Picosecond Z-scan measurements on bulk GaN crystals

Bulk GaN crystals were characterized by using picosecond laser pulses at $\text{λ=0.527\hspace{0.05em}μ}\mathrm{m}$ 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 ${\mathrm{(n}}_2{\text{=−4×10}}^{\text{−12}} \mathrm{esu})$ and free ${\mathrm{(\sigma }}_r{\text{=−1.0×10}}^{\text{−20}} {\mathrm{cm}}^{\mathrm{3}})$ electrons in that material were determined.