Abstract

The n₂ of BaF₂, MgF₂, LiF, sapphire (Al₂O₃), and diamond are measured with picosecond pulses from a Q-switched, mode-locked Nd:YAG laser at wavelengths of 1064 nm, 532 nm, 355 nm, and 266 nm with the corresponding pulsewidths of 30 ps (FWHM), 21 ps (FWHM), 17 ps (FWHM), and 15 ps (FWHM), respectively. The experimental data is compared to the recently published two-band model,¹ which predicts the nonlinear refractive index as a function of the incident photon energy to the material's energy gap, ħω/E_g. The two-photon absorption coefficient is measured in diamond at 355 nm and 266 nm and yields values of β = 0.3 cm/GW and β = 0.7 cm/GW, respectively. The dispersion of n₂ is measured by the Z-Scan technique,² where the transmittance of a focused Gaussian beam with constant energy is monitored through an aperture in the far-field as a function of the sample's position with respect to focus. This technique gives a direct measurement of the sign and magnitude of the nonlinear refractive index. By removal of the far-field aperture, the two-photon absorption (2PA) coefficient of the material can be measured. For photon energies in the range E_g/2<ħω < E_g, the material will exhibit 2PA.

© 1992 Optical Society of America