Two-photon absorption spectroscopy and polarization dependence of Gallium phosphide

The two-photon absorption (2PA) coefficient of Gallium phosphide (GaP) was measured by open-aperture z-scan in the spectral range 650 – 1200 nm. Anisotropy in the polarization dependence of 2PA is then used to determine the two independent components of the imaginary part of the third-order nonlinear susceptibility tensor corresponding to the zinc blende crystal structure of GaP. These results are then compared to first-principles calculations using length gauge theory. The strong 2PA in GaP qualifies this material to be used as a reference when extending this experimental and theoretic technique to other crystal and nanocrystal materials.