Investigation on the optical characteristic of calcite crystal and α-BBO crystal in direct third-harmonic generation process

Calcite crystal and α-BBO crystal of which the microstructure possesses symmetric center with delocalized π bond were selected to produce phase-matched direct third-harmonic. In order to obtain a high third-order nonlinear conversion efficiency, the parameters including the walk-off angles, phase-matching angles, acceptance angles, acceptance wavelengths and group-velocity index in calcite crystal and α-BBO crystal were calculated within I (ooo→e), II (ooe→e) and III (oee→e) phase-matching types. The tinier walk-off angle and the larger acceptance angle in α-BBO crystal may lead to a better third-order nonlinear interaction in UV region. The smaller phase-matching angle in calcite crystal makes it possible to produce third-harmonic with shorter wavelength even in deep UV region. In the experiment, phase-matching angles and phase-mismatching angles of type-II which was the most efficient phase-matching type were measured in calcite crystal and α-BBO crystal. The results may provide a reference for further study on direct third-harmonic generation.