, one self-doping, the other impurity doping. In the second case, maser action is based primarily on a favorable frequency ratio of signal and idler transitions. Maser experiments using this approach are described. In addition, excitation of unidirectional gain and attenuation by circular polarization is discussed. Properties of practical isolator materials are surveyed; they include high-concentration paramagnetic and polycryslolline ferrimagnetic materials.
In a three-level maser, a transition between two energy levels is saturated in order to produce an induced emission of power at a lower frequency corresponding to a transition between an intermediate energy level and one or the other of the two saturated levels. In this paper, certain effects are discussed which cannot be predicted from a theory in which the population differences alone are considered in this process. For instance, it is shown that, in some cases it is possible to obtain for the same system an induced emission of power at two frequencies given by the resonances between the intermediate energy level and both of the saturated levels. Also, it is shown that, even in the absence of inhomogeneous broadening of the spectral line, one can obtain a net induced emission at some portions of a resonant line and a net absorption at other frequencies within the line widths. Such effects become important even in early stages of saturation in cases where T1=T2{T}_{1}={T}_{2}T1​=T2​ which is true for ordinary gaseous systems and individual spin systems in a majority of very dilute paramagnetic solids. A complete theory is discussed for a gaseous system and extended to two limiting cases of paramagnetic materials with T1=T2{T}_{1}={T}_{2}T1​=T2​ and ${T}_{2}\ensuremath{\ll}{T}_{1}$ for each individual spin system. Furthermore, it is shown that for a saturating field of fixed frequency, the integral of induced power over the entire line is in full agreement with the results of a semiclassical treatment in which the population differences alone are considered. Some remarks are made as to the practicability of certain systems of this type.
The transient behavior of three-level paramagnetic masers is investigated theoretically. Exact solutions to three-level maser rate equations are developed and their significance discussed. The results indicate the possibility of obtaining enhanced maser action at low saturation by pulsed pumping. A new method for the measurement of spin-lattice relaxation times for a three-level maser is proposed. The method, in addition to certain well-known static measurements, requires the observation of the decay of the signal strength when the pumping field is switched off.
Masers, the microwave analog of lasers, are an important class of devices that can be used to generate highly coherent and stable microwave signals as well as quantum-limited amplification of microwave photons. Recently, the interest in such devices has increased, probably owing to experiments demonstrating the successful operation at room temperature of solid-state maser employing defects in diamond. These defects, called nitrogen vacancy (NV) centers, are comprised of a negatively charged substitutional nitrogen that replaces one of the diamond crystal lattice’s carbons and a vacancy adjacent to the excluded carbon atom. The performance of diamond-based masers, in terms of gain, noise, bandwidth, frequency of operation, and saturation, greatly depends upon the specific diamond crystal composition and its geometry in the respective microwave device. The currently available data on these issues are scarce and sometimes conflicting. It is, therefore, important to provide additional experimental data, complemented by theoretical analysis, to further optimize the required diamond material and thus enhance the capabilities of diamond-based maser technology. The latter is currently limited to operate only as an oscillator at very small bandwidths, with low saturation power. Here, we provide experimental results on a set of important parameters affecting diamond maser operation, such as the population and linewidth of the electron spin energy levels of the NVs under light illumination, as well as their relaxation times for several different diamond material compositions. These results are then used to point out which diamond material compositions and crystal geometry may be best suitable for maser-type applications.
By A. E. Siegman London: McGraw-Hill Publishing Co. Ltd. 1964. Pp. 582. Price 143s. 6d. This is a postgraduate book, partly for electronics engineers, who would find a good deal of the material difficult, because it is a comprehensive account of the subject which demands a good deal of familiarity with quantum mechanics. The contents include chapters on internal structure and energy levels of atoms, the magnetic properties of atoms, energy levels of paramagnetic maser materials, paramagnetic resonance and the dynamics of spin systems, the three level solid state maser and travelling wave maser.
The practical application of masers is often hindered by saturation effects of the maser material. Any strong signal frequency (e.g., the transmitter pulse of a radar system leaking into the maser) will cause saturation. This paralyzes the maser for a period of time, too long for most intended applications. In this report, a method is described which has been used in desaturating a four-level ruby maser. The saturation is caused by equalizing the populations of levels one, two, and three of the maser crystal by the leakage of the transmitter pulse, in addition to the action of the maser pump. The application of a desaturation pulse between levels one and four will then desaturate the maser restoring the excess population density between levels two and one. In a cavity maser, full recovery of the amplification capabilities has been achieved within less than 1 msec after the maser has been saturated. Repetition rates up to 120 cps are tolerable.
The paramagnetic resonance properties of Cr+++ions in Al2O3(ruby) were investigated theoretically and experimentally in order to obtain information necessary for the application of this material as active material in a three-level solid-state maser (3LSSM). Numerically computed energy levels, together with their associated eigenvectors, are presented as a function of applied magnetic field for various orientations of the magnetic field with respect to the crystalline symmetry axis. A more detailed discussion is devoted to energy levels, eigenvectors and transition probabilities at angles O°, 54.74° and 90°, where certain simple relations and symmetries hold. Paramagnetic spectra for signal frequencies between 5 and 24 kmc are shown; agreement between computed and measured resonance fields is satisfactory.