Picosecond Z-scan measurements on bulk GaN crystals

Bulk GaN crystals were characterized by using picosecond laser pulses at λ=0.527 μm and Z-scan techniques. The role of the free-carrier absorption was evaluated by a dynamical, pump-and-probe-type transmitivity measurement. The values of two-photon absorption coefficient (17–20 cm/GW) and refractive index changes at high optical irradiances due to bound (n2=−4×10−12 esu) and free (σr=−1.0×10−20 cm3) electrons in that material were determined.