【作者】 张芳；

【导师】 许心光； 王正平；

【作者基本信息】 山东大学 ， 凝聚态物理学， 2016， 博士

【摘要】 二维纳米材料的非线性吸收性能是近年来的研究热点。利用二维材料的可饱和吸收性能可以将其制成被动调Q或锁模元件,应用于全固态激光器和光纤激光器中；其双光子吸收性能可以实现材料的光限幅效应,有望用于人眼及敏感光学元件的防护中。目前,人们比较关注的二维材料有石墨烯、氧化石墨烯、功能化的石墨烯、过渡金属硫化物、氮化硼以及黑磷等等。石墨烯是人们研究的最早的二维材料,它最先体现出来的性能,如透过率、电子迁移率、热导以及延伸率等等,刷新了人们对于纳米材料的认识。石墨烯具有特殊的零带隙结构,使其能够实现宽波段能带跃迁。过渡金属硫化物的带隙宽度随层数而变化,大约在1 eV-2 eV之间。氮化硼属于宽带隙材料,可以被看作是绝缘体,禁带宽度在5 eV以上。2014年,禁带宽度在0.3 eV～2 eV的黑磷的出现,填补了其他二维纳米材料遗留下来的带隙方面的空白。非线性吸收是一种基于强相干光条件下的非线性光学效应,在激光出现之后才得到广泛研究。根据不同的吸收机理,可以将非线性吸收分为饱和吸收、双光子吸收、自由载流子吸收以及反饱和吸收等等。测量非线性吸收的方法有很多,Z扫描法即为其中之一。这种方法所需装置简单,精确高,理论相对成熟。本论文采用Z扫描法对多种二维材料进行了非线性吸收性能的测试及分析,主要研究内容如下：(1)用OPO激光器(410 nm～2300 nnm)研究了石墨烯宽频带的可饱和吸收性能。发现调制深度仅依赖于石墨烯的层数,与入射光子能量无关。在低频光子激发时,石墨烯表现出低的饱和强度以及高的饱和载流子浓度,说明石墨烯光电器件在中红外、远红外甚至太赫兹波段具有应用潜力。证明了石墨烯的沙漏型能带结构以及狄拉克点的存在。总结分析了三种情况下饱和强度的数量级变化：(i)对于稳态、大尺寸光斑,Is的数量级为1MW/cm2。 (ii)对于稳态、小尺寸光斑,Is的数量级为10MW/cm2。(iii)对于瞬态情况,Is的数量级为100 MW/cm2。(2)研究了掺N石墨烯、羧基化氧化石墨烯以及掺氟氧化石墨烯的非线性吸收性能。掺氮石墨烯表现出明显的饱和吸收性能。由于氮原子对石墨烯能带结构的影响,掺氮石墨烯表现出比纯石墨烯更低的饱和强度以及更大的双光子吸收系数。通过两种激发波长的非线性吸收性能的比较,证明了掺氮石墨烯是一种优良的红外非线性材料。与掺氮石墨烯相比,羧基化氧化石墨烯以及掺氟氧化石墨烯的饱和吸收性能不明显,但却表现出强烈的双光子吸收现象。与纯氧化石墨烯相比,羧基化氧化石墨烯中羧基的存在增强了氧化石墨烯在短波长波段的双光子吸收性能,在532 nm波长激发时的双光子吸收系数是1064 nm波长激发时的8.5倍,因此,羧基化氧化石墨烯是一种潜在的可见和紫外光波段光限幅材料。对于掺氟的氧化石墨烯而言,F的掺杂,使其表现出比氧化石墨烯更强的非线性吸收性能,双光子吸收系数的数量级达到石墨烯的水平。不同激发波长下的非线性拟合结果显示,掺氟的氧化石墨烯是一种潜在的红外光限幅材料。(3)研究了羟基化氮化硼纳米片的双光子吸收性能。由于羟基的存在,加强了纳米片的水融性,增强了B-N原子的极化强度,改变了其分子偶极矩,使之表现出良好的双光子吸收性能,羟基化的氮化硼纳米片有望在未来的非线性光学领域(尤其是光限幅方面)获得应用。(4)研究了二硫化物XS2纳米片(MoS2, WS2)的非线性吸收性能。与MoS2纳米片相比,WS2纳米片表现出更强的非线性吸收效应(包括饱和吸收以及双光子吸收),更适合用作被动调Q、锁模、以及光限幅材料。(5)研究了三种激发波长下黑磷纳米片的非线性吸收性能。结果表明通过溶液浓度的增加可以有效提高黑磷纳米片的饱和强度及双光子吸收系数,并且与可见波段相比其在红外波段表现出更强的非线性吸收性能。更多还原

【Abstract】 In recent years, the nonlinear absorption properties of two-dimensional (2D) materials are the hot topic. The saturated absorption performance of 2D material can be used to produce the saturable absorber for the application in solid state mode-locked laser and fiber laser. Using the two photon absorption property,2D material can be used as the optical limiting material for the protection of eyes and sensitive optical device.In past decade, a series of 2D materials were be found in succession, such as graphene, graphene oxide, functionalized graphene, transitional metal dichalcogenides, boron nitride, black phosphorus, and so on. Graphene is the first founded 2D material, which was researched by scientists. Many rare properties performed by graphene, such as high optical transparency, flexible mobility, elongation and thermal conductivity, refreshed the understanding of nano-materials. The most outstanding property of graphene is the zero band gap structure, which can realize the transition of free electron in a very wide waveband. The band gap of transitional metal dichalcogenides is in a range of 1 eV-2 eV. Due to that the band gap width changes with the layer number of nanosheets, the transitional metal dichalcogenides own a quasi-continuous band gap structure. Boron nitride nanosheets can be regarded as a nonconductor, because of the wide band gap (over 5 eV). In 2014, scientist found the black phosphorus nanosheets with the band gap of 0.3 eV-2 eV, which filled up the blank leaved by other 2D nano-material.The nonlinear absorption is a kind of nonlinear effect, which only can be performed by strong coherent light, so it was found after the presence of the laser. According to the difference of absorption mechanism, nonlinear absorption can be defined as saturated absorption, two photon absorption, free charge carrier absorption, reverse saturable absorption, and so on. There are kinds of methods of measuring nonlinear absorption, but Z-scan method is the most popular method, due to the simple experimental facility, high accuracy and relatively developed theory. We chose Z-scan method to research the nonlinear absorption properties of several kinds of 2D materials, which can be summarized as follows.(1) Using the OPO laser, we researched the saturated absorption performance of graphene with different number of layers in the waveband of 410 nm～2300 nn. The modulation depth of graphene has no relationship with excitation wavelength and only depends on the number of layer. Graphene performed lower saturated intensity and high carrier concentration with the excitation of long wavelength compared to short wavelength excitation, which outstand the huge application value of graphene in intermediate infrared, far infrared, even the terahertz waveband. At the same time, we demonstrated the hourglass band structure of graphene and the existence of Dirac point. Also, we discussed the change of saturated intensity Is in three different conditions. For steady state and large facula excitation, Is is in 1 MW/cm2 order of magnitude. For steady state and small facula excitation, the order of magnitude of Is is 10 MW/cm2. In the condition of transient state excitation, Is is in 100 MW/cm2 order of magnitude.(2) We studied the nonlinear absorption properties of N-doped graphene, carboxyl of graphene oxide and F-doped graphene oxide. N-doped graphene performed obvious saturated absorption property. Due to the influence of band gap structure caused by N atoms, N-doped graphene displayed lower saturated intensity and larger two photon absorption coefficient compared to pure graphene. Through the comparison of nonlinear absorption properties with two different wavelengths excitation, N-doped graphene is kind of outstanding nonlinear material applied in infrared band.Compared to N-doped graphene, the saturated absorption property of carboxyl of graphene oxide and F-doped graphene oxide is not much obvious, but the two photon absorption performance is very strong. Compared to pure graphene oxide, carboxyl of graphene oxide can promote the nonlinear absorption performance in short wavelength waveband due to a large amount ofcarboxyl. The two photon absorption coefficient with 532 nm excitation was 8.5 times larger than that with 1064 nm excitation. Carboxyl of graphene oxide is a kind of outstanding optical limiting material applied in visible and ultraviolet band.For F-doped graphene oxide, it performs stronger nonlinear absorption property compared to pure graphene oxide due to the doping of F atoms. The fitted two photon coefficient is in the same level compared to pure graphene. Through the comparison of nonlinear absorption properties with two different wavelengths excitation, F-doped graphene oxide is kind of outstanding nonlinear material applied in infrared band.(3) The two photon absorption properties of OH-functionalized BN nanosheets were researched. The existence of hydroxyl promotes the solubility of nanosheets. In microcosmic field, hydroxyl enhances the electric polarization of B-N atoms and changes the molecule dipole moment, which motivates the two photon absorption performance. The hydroxylated boron nitride nanosheets are a potential material, which can be widely used in nonlinear optics sector as optical limiting material.(4) The nonlinear absorption properties of MoS2, WS2 were researched. Compared to MoS2 nanosheets, WS2 nanosheets displays stronger nonlinear absorption property including the saturated absorption and two photon absorption. WS2 nanosheets are more suitable to be applied in passive mode locked lasers, Q-switched lasers and eye and sensitive optical devices.(5) The black phosphorus nanosheets were successfully exfoliated adopting liquid phase exfoliation method. The nanosheets had a thickness of 7 nm-10 nm with a size of few micrometers. The nonlinear absorption performance was studied using three different excitation wavelengths. We demonstrated that black phosphorus nanosheets own better nonlinear absorption compared to graphene. Based on the analysis of different nonlinear results with three excitation wavelength (355 nm,532 nm,1064 nm), we demonstrated the huge potential of black phosphorus nanosheets in the application in infrared band as nonlinear absorption material. The black phosphorus nanosheet is a kind of broadband nonlinear absorption material.更多还原