几种光功能材料的超快光激发和光动力学过程

【作者】 刘康俊；

【导师】 钱士雄；

【作者基本信息】 复旦大学 ， 光学， 2008， 博士

【摘要】 本文从有机物和无机物两个方面研究了两种蓝色荧光类材料。这两种材料均有用于蓝色发光二极管领域的应用前景。本文通过800纳米的飞秒激光激发,详细研究了这两类材料在非线性激发下的发光特性,并利用泵浦探测技术和时间分辨荧光技术研究了其荧光发射和粒子数演变等瞬态行为,分析了非线性荧光的产生机理。此外,本文还研究了紫细菌外周捕光天线LH2中的能量传递过程。主要的研究内容和主要创新点如下:1.有机材料:联六苯纳米棒对一类PPP低聚物做为基本单元而生长成的纳米棒阵列薄膜的荧光特性作了研究,其在单光子和双光子激发下的荧光研究结果,均显示出很强的激发和荧光发射各向异性,不论单光子或双光子激发,只有当激发光的偏振方向垂直于纳米棒的取向时,才能达到最佳激发效果。另外通过对其双光子激发出的荧光的偏振性研究,我们发现其出射荧光也具有很强的偏振性,偏振方向亦垂直于纳米棒。通过对荧光空间分布的研究,发现其发光方向主要集中在两个相互垂直的平面中。这些研究结果表明,此类材料对于制作有偏振特性的有机发光二极管具有很大的应用前景。其空间分布研究对于其应用也有很大的参考价值。此外,我们还研究了纳米棒薄膜在400nm激光激发下的动力学,其动力学过程包括了一个2ps左右的快的激发态带内弛豫和一个25ps左右的激发态向基态弛豫两个阶段。2.无机材料:氧化锌我们利用800纳米飞秒激光激发氧化锌单晶和纳米棒材料,均发现其荧光发射有受激发射现象。对于氧化锌单晶材料,我们在测量荧光的同时,也研究了其在相同激发能量密度条件下激发态粒子数的弛豫过程,其荧光在受激发射后峰位发生了红移,并且此时的动力学过程呈现出一个4皮秒的快过程,说明此时的受激发射激励是来源于载流子浓度超过Mott浓度而形成电子空穴等离子体。对于氧化锌纳米棒,当激发能量密度超过一定阈值时,其荧光发射产生也会受激发射现象,表现为荧光的急剧增加但峰位不变,通过分析此时的瞬态荧光光谱,我们发现这一受激发射过程建立时间＞5ps,因此此时的光学增益的来源主要为激子-激子散射所造成,随着能量密度的进一步提高荧光峰位显著红移,根据此时的瞬态荧光谱,荧光发射的建立过程明显缩短,说明此时电子空穴等离子体已取代激子-激子散射成为主要的增益效应,导致能带重整化,最终使得荧光峰红移。本文研究结果表明这两类材料都具有很好的荧光特性,对有机材料的荧光特性研究表明其具有成为偏振发光二极管的特性。对氧化锌材料的研究对于人们深入了解其内在增益机理有所帮助。3.此外,本文还研究了紫细菌外周捕光天线LH2中的能量传递过程,通过分析激发B800带和B850带所表现出的丰富变化的光动力学特性,我们认为B850环的双激子带对B850的动力学过程有着大的贡献。通过八能级系统速率方程拟合了LH2在不同激发波长处的超快光动力学实验结果。高光强辐照所产生的氧化作用也对LH2的光动力学过程有重大的影响。其结果表明光氧化虽未改变B800向B850高速的能量传递,但B850环上分子被氧化而成的细菌叶绿素阳离子所形成的更低的能量陷获态的产生,导致了B850所表现出来的超快光动力学有了很大的改变,其能量传递途径由原先的经带内弛豫到荧光发射态的传递,在有光氧化时,变为激发能量以更快的弛豫速率被陷获态所俘获。更多还原

【Abstract】 The blue light-emitting diode (LED) is a rapidly expanding research field in recent years. In this thesis, we focus on two blue fluorescent materials, organic and inorganic, which could be the promising candidates for blue LED application. Using femtosecond (fs) pulses, we studied the photoluminescence (PL) emission from these materials under nonlinear optical excitation, and used the fs pump-probe and time-resolved photoluminescence (TRPL) techniques to investigate the mechanisms of these processes. We also studied the energy transfer processes in light harvesting complexes (LH2) from Rb.Sphaeroides by using femtosecond pump-probe technique.The main research and the innovative results are given below.1. Organic material: Para-sexiphenyl (PSP) nano-needlesThe PSP nanoneedle film is a long-range ordered structure consisted of oriented nano-needles, which are parallel to each other in a limited area and turn to a tilted orientation in adjacent areas. We studied the PL emission of PSP nano-needles under linear and nonlinear optical excitation. Excited by the femtosecond pulses, two-photon absorption (TPA) induced PL emission spectrum shows several peaks corresponding to the vibronic transitions in PSP molecules. Polarization-dependent PL measurements reveal that the TPA induced PL in the highly-ordered structures, similar with the linear excitation case, is strongly dependent on the excitation polarization. The measurement of PL emission with different polarizations and at different detection directions reveals that the PL emission from these highly-ordered structures has strong dependence on the polarization and emission direction. We also studied the population evolution in this nano-needles film under 400 nm excitation. The ultrafast dynamics show a 2 ps relaxation process of the excitation state and a 25 ps transition from the singlet state to the ground state.2. Inorganic material: Zinc oxide (ZnO)The multi-photon absorption induced ultraviolet stimulated emission were observed from both ZnO bulk crystals and ZnO nanorods under the pumping of the femtosecond pulses at wavelength of 800nm. For ZnO bulk crystals, the PL emission peak shows a red-shift with the increasing pumping, while the ultrafast dynamic shows a lifetime of about 4 ps, so the optical gain from ZnO crystal is attributed to the electron hole plasma (EHP). For ZnO nanorods, the formation time of the stimulated emission peak was determined to be longer than 5 ps, and the mechanism of the stimulated emission is attributed to the exciton-exciton scattering (EE) process. With the increase of the excitation flux, the PL peak shows an obvious red-shift, and the formation time decreases, so the EHP becomes the main optical gain mechanisms under high excitation flux.The research shows that two materials are promising candidates in the blue LED field. The organic material PSP may have potential applications in polarized LED.3. We also studied the energy transfer processes in light harvesting complexes (LH2) from Rb.Sphaeroides by using femtosecond pump-probe technique. At excitation wavelength around 835 nm, a sharp photobleaching signal was observed which was assigned to the contribution of the two-exciton state, further confirmed by the power dependence measurement. Rate equations with eight-level scheme were used to simulate the population evolution in LH2 and the transient dynamics under femtosecond pulse excitation. The photo-oxidation effect of LH2 was investigated by using the absorption spectroscopic and fs pump-probe techniques. The experimental results reveal that in comparison with the unoxidized BChl-B850 molecules, BChl-B850 radical cation in oxided B850 could act as an efficient relaxation channel to rapidly trap the excitation energy, but the B800→B850 energy transfer process is almost unaffected in the oxidation process.更多还原