超分辨结构中的光学非线性
文献类型：学位论文
| 作者 | 刘静 |
| 学位类别 | 硕士 |
| 答辩日期 | 2010 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 魏劲松 |
| 关键词 | 超分辨，相变材料，复合薄膜，非线性，能带结构 |
| 其他题名 | Optical Nonlinearity in the Super Resolution Structures |
| 中文摘要 | 本文系统地研究了用于高密度光存储以及激光直写的超分辨结构中材料的非线性性质以及非线性效应。利用改进型的z扫描装置，在实验中测量出相变材料以及纳米复合材料在低功率激光激励下的非线性效应，同时对各种类型非线性内部形成机理进行了较为深入的分析，最后利用扫描近场光学显微镜(SNOM)对这些材料的非线性超分辨结构效应进行了探讨。 针对传统的z扫描装置所存在的不能调谐的弱点，一种改进型的z扫描装置被提出；作为激励光源的连续光被声光调制器以及信号发生器调制成功率以及脉宽可变，同时，在整个的z扫描系统中增加了观察部分，使得可以实时地对被测样品表面的形貌进行观察；通过对开孔及闭孔数据的模拟得到材料的非线性折射及非线性吸收系数。 晶态Sb2Te3以及Sb，Ge-Sb-Te等相变材料均呈现非线性饱和吸收特性。这些材料的非线性吸收特性都是来源于其直接带隙跃迁的带填充效应。采用Sb2Te3作为这些材料的代表，基于第一性原理，我们计算了晶态Sb2Te3的能带结构、态密度分布，以及电荷密度分布，分析表明晶态Sb2Te3的饱和非线性吸收性质是来源于直接带隙中的带填充效应；而非线性折射系数则是激光作用下材料吸收热量导致折射率发生变化的结果。另外，结合晶态Sb2Te3的非线性折射性质以及改进型的z扫描装置，我们提出了一种测量材料热光系数的方法，其测量结果和利用变温椭偏装置测量结果以及理论计算结果吻合较好。 Sb以及Ge-Sb-Te均呈现非线性饱和吸收性质，结合Sb以及Ge-Sb-Te的能带，我们也认为其非线性吸收的来源是带隙中的带填充效应；光子激发的载流子将占据导带，使得价带的电子很难再跃迁到导带，从而使得在激光强度越大的情况下，材料的吸收反而越小。 与晶态Sb2Te3等材料的的饱和非线性吸收性质相反的，相变材料AgInSbTe具有典型的反饱和吸收性质。通过对实验参数的调整以及内部机理的分析，我们认为载流子吸收导致等效多光子吸收。非线性吸收系数随激光功率的增加而增大，有效吸收光子数则在不断减小。 作为超分辨结构中经常使用的多晶硅，具有正的非线性吸收系数以及正的非线性折射系数，而且多晶硅的非线性系数不随着激励激光功率的增加而改变。结合计算得到的多晶硅的间接带隙的能带结构，我们认为其非线性来源是激发态吸收效应。 与其他的非线性来源不同，我们还采用共溅的方法制备了金属-半导体纳米复合薄膜Ag-Si，z扫描测量显示这中复合薄膜呈现非线性饱和吸收与自聚焦效应。通过分析，我们认为其非线性的来源是表面等离子共振；激光的初始作用使得样品中的金属粒子发生聚集，然后在激光的作用下，大尺寸粒子的散射与金属粒子表面的表面等离子体相耦合，实现了在低功率激光激发下的巨大非线性。 为验证这些材料在光学超分辨特性，我们采用扫描近场光学显微镜对透过材料的激光光斑进行近场扫描。对AgInSbTe材料，我们在实验中明显观察到了近场光斑的限幅效应，高斯光斑顶端被钝化；相应地，在对材料吸收光斑的模拟中，我们发现AgInSbTe的吸收光斑可以得到明显的减小；基于AgInSbTe材料的等效多光子吸收效应以及熔化阈值效应，数值计算表明在激光直写领域AgInSbTe可以实现纳米量级的尺寸记录。 同时，我们还发现透过晶态Si、非晶态Sb2Te3等材料的激光光斑并没有明显的减小；而透过晶态Sb2Te3、Ag-Si复合材料的激光光斑有明显的减小。结合这些材料的非线性性质，我们认为，超分辨结构中的超分辨效应主要是来源于材料的饱和非线性吸收特性，而材料的自聚焦效应影响不是非常明显。 |
| 英文摘要 | In this dissertation, the optical nonlinearity of the materials applied in the super resolution structure is studied and the nonlinear effects in the super resolution structures are calculated and discussed. Based on the improved z-scan apparatus, giant optical nonlinear coefficients (including nonlinear absorption coefficient and nonlinear refractive index) of phase change materials and nanocomposites were measured under the low laser power excitation. Different nonlinear kinds of these materials are enumerated and the internal mechanisms are discussed respectively. Considering the giant nonlinearity in the super resolution structure, there should be some improvement about the light transportation in the structures and the scanning near-field optical microscopy measurements are taken. Some improvements are proposed based on the traditional z-scan method. The acousto-optic modulator and the signal generator are added to modulate the cw laser beam into pulses; and in the z-scan system the observation parts are added to get the real time optical picture of the sample while measuring. Nonlinear absorption coefficient and refractive index can be got from the open mode and closed mode transmittances respectively. As the typical kinds of band filling type nonlinearity, crystalline Sb2Te3, Sb, Ge-Sb-Te possess nonlinear saturable absorption characteristic. These materials have narrow direct band gaps and under the laser irradiation, the photon absorption induced free carrier on the conduction band will prevent the electron jumping from valence band onto the conduction band. In this part, we take crystalline Sb2Te3 as an example and discuss the internal mechanism. Based on the first principle theory, the band structure and density of states, as well as the partial charge distribution are calculated, the nonlinear absorption characteristic is regarded to come from the band filling model, while the nonlinear refraction comes from the therma-optical effect, the theoretical calculations are in good accordance with the experimental results. In addition, based on the nonlinear refraction of Sb2Te3 and the improved Z-scan appratus, a new method for measuring the thermo-optic coefficient was proposed. For the free carrier absorption type optical nonlinearity, the typical example is the AgInSbTe material, which possesses reverse saturable nonlinear absorption characteristic and positive nonlinear absorption coefficient. For this kind material, an effective multi-photon absorption mechanism is proposed and the numerical expression is given. Under the laser irradiation, the free carrier would absorb photon subsequently, which is the effective multi-photon absorption totally. Exited state absorption model is taken as another type of the reverse saturable nonlinear absorption to explain the nonlinear absorption behaviors of polycrystalline silicon. In the measurements, the nonlinear absorption coefficient does not change with different laser intensity. We think this type nonlinearity results from the larger absorption cross section for the excited state comparing with that for the ground state in the indirect band gap structures. Surface plamon resonance is responsible for the giant optical nonlinearity of the metal-semiconductor nanocomposites under low power excitation. The nonlinearity of Ag-Si includes saturable nonlinear absorption and self-focusing effect. Z-scan measurements indicates that under the primary laser irradiation, the metal particles aggregated and in the following repeated measurements the scattering effect from the aggregated particles couple with the surface plasmon, deducing enhancement of the local field. To get the exact super-resolution effects of the above discussed materials in the super resolution structures, we take scanning near-field optical microscopy to get the near field optical spot pictures through these materials. For the reverse saturable absorption material AgInSbTe, optical limiting effect and transmitted beam expansion are observed in the experiments; correspondingly the simulation indicates that the effective absorption spot can shrink effectively. Based on the melting threshold property and the former effective multi-photon absorption property discussed above, numerical simulation suggests that optical recording or lithography beyond diffraction limit can be realized in the AgInSbTe material. For the nonlinear materials possessing saturable absorption characteristic, we concluded that after these materials the transmitted laser spot would shrink effectively, despite of the nonlinear refraction. Thus we think that in the super resolution structure the super resolution effect comes from the nonlinear absorption property, other than the previous reported nonlinear refraction effect. Numerical simulations are in good accordance with the experimental observations. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/16671]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 刘静. 超分辨结构中的光学非线性[D]. 中国科学院上海光学精密机械研究所. 2010. |
入库方式： OAI收割
来源：上海光学精密机械研究所
浏览0
下载0
收藏0
其他版本
除非特别说明，本系统中所有内容都受版权保护，并保留所有权利。