飞秒激光诱导的发光现象及其机制与应用
文献类型：学位论文
| 作者 | 王新顺 |
| 学位类别 | 博士 |
| 答辩日期 | 2008 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 徐至展 |
| 关键词 | 多光子过程 上转换 缺陷 束缚激子 限幅效应 电荷迁移态 价态变化 Bloch方程 三维立体显示 |
| 其他题名 | Phenomena, Mechanisms and Applications of Femtosecond Laser Induced Luminescence |
| 中文摘要 | 飞秒脉冲激光与物质的相互作用是当今物理前沿的研究热点之一。多光子过程是物质在超强激光脉冲作用下产生的高阶非线性作用，它是一个阈值过程，其吸收强度与激发光光强成n次方关系，其泵浦波长远小于单光子过程。正是由于这些特点，多光子过程相对于单光子过程才具有散射损失小、穿透深度大、作用区域小（有可能突破衍射极限）、空间分辨率高和空间选择性强的优点。随着超短脉冲激光技术的发展，双光子过程已经在荧光显微成像、三维光信息存储、光学微加工、频率上转换激射、光学限幅、三维立体显示以及光生物学等许多领域得到了广泛的应用。三光子及三光子以上的多光子过程也已随着飞秒激光技术的发展而在很多介质中得以实现，三光子及三光子以上的多光子过程的研究已越来越受到人们的重视。 本论文研究了一些无机光功能晶体材料在飞秒激光作用下产生的与多光子过程有关的发光现象及其机制与应用。主要的创新性研究成果有： 1．氧化锌晶体的双光子和三光子吸收特性研究： (1) 首次发现飞秒激光在氧化锌晶体中诱导产生了束缚激子和缺陷。实验中发现氧化锌晶体在800nm飞秒激光激发下的紫外发光与267nm的单光子激发相比发射峰发生了红移，而蓝绿发光则发生了蓝移。利用光致缺陷及束缚激子的模型，解释了这一现象。我们认为，飞秒激光在氧化锌晶体中由于三光子过程产生的自由电子而诱导了束缚激子及锌空位和锌间隙缺陷；束缚激子发光导致了紫外发光峰的红移，而新的缺陷发光则导致了蓝绿发光峰的蓝移。氧化锌的偏振发光实验及发光随泵浦光波长变化的实验验证了这一想法。 (2) 由实验测得的上转换发光强度随泵浦光强度变化的关系，推断出氧化锌晶体在800 nm飞秒激光激发下诱导的395 nm上转换发光是三光子带-带激发过程，而500 nm的上转换发光是直接双光子缺陷吸收过程。 (3) 根据实验测得的氧化锌入射激光强度与透射激光强度的关系，采用非线性拟合曲线的方法拟合出了双光子和三光子吸收的耦合系数和的数值，并计算了氧化锌晶体在800nm处的双光子和三光子吸收截面2和3。其值分别为=0.2018cmGW-1、 =7.102×10-3cm3GW-2、2=1.189×10-51cm4s和3 =1.040×10-80 cm6s2。由于氧化锌在800nm处具有很强的双光子和三光子吸收特性，所以它对入射光具有很好的限幅和稳幅作用。 2．飞秒激光在Yb:YAG晶体中诱导的色心及价态变化研究： (1) 首次发现飞秒激光在Yb3+:YAG晶体中诱导了Yb3+离子的永久性价态变化及色心的形成。实验发现Yb3+:YAG晶体在800nm飞秒激光作用下诱导的上转换发光不同于文献中Yb3+离子的电荷迁移态发光，而与Yb2+离子的5d到4f的发光及缺陷发光相一致。这说明飞秒激光在Yb3+:YAG晶体中诱导了Yb3+离子的永久性价态变化及色心的形成。通过Yb3+:YAG晶体在800nm飞秒激光辐照前、后及辐照后再退火的吸收谱验证了这一结论。 (2) 利用光激发空穴模型解释了Yb3+离子的永久性价态变化及色心的形成。永久性Yb2+离子的形成是由于处在基态的Yb3+经800nm飞秒激光激发到电荷迁移态（CTS）后，再经激发而使配位体O2-中的正空穴脱离Yb2+离子的束缚到达价带而形成的，而Yb:YAG晶体中F色心的形成是由氧离子空位俘获由飞秒激光通过多光子过程在Yb:YAG晶体中诱导的自由电子而形成的。 3．飞秒激光诱导的Nd:YVO4晶体的上转换发光研究： (1) 根据实验测得的飞秒激光诱导的上转换发光强度随泵浦光强度的变化关系，推断出了Nd:YVO4晶体在800nm飞秒激光激发下诱导的上转换发光在泵浦光功率比较低时来自于三光子激发过程，而在泵浦光功率比较高时则可能自于四光子、五光子或六光子激发过程。 (2) 实验上首次发现飞秒激光诱导Nd:YVO4晶体的上转换发光强度大于线性荧光强度，而半导体连续激光诱导的上转换发光强度却远远小于线性荧光强度。这说明飞秒激光诱导的上转换发光效率比同样波长的半导体连续激光诱导的上转换发光效率高。 (3) 根据Nd3+离子的三能级模型，利用Bloch方程计算出了在800nm飞秒激光作用下Nd3+各个能级的布居数随着时间的变化，发现与上转换发光强度成正比的最高激发态的布居数比与线性荧光强度成正比的中间激发态4F5/2的布居数多，即从理论上说明了飞秒激光诱导的上转换发光强度比线性荧光要强，这是与实验结果相一致的。 4．飞秒激光诱导的Er:YAG晶体和Pr3+:Y2SiO5晶体的上转换发光与单光束上转换真三维立体显示研究： (1) 首次发现了Pr3+:Y2SiO5晶体的直接三光子上转换发光。根据实验测得的Er3+:YAG和Pr3+:Y2SiO5晶体的上转换发光强度随泵浦光光强的变化关系，判断出800nm飞秒激光诱导的Er3+:YAG晶体的上转换发光来自于逐级的双光子过程，而Pr3+:Y2SiO5晶体的上转换发光则来自于直接的三光子过程。 (2) 实验发现Er3+离子的550nm上转换发光强度远大于其它可见发光，这使Er 3+:YAG晶体适宜作单频单束激光扫描静态体三维显示技术的绿光上转换材料。而Pr3+:Y2SiO5晶体的紫外上转换发光远大于可见发光强度，这使得其不适合作单频单束激光扫描静态体三维显示技术的上转换发光材料，而适合作上转换可调谐紫外激光晶体。单束激光扫描静态体三维显示实验验证了这一结果。 |
| 英文摘要 | The interaction of intense femtosecond laser pulses with matter is one of the most important frontiers in the high-field physics. The multi-photon process is based on the higher order nonlinear effect which is produced under the excitation of the ultra intensive laser pulses. Because of its threshold process, its longer pump laser wavelength and its n power dependence on the input laser intensity, n-photon process possesses the advantages of less dispersion loss, longer penetration length, smaller interaction area(which possibly overcomes the diffraction limit), stronger spatial confinement, higher contrast in imaging and stronger spatial selectivity comparing with single-photon excitation process. With the development of the ultrashort pulse laser, two-photon process has been widely applied in many areas, such as infrared-pumped visible laser, frequency-upconversion imaging and microscopy, three-dimensional optical data storage and processing, three-dimensional display, optical micro-fabrication, optical power limiting and stabilization. Much attention has been paid to three-photon process with the advent of the femtosecond laser in recent year. In this thesis, luminescence phenomena in some optical functional crystals based on the multi-photon process by femtosecond laser excitation were investigated. The main work is given as follows: 1．Two- and three-photon absorption characteristics of ZnO crystal. (1) It was found that the defects and the exciton (D0X) bound to neutral donors had been induced under the 800nm femtosecond laser radiation. In our experiment, the ultraviolet peak has a red shift and the visible one has a blue shift in the spectrum of ZnO crystal excited by 800nm femtosecond laser comparing with that excited by 267nm monochromatic light. According to the models of the exciton (D0X) bound to neutral donors and the defects induced by photon excitation, the above-mentioned phenomenon was explained. The zinc vacancy, zinc interstitial defects and the exciton (D0X) bound to neutral donors have been induced under the 800nm femtosecond laser radiation. The new defects result in a blue shift and the exciton (D0X) bound to neutral donors leads to a red shift relative to the luminescence excited by the 267nm monochromatic light. This idea was proven by the polarization-dependent and pump-wavelenghth-dependent luminescence experiments (2) According to the experiments of the log-log relationship between pumping power density of the femtosecond laser and fluorescence intensity of ZnO, the luminescence peaking at 500nm excited by 800nm femtosecond laser is attributed to simultaneous two-photon defect-absorption process and the one at 395nm is assigned to the three-photon sequential band-to-band absorption process. (3). According to the experiments of the relationship between incident power density and output laser intensity of the femtosecond laser, the two- and three-photon absorption coefficients of ZnO crystal were measured to be 0.2018cmGW-1 and 7.102×10-3cm3GW-2, respectively. The two- and three-photon cross sections were calculated to be 1.189×10-51cm4s and 1.040×10-80 cm6s2, respectively. Owing to the intensive two- and three-photon absorption effects of ZnO crystal, a good optical power limiting performance at 800nm has been demonstrated. 2．Defect centers and valence state change induced by infrared femtosecond laser in Yb:YAG crystals (1) It was found that defect centers and permanent valence state change had been induced by infrared femtosecond laser in Yb: YAG crystals. In our experiment, the up-conversion luminescence induced by 800nm femtosecond laser excitation in Yb3+: YAG crystal is different from the charge transfer luminescence of Yb3+ and almost consistent with the 5d→4f luminescence of Yb2+. This revealed that defect centers and permanent valence state change had been induced by infrared femtosecond laser in Yb:YAG crystals, which was proven by the absorption spectra of Yb3+:YAG crystal before and after femtosecond laser irradiation, and after further annealing at 1600 °C in air for 24 hours. (2) According to the photon excitation model of the hole in ligands O2-, the above-mentioned phenomenon was explained. It is thought that Yb3+ at ground state is firstly pumped into the charge transfer state of Yb3+ by 800nm femtosecond laser excitation, then, the positive hole in ligands O2- become a free hole in the valence band by further laser excitation and the Yb2+ in the charge transfer state of Yb3+ turns into the permanent Yb2+. The defect center is formed after the oxygen vacancy in YAG crystal capture a free electron in conduction band induced by three-photon excitation. 3．Multiphoton-excited upconversion luminescence of Nd: YVO4 (1) According to the experiments of the log-log relationship between pumping power density of the femtosecond laser and fluorescence intensity of Nd:YVO4 crystal, the up-conversion luminescence excited by 800nm femtosecond laser is attributed to three-photon excitation process at low pump power and four-, five-, or six-photon excitation process while at high pump power. (2) It was found that the upconversion fluorescence intensity of Nd3+ excited by the femtosecond laser is higher than the downconversion one; by contraries the downconversion luminescence such as 1064nm is much stronger than the upconversion one for the diode laser excitation. This shows that the upconversion efficiency pumped by the femtosecond laser is higher than that by the CW diode laser. (3) According to the three-level model of Nd3+, the time-dependent population at each level was calculated based on the Bloch equations. It was found that the population at the highest excited state which is proportional to upconversion fluorescence intensity is more than the one at intermediate level which is proportional to downconversion fluorescence intensity. This reveals that the upconversion fluorescence intensity of Nd3+ excited by femtosecond laser is higher than the downconversion one, which is consistent with the experimental results. 4．Upconversion luminescence of Er:YAG and Pr3+:Y2SiO5 crystals excited by femtosecond laser and three-dimensional display based on upconversion luminescence with one beam laser excitation. (1) Simultaneous three-photon absorption induced upconversion in Pr3+:Y2SiO5 crystal by femtosecond laser irradiation was found for the first time. According to the experiments of the log-log relationship between pumping power density of the femtosecond laser and fluorescence intensity of Er:YAG and Pr3+:Y2SiO5 crystals, the up-conversion luminescence of Er:YAG crystal excited by 800nm femtosecond laser is attributed to two-photon excitation process while the one of Pr3+:Y2SiO5 crystals is from simultaneous three-photon excitation process. (2) It was found that the fluorescence intensity of Er3+ peaking at 550nm is much stronger than any other fluorescence, which makes Er:YAG crystal suitable for three-dimensional display based on upconversion luminescence with one beam laser excitation as the green upconversion fluorescent material and that the ultraviolet fluorescence intensity of Pr3+ is much stronger than any other fluorescence, which makes Pr3+:Y2SiO5 crystal not suitable for three-dimensional display based on upconversion luminescence with one beam laser excitation, but suitable for tunable ultraviolet laser as upconversion laser material. The three-dimensional display experiments based on upconversion luminescence with one beam laser excitation have proven the above results. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15234]  |
| 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 | 王新顺. 飞秒激光诱导的发光现象及其机制与应用[D]. 中国科学院上海光学精密机械研究所. 2008. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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