Noncollinear interactions in urea crystals used for nonlinear optical frequency conversion

Simple general relationships are obtained for the calculation of noncoUinear interactions of light beams in nonlinear single crystals. These relationships are used to determine the optimal (from the point of view of the effective nonlinear susceptibility) directions of propagation of optical waves in urea single crystals used for frequency doubling of neodymium laser radiation and addition of the resultant second harmonic to the fundamental-frequency radiation. It is shown that for all types of interaction (e^ωe^ω → o^2ω;e^ωe^2ω → o^3ω; e^ωo^ω → o^2ω; e^ωo^2ω → o^3ω; e^ωo^2ω → o^3ω) there is a wide range of directions for which the relative effective nonlinear susceptibility exceeds 0.9 of its maximum value, which is 1.5–2 times greater than in the collinear cases