Wood’s anomalies are investigated by using a microwave spectrometer operating at a wavelength of 3.8 mm. For S-polarization, high surface conductivity, periodicities of 2λ and 3λ, and groove depths from λ/8 to (5λ)/4, rectangular profile gratings are shown to exhibit anomalies which can be explained by simple interference between the orders due to the incident beam and the orders due to the passing-off order if it is considered as a second beam incident at ±90°. Artificial passing-off orders are formed, and these permit one to change the natural anomalies from bright to dark and vice versa. Interference of the orders owing to the incident and imposed beams at anomalous angles is demonstrated by measuring the power in each of the overlapping orders separately and comparing the sum of their amplitudes with the value measured at the anomalous angle. For all cases tested the results are satisfactory for either bright or dark anomalies. By adjusting the angle of a second incident beam to calculated values, artificial anomalies, i.e., anomalies at angles of incidence where they do not normally occur, are created. These anomalies cannot be distinguished from their natural counterparts.
Frederick C. Evering, Jr., "Artificial Diffraction Anomalies for Gratings of Rectangular Profile," Appl. Opt. 5, 1313-1317 (1966)