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A perturbation expansion of the sublattice magnetization of a spin ½ antiferromagnet is developed. The theory proceeds from a Fermion representation of the spin operators for spin ½, in which an auxiliary (drone) set of Fermi operators is introduced to account for the boson character of spin operators attached to different lattice sites. The development of the present theory is parallel to that derived earlier by Mills, Kenan, and Korringa, with the role of the ``contracted lines'' of the earlier theory being played by the drone lines of the present theory. Integral equations are derived for the single-particle spin propagator. A resummation over a class of graphs contributing to the kernel, which gives the spin-wave result at low temperatures, is outlined.

Performance-limiting asymmetric distortion is observed in the spectra of fundamental pulses transmitted through GaAs-Al(0.9)Ga(0.1)As multilayer waveguides designed for surface-emitted second-harmonic generation. This behavior is attributed to refractive-index changes resulting from the accumulation of free carriers created by two-photon absorption in the GaAs layers. Numerical simulations of the intensity-dependent spectra by use of the separately measured two-photon absorption coefficient are shown to be in good agreement with the observed spectra.

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Normal-incidence planar-optical waveguide-imbedded phase gratings of finite aperture width and length are analyzed with Svidzinskii's (Sov. J. Quantum Electron. 10, 1103 (1980)] two-dimensional Braggdiffraction theory. Svidzinskii's characteristic-grating equations are adapted for the rectangulargrating case, and an overlap integral is used to extend the theory to account for the mode structure of the waveguide. The combined theory is used to optimize the throughput of a system composed of an input grating coupler, a waveguide, and an output grating coupler for both the highly multimode (thickwaveguide) and the few-mode (thin-waveguide) cases.