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The relative efficiency of a number of materials which generate the second harmonic of the ruby laser line has been measured. Potassium dihydrogen phosphate (KDP) was the most efficient crystal measured. There seems to be no general correlation between the second harmonic intensities of the crystals investigated and their physical properties, e.g., crystal symmetry, and whether or not the crystal is ferroelectric. In all cases where a second harmonic output was expected it has been observed. Of particular interest is guanidine carbonate which did not exhibit second harmonic generation. This result is in agreement with recent theoretical considerations which predict that second-order polarization should not occur in crystals with point symmetry D4 or D6 even though these crystals may be piezoelectric.
Savage et al. (Sat,) studied this question.