金属复合纳米材料及宽带半导体材料的超快光物理研究

【作者】 董志伟；

【导师】 钱士雄；

【作者基本信息】 复旦大学 ， 光学， 2008， 博士

【摘要】 本论文主要分为两个部分,第一部分研究了磁控溅射制得的以氮化硅为基质的含银金属纳米复合薄膜及离子交换得到的掺银,铜复合玻璃的超快能量弛豫过程和三阶非线性光学性质,以及热处理的影响。采用的是z-scan及飞秒时间分辨的泵浦-探测,光学克尔效应等实验技术。第二部分是对氧化锌晶体及微米管在800 nm附近强的飞秒光脉冲作用下的多光子激发过程的研究,并对实验结果进行了一些相关的理论模拟。主要研究内容和相应创新结论如下:1.通过飞秒泵浦.探测及z-scan等实验技术,研究了热处理对由磁控共溅射法制备的Ag∶Si₃N₄纳米复合薄膜的光学非线性及超快能量弛豫过程的影响。结果表明,虽然热处理对材料的三阶光学非线性的值影响不大,但是因为热处理后材料的吸收降低（800 nm处）,其品质因数相应提高。对材料的超快能量弛豫过程的研究表明,热处理后材料的能量弛豫过程明显加快。我们认为这主要是退火后银纳米颗粒尺寸明显增大,导致自由电子振荡频率和声子德拜频率的重叠加大,进而导致更强的电子-声子耦合,加速了快过程的弛豫速率。而慢的弛豫过程加速的原因被认为与热处理后Si3N4基底的相变使得其导热性能得到提高有关。2.利用相关实验技术,研究了离子交换得到的单独掺银,单独掺铜及同时掺银和铜的复合玻璃的光学非线性及超快能量弛豫过程。发现在单独掺银的复合玻璃中,热处理不仅能够明显增大材料在表面等离子共振（SPR）附近的光学非线性,还可以加速材料的快能量弛豫过程。其原因在于热处理不但能够促进银纳米颗粒的聚集,还有利于玻璃基底中银离子向银原子间的转化。在退火后的样品中可能存在银纳米颗粒内部的电子与周围玻璃基底的声子问耦合等过程,这有利于材料能量弛豫过程的加速。对掺铜复合玻璃的时间分辨的结果分析可以得出,材料激发后电子布居的改变而造成的复合材料SPR红移及展宽是造成不同波长出现光致吸收与漂白转化的根本原因。对同时掺银和铜的复合玻璃材料的时间分辨研究表明,玻璃基底中的铜和银纳米颗粒在不同波长处对材料的超快能量弛豫过程起主要贡献。3.对氧化锌单晶及微米管两种材料在波长为800 nm附近的飞秒强激光激发下多光子吸收过程的研究结果表明,即使存在双光子失谐,在强的飞秒激光激发下,双光子吸收而不是三光子吸收在氧化锌材料的激发过程中占主要影响。我们认为极强光电场作用下出现的一些非线性效应可能会极大地提高氧化锌的双光子吸收效率。对双光子Rabi振荡参与下的氧化锌激发过程的理论模拟结果表明,在失谐条件下,一定消相过程的存在有利于二能级系统中上能级粒子数的布居。总之,本论文通过对热处理前后两类金属复合纳米材料的光学非线性及超快能量弛豫过程的研究,证明了适当温度下的热处理有利于提高该类材料的光学非线性性能及超快光响应。对不同种类氧化锌材料在飞秒激光强光场作用下的研究结果表明,多光子过程同样能有效地激发氧化锌这类宽禁带半导体材料,强光场下出现的一些非线性光学效应可能在材料的激发过程中占有重要的影响。本论文的研究结果有助于人们加深对金属纳米复合材料体系光物理特性的认识。有关氧化锌材料的多光子激发过程的研究有助于揭示材料在强场作用下的出现的一些奇异特性。更多还原

【Abstract】 In this thesis,first,we investigate the energy relaxation dynamics and third-order nonlinear optical susceptibilities of noble metal nanocomposites by using femtosecond（fs） pump-probe,time-resolved optical Kerr effect（OKE） and z-scan techniques.Then,multi-photon absorption processes in ZnO materials,including single crystal and microtubes,were studied under the excitation of intense fs pulses near 800 nm.Relative simulation has also been made.The main innovative results and conclusions are as follows:1.The energy relaxation dynamics and nonlinear optical properties of Ag:Si₃N₄ nanocomposite films fabricated by magnetron co-sputtering were investigated by utilizing fs pump-probe and z-scan techniques.After thermal treatment,the figure of merit was improved although the third-order susceptibility didn’t change much.Heat treatment is proven to be a feasible way to improve the performance of ultrafast response for this material.The acceleration of the fast relaxation is attributed to the intensification of the coupling between the electrons and the phonons in Ag nanoparticles due to the diminishing of the detuning of the electron-oscillation phonon resonance overlap,caused by the size increase of Ag nanoparticles after annealing.The improvement of thermal conductivity properties of the Si₃N₄ substrate after annealing may be responsible for the acceleration of the slow relaxation process in the annealed samples.2.Effects of thermal treatment on the optical nonlinearity and ultrafast dynamics of Ag and Cu nanoparticles embedded in soda-lime silicate glasses were investigated.For the glass embedded with Ag nanoparticles,after the annealment,the figure of merit near the SPR increases obviously due to the increased content of Ag particles inside the glass.In addition to this,the relaxation is accelerated after the annealment,which could be attributed to the efficient interfacial coupling between hot electrons in Ag and phonons in SiO₂ substrate.The time-resolved experimental results indicate that the change of electrons population results in the red shift and the broadening of SPR,leading to the conversion from photobleaching to photoabsorption in the glass embedded with Cu nanoparticles.While Cu and Ag nanoparticles dominate the transient absorption properties of the nanocomposites at different wavelength in the glass embedded with both Cu and Ag nanoparticles.3.Multi-photon absorption in ZnO single crystal and microtube were investigated by utilizing intense fs pulses at wavelength near 800 nm.Nonlinear optical effects that may emerge under extremely intense field are attributed to be responsible for the efficient two-photon absorption process under detuned excitation.The simulation results of the two-photon Rabi effect show that there would be the population remained in the upper state if dephasing is taken into consideration even under detuning,while no population is found in the upper state after excitation when there is no dephasing effect.In conclusion,the ultrafast dynamics and the nonlinear optical response of metal nanocomposites were investigated.Heat treatment has been proven to be a feasible way to improve the performance of ultrafast response for this kind of materials. Based on the experimental results of ZnO materials excited by intense fs pulses near 800 nm,nonlinear optical effects that may emerge under an intense field are attributed to be responsible for the efficient two-photon absorption process under detuned excitation.Our research results are meaningful in accumulating the information of the optical properties of metal nanocomposite and the relative researches on the multi-photon absorption processes in ZnO materials are benefit for further understanding of the light-matter interaction under ultrashort intense excitation.更多还原