纳秒脉冲激发下相变材料的热致非线性吸收及其机理研究
文献类型：学位论文
| 作者 | 刘爽 |
| 学位类别 | 博士 |
| 答辩日期 | 2012 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 干福熹 ; 魏劲松 |
| 关键词 | 非线性吸收 相变材料 带填充效应 带隙缩短 |
| 其他题名 | Thermal-induced nonlinear absorption of phase change materials stimulated by nanosecond laser pulse |
| 中文摘要 | 硫系相变材料广泛应用于相变光存储、相变随机存储（PCRAM）、近场超分辨结构掩模层和激光直写微纳图形和结构制造中的热刻蚀材料，这些应用都涉及到相变材料与纳秒脉冲光的相互作用和纳秒时间内的相变过程。传统的观点认为，在这些应用中相变材料的光学性质是不变的，而近年来的研究结果表明，在纳秒脉冲激发下相变材料表现出光学非线性特性，尤其是非线性吸收特性。目前，针对相变材料光学非线性性质及其微观机理还缺乏系统的研究。本论文将纳秒脉冲激光作为激发源，利用改进型Z扫描装置测量了典型的相变材料的非线性吸收特性。为了研究非线性吸收效应的微观机理，本文利用变温椭偏仪测量了相变材料的吸收系数随温度的变化，证明了温度升高导致的吸收系数变化是上述非线性吸收的原因。本文还利用变温XRD和变温傅里叶红外光谱估算了晶格膨胀和带隙萎缩导致的吸收系数变化。同时计算了相变材料的能带结构，探讨了电子跃迁在上述非线性吸收中可能发挥的作用。本论文主要的内容包括以下几点： 1. 利用改进型Z扫描测量了非晶态Sb70Te30、Sb2Te3、Ge2Sb2Te5和AgInSbTe在405nm和658nm纳秒脉冲激发下的非线性吸收特性。测量结果表明，非晶态Sb70Te30、Sb2Te3、Ge2Sb2Te5和AgInSbTe在405nm和658nm都表现出反饱和反射特性和反饱和吸收特性。通过观察非晶态Sb70Te30、Sb2Te3、Ge2Sb2Te5和AgInSbTe薄膜在不同入射功率作用后的光学照片和原子力形貌图可以知道，在合适的入射功率下材料的激光作用点发生了部分晶化。 2. 利用了改进型的Z扫描装置测量了晶态相变材料在405nm和658nm波长纳秒脉冲激发下的非线性吸收特性。测量结果表明，晶态相变材料的非线性吸收可以分为两类效应：1）晶态Sb，Sb70Te30和Sb2Te3在405nm和658nm都显示出非线性饱和吸收特性，也就是入射功率越高，薄膜透射率越高，对应的非线性吸收系数 在102 m/W数量级。2）Ge2Sb2Te5和AgInSbTe在658nm显示出反饱和吸收特性，另外Ge2Sb2Te5和AgInSbTe在405nm显示出饱和反射特性和反饱和吸收特性。这两个效应都表明也就是入射功率越高，薄膜透射率低，对应的非线性吸收系数 在102 m/W数量级。 3. 利用变温椭偏测量了晶态Sb，Sb2Te3，Sb70Te30，Ge2Sb2Te5和AgInSbTe在405nm和 658nm的吸收系数随温度的变化。测量结果可以分为两种现象：1）Sb，Sb2Te3和Sb70Te30的吸收系数随温度升高而降低，对应于非线性饱和吸收和反饱和反射现象。2）Ge2Sb2Te5和AgInSbTe的吸收系数随温度升高而增大，对应于非线性反饱和吸收和饱和反射现象。进一步的数值计算表明，热致吸收系数的变化导致的非线性吸收系数 绝对值在102 m/W数量级，与上述Z-scan非线性测量结果吻合的较好，从而证明了热效应是非线性吸收现象的起源。 4.任何激发现象都涉及到电子的跃迁，为了探讨电子跃迁对相变材料非线性吸收的贡献，本文中计算了Sb、Sb2Te3和Ge2Sb2Te5的能带结构和态密度分布。计算结果表明Sb和Sb2Te3是直接带隙的窄带半导体材料，而Ge2Sb2Te5是非直接带隙半导体材料。另外，本论文还利用变温傅里叶红外光谱测量了Sb，Sb70Te30和Sb2Te3的光学带隙随温度的变化和利用变温椭偏仪测量了Ge2Sb2Te5和AgInSbTe的光学带隙随温度的变化，测量结果表明Sb，Sb2Te3，Sb70Te30，Ge2Sb2Te5和AgInSbTe的带隙都随着温度的升高而缩短。更进一步的数值计算表明，对于直接带隙的Sb和Sb2Te3材料，一方面在入射激光作用下材料存在着带填充效应，导致一个102 m/W数量级的非线性吸收系数，另一方面，材料的能带带隙随温度升高而减小导致吸收系数增大，不过带隙缩短导致的吸收系数增大幅度很小。另外，对于非直接带隙的Ge2Sb2Te5，带隙缩短导致的吸收系数变化远小于热效应的影响。为了解释Ge2Sb2Te5的非线性吸收系数 随入射功率升高而反转，我们提出了一个五能级模型。 |
| 英文摘要 | Chalcogenide phase change materials (PCMs) has been extensively used as the recording materials for optical data storage and phase change random access memory （PCRAM）, the mask layer of super-resolution near-field structure (Super-RENS) and thermal-lithography materials of micro pattern and structure fabrication. All these applications involve the interaction between PCMs and laser pulses. The conventional viewpoint assumes the invariant of optical properties of PCMs under laser pulse irradiation. However, recent reports strongly suggest that nonlinear optical properties, especially the nonlinear absorptions, play an important role in the function of PCMs, little work concerning the nonlinear optical properties of PCMs has been conducted. In this dissertation, the nonlinear absorption characteristics of typical PCMs have been investigated by an improved Z-scan technique with nanosecond laser pulse used as excitation sources. To explore the internal mechanism of nonlinear absorption characteristics, the varying-temperature ellipsometry measurements have been performed to study the variance of absorption coefficient with temperature, The measurements suggest that the nonlinear absorption characteristics of PCMs originate from thermally-induced absorption change. Furthermore, the varying-temperature X-ray diffraction and Fourier transformed infrared spectroscopy (FT-IR) measurements have been performed to estimate the contribution to absorption coefficient change from lattice expansion and band gap shrinkage. Besides, the band structures are calculated with first-principle theory, and the possible electronic contributions to nonlinear absorption are discussed. The main content of this dissertation is listed below: 1. The nonlinear absorption characteristics of amorphous Sb70Te30, Sb2Te3, Ge2Sb2Te5, and AgInSbTe films have been investigated with improved Z-scan technique at 405nm and 658nm laser wavelength. The measurement results show that these films exhibit an anti-saturable reflection (ASR) and an anti-saturable absorption (ASA) at 405nm and 658nm, respectively. To inspect the structural change of amorphous PCMs after the laser pulse irradiation, the optical microscope (OM) and atomic force microscope (AFM) pictures of irradiation area has been taken. The pictures suggest the amorphous PCMs films can be partially crystallized under proper incident laser intensity. 2. The nonlinear absorption characteristics of crystalline Sb, Sb70Te30, Sb2Te3, Ge2Sb2Te5, and AgInSbTe films has been investigated with improved Z-scan technique at 405nm and 658nm laser wavelength. The measurement results show the nonlinear absorption characteristics of these films, it can be separated into two types: 1)the crystalline Sb, Sb70Te30, and Sb2Te3 films exhibit saturable absorption (SA) at both 405nm and 658nm, which means larger transmittance when the incident laser intensity becomes higher. Data fitting produces a nonlinear absorption coefficient β as large as102 m/W; 2) the crystalline Ge2Sb2Te5 and AgInSbTe film exhibit ASA and saturable reflection (SR) at 405nm, and crystalline Ge2Sb2Te5 and AgInSbTe film exhibit ASA at 658nm, these effects means smaller transmittance when the incidence intensity becomes higher. Data fitting produces a β as large as102 m/W. 3. The linear absorption coefficient of crystalline Sb, Sb70Te30, Sb2Te3, Ge2Sb2Te5, and AgInSbTe films at 405nm and 658nm has been determined with varying-temperature ellipsometry measurement. The measurement results can be separated into two categories: 1) the absorption coefficient of crystalline Sb, Sb70Te30, Sb2Te3 decrease with temperature increase, suggesting a SA effect; 2) the absorption coefficient of crystalline Ge2Sb2Te5 and AgInSbTe films increases with temperature increase corresponding to nonlinear anti-saturable absorption and saturable reflection. Further numerical estimation shows that the thermally-induced nonlinear absorption coefficient |βth| is in the order of 102 m/W, which is consistent with the nonlinear absorption β determined by Z-scan measurement. This agreement indicates that thermally-induced absorption variation is the origin of nonlinear absorption characteristics. 4. Since all excitation involves the transition of electrons, the electronic contribution to nonlinear absorption is discussed. The band structures and distribution of density of states (DOS) of Sb, Sb2Te3, and Ge2Sb2Te5 are calculated, the calculation shows that Sb and Sb2Te3 are direct band gap semiconductor, whereas Ge2Sb2Te5 is indirect band gap semiconductor. Besides, the temperature dependence of optical band gap of crystalline Sb, Sb70Te30, and Sb2Te3 have been determined with varying temperature Fourier transformed infrared spectroscopy, and the temperature dependence of optical band gap of crystalline Ge2Sb2Te5, and AgInSbTe films have been determined with varying temperature Ellipsometry. The measurement shows that the optical band gap shrinks with increasing temperature. Further numerical calculation shows that for crystalline Sb and Sb2Te3 with direct band gap, the band filling effect may induce a nonlinear absorption coefficient in the order of 102 m/W, while the band gap shrinkage with temperature has little effect on the absorption coefficient. For crystalline Ge2Sb2Te5 with indirect band gap, the absorption coefficient change induced by band gap shrinkage is much smaller than corresponding value induced by thermal effect. To explain the reversal of nonlinear absorption of crystalline Ge2Sb2Te5, a five-level band model is proposed. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15719]  |
| 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 | 刘爽. 纳秒脉冲激发下相变材料的热致非线性吸收及其机理研究[D]. 中国科学院上海光学精密机械研究所. 2012. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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