This paper is concerned with the experimental determination and the interpretation of the relative magnitudes of the two independent coefficients, $d_{11}$ and $d_{14}$, that appear in the tensor which describes the symmetry of second harmonic generation (SHG) of optical maser beams in quartz. Experimental data on these co-efficients aid in determining the physical process involved in optical SHG. Data have been obtained for both ruby (6934 Å) and CaW${\mathrm{O}}_4$: ${\mathrm{Nd}}^{+3\mathrm{}}$ (10 582 Å) unfocused laser beams. These experiments, which failed to give any evidence of SHG due to $d_{14}$, show that $\frac{d_{14}}{d_{11}}<\frac{1}{30}$ for the ruby laser, and $<\frac{1}{40}$ for the Nd laser. The result $d_{14}\ll d_{11}$ shows that the mechanism involved in SHG in quartz is nearly lossless and dispersionless at the frequencies of the laser beams and their second harmonics. This further shows that in quartz the linear electro-optic effect and optical SHG cannot be due to the same mechanism. It is concluded that optical SHG in this material is due to a high-frequency electronic mechanism.

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