基于飞秒激发Z扫描实验技术, 研究了氮化镓薄膜和不同铝掺杂含量的掺铝氮化镓(以下简称铝镓氮)薄膜的超快非线性光学响应特性。在开孔Z-scan测试中, 纯GaN晶体薄膜表现出典型的双光子吸收特性, 双光子吸收系数为3.5 cm/GW, 且随着激发光强的增大而逐渐减小。随后测试了不同铝掺杂含量的AlxGa1-xN薄膜的非线性吸收系数。结果表明, 随着铝掺杂摩尔分数的提高(0, 19%, 32%, 42%), 非线性吸收系数逐渐减小(18, 10, 6, 5.6 cm/GW)。结合半导体非线性吸收理论分析, AlxGa1-xN薄膜材料的非线性过程主要是双光子吸收主导非线性响应物理过程。实验结果与半导体双光子吸收过程Sheik-Bahae理论符合得很好。

The ultrafast nonlinear absorption response of GaN and AlxGa1-xN films was studied by employing conventional femto-second Z-scan measurements. In the Z-scan, GaN films exhibit typical two-photon absorption property and with a two-photon absorption coefficient of 3.5 cm/GW. Simultaneously, the two-photon absorption coefficient decreases with the increase of the excitation intensity. GaN films possess excellent nonlinear optical property which is dominated by two- or multi-photon absorption. The Z-scan measurement was further used for AlxGa1-xN films with different Al element content. It is found that the two-photon absorption coefficient of AlxGa1-xN films closely dependent on the mole fraction of Al element. The two-photon absorption coefficient decreases from 18 cm/GW to 5.6 cm/GW with the increase of Al mole fraction from 0 to 0.42. Considering the conventional semiconductor nonlinear absorption theory, it is believed that the observed nonlinear absorption originates from the two-photon absorption dominant nonlinear response. The experimental results are in good agreement with the Sheik-Bahae theoretical prediction for two-photon absorption coefficient.