采用等离子体增强化学气相沉积和后退火的方法制备了纳米锗/氮化硅(nc-Ge/SiNx)多层薄膜。借助Raman光谱仪对其微结构进行表征, 测得样品的晶化率大于46%。由样品的光吸收谱可知, nc-Ge的尺寸越小, 其光学带隙越大。利用Z扫描技术对样品的非线性光学特性进行研究, 以波长为1 064 nm、脉宽为25 ps的锁模激光作为激发光, 测得样品的非线性折射率系数在10-10 cm2/W数量级。实验结果表明, 通过改变nc-Ge的尺寸可以使材料的非线性光学折射率由自散焦转变为自聚焦特性, 而负的非线性折射率系数可归因于两步吸收产生的自由载流子散射效应。当激发光强增大时, 在锗层厚度为6 nm的多层膜中同时存在两步吸收过程和饱和吸收过程。两种非线性光学吸收过程之间的竞争是样品呈现不同非线性光学特性的主要原因。

Nanocrystalline Ge (nc-Ge)/SiNx multilayers were fabricated by plasma enhanced chemical vapor deposition technique and post-thermal annealing method. The microstructure was characterized through Raman spectroscopy, and the calculated crystallinity was higher than 46%. The optical absorption spectra reveal that the band gap enlarges as the grain size of nc-Ge decreases. The nonlinear optical properties of nc-Ge/SiNx multilayers were investigated through Z-scan technique by using a mode-locking laser with 25 ps pulse duration at 1 064 nm as the pump laser. The measured nonlinear refractive coefficient was about 10-10 cm2/W. It is interesting to found the nonlinear optical refraction changes from the self-defocusing to self-focusing with the increasing of the grain size of nc-Ge, and the negative nonlinear refractive coefficient can be attributed to the free carries dispersion generated by tow-step absorption process. Moreover, two-step absorption and saturation absorption process coexist in the multilayers with 6 nm Ge layer as the pump intensity increases. The different optical nonlinearities are mainly originated from the competition between the two nonlinear absorption processes.