Artificial index gratings, which are composed of binary microstructures of sizes less than the incident wavelength, are analyzed as functions of the filling factor or duty cycle of the microstructures. Different models for calculating the optimum duty cycles to produce high blazed diffraction efficiency are compared. Blazed binary grating designs in a material with a refractive index n = 2 show theoretical diffraction efficiencies as high as η = 80%. In the semiconductor material silicon, which has a refractive index n = 3.4, theoretical diffraction efficiencies as high as η = 70% are predicted.