HIGHER-ORDER   NONLINEAR   EFFECTS

As we have mentioned earlier, not all materials have a nonlinear a₂ coefficient only 10% of all natural crystals do but all materials, including glass, have a third-order coefficient. However, as this coefficient is extremely small compared

Figure 1.1. Generation of high-order harmonics.

to a second-order coefficient, much less efficiency is expected for third-harmonic generation. Calcite, for example, can be used as a third-harmonic generator with efficiencies of less than 10^-5. Alternatively, series nonlinear crystals can be used to generate third-harmonic light: the first crystal producing second-harmonic light at 532 nm and the second crystal mixing this light with the original 1064-nm fundamental wavelength, the second crystal often being type I phase-matched. Both methods are outlined in Figure 1.1.