可见光波段负折射材料的研究

	可见光波段负折射材料的研究
	贾丹
学位类型	博士
导师	宣丽
	2014-07
学位授予单位	中国科学院大学
学位专业	光学
摘要	负折射是一种违反常规的电磁波穿越光学界面的现象，表现为入射光和折射光位于界面法线的同侧。负折射平板材料制备的透镜，其成像分辨率可以突破衍射极限，还可以用于具有军事意义的隐身设备。除此之外，负折射材料在位相补偿、纳米波导等一系列光学领域，具有巨大的潜在应用价值，因此成为近十年的研究热点。2000年，D.R.Smith等人把金属微结构的线阵列和开口谐振环阵列组合，首次在实验上观测到负折射，其现象出现在微波波段。之后有人采用金属渔网结构的材料把负折射波段推至可见光的边缘。但是，人们所设计的负折射材料结构精细复杂，尤其当负折射出现在波长更短的可见光波段的话，材料制作需要高端纳米技术，制备方法极其困难，限制了负折射材料的应用。本论文首先提出利用液晶分散银纳米球的材料在可见光波段实现负折射。理论分析表明，液晶的Δn越大，获得负折射的波段越宽；预计Δn为0.22的E7液晶/银纳米球复合材料的负折射波段应该在460nm-520nm范围，其中银纳米球的半径要小于25nm、间距小于70nm。但是，实验中银纳米球极易团聚，导致这种负折射材料很难制备。根据液晶分子上的共轭π电子很容易离域运动，可以在光波的电场中产生诱导偶极子，即类等离子体，预计也能产生负折射现象，同时液晶材料对可见光的吸收基本可以忽略，因此液晶负折射材料将具有高透过率。实验上制备了液晶取向不同的器件，证明液晶的取向方向影响负折射角，且液晶中的最大负折射角与液晶的Δn值近似呈线性关系。实验上获得波长532nm的TM激光在Δn=0.42的液晶材料中的最大负折射角为-14°，约为已有文献报道值的2倍，负折射的临界入射角达-24°。由于液晶分子上的偶极子受限于长轴方向，只有特定方向入射的光束才能产生负折射；另外液晶分子偶极矩相对金属线的感应电矩来说极小，尽管液晶分子偶极子的密度远大于金属线阵列密度，但结果仍是负折射角度偏小。同时金属结构中的自由电子可以在各个方向被诱导振荡，应该是实现全方位负折射的基本材料，只要能控制材料的光损耗即可。因此理论分析了银纳米线阵列材料的比表面积，设计了低能量损耗的多孔氧化铝/银纳米线阵列材料，其中银纳米线间距为100nm、银纳米线阵列半径为25nm，负折射波段范围600nm-800nm。但是实验发现，厚度仅为20μm的材料的光透过率几乎还是为0。针对氧化铝/银纳米线的能量损耗问题，依据Viktor A. Podolskiy等人提出的电磁波作用于银纳米线的趋肤深度理论，通过增大银纳米线的比表面积能够大幅降低能量损耗。进一步提出在反六角液晶分子模板中制备比表面积更大的银纳米线阵列，其中银纳米线半径只有10nm、线间距为40nm，介电常数虚部值降至氧化铝/银纳米线的1/6，故负折射的透过率应数倍增加。理论计算表明在400nm-800nm宽波段可见光范围内能够产生负折射。初步实验获得了反六角溶质液晶/银纳米线阵列的复合结构材料。本工作深入金属等离子体和分子偶极子的电子特性，定性分析了振荡激元产生负折射的微观机理。在此基础上，借助介电常数计算模型给出一些微结构的负折射预测，探索了能够覆盖整个可见光波段的全方位负折射材料。同时，对金属材料的能量损耗与比表面积的关系进行了分析，解明只有金属纳米线的半径小到10nm时才有可能获得有价值的负折射材料。综上，金属中的自由振荡等离子体虽然能强烈作用于电磁波、使其传播方向发生负折射偏转，但金属的光能损耗也是很难突破的瓶颈问题；而液晶中的类等离子体振荡效应，没有光能损耗问题，虽然其类等离子体的振荡目前不能在全方位产生，但有希望通过控制液晶分子的取向形式来改善，这是本研究今后要开展的工作。
其他摘要	Negative refraction is a kind of novel electromagnetism phenomenon. In visible optical frequecies, the propagation paths of refractive wave and incident wave are at the same side of the normal of the inerface. A flat lens mad of negative refraction materials (NRM) can foucs light and the resolution of the image can be smaller than the diffraction limit. NRM also can be used in invisible cloak devices which qualify NRM significance in future military. Besides, NRM bring out plenty of potential applications in visible optical frequecies, such as phase compensation, nano waveguide and so on. The research of NRM in optical frequecies has become one of the most hottest fields in decade.The artificial NRM composed of metallic wires arrays and metallic slit rings arrays was firstly detected experimentally at microwave range by D. R. Smith et al. in 2000. Up to now, fishnet metamaterial has realized negative refraction at 780nm. However，because of the sophistication and narrow response waveband of the magnetic cells, the NRMs became challenge to nano processing technology. Besides, the issue of strong energy loss is existed generally in the artificial NRMs, which restrict their practical applications.First, silver nanospheres dispersed in liquid crystals is proposed to realize negative refraction in visible optical wavelength, which makes use of the optical anisotropy of liquid crystals and the plasma of silver nanospheres. The theory turns out that the bigger the Δn of the liquid crystals ,the wider wavelength band of negative refraction. The negative refraction band of E7 liquid crystals (Δn=0.22)/silver naospheres composite material is 460nm-520nm. It demonstrates that silver nanospheres arrays with the diameter smaller than 25nm and the distance smaller than 70nm can realize negative refraction in visible optical wavelength. Liquid crystals/silver naospheres composite material are prepared in experiment. But the device confronts with the problem of stability due to the easy reunion of the nanospheres.The conjugate π electrons in liquid crystal molecule are delocalized in applied electromagnetic fields and the dipole is induced to react to the electromagnetic fields. The dipole response to electromagnetic fields is a similar physical process compared with plasma response. Liquid crystals (LC) are nearly transparent for visible optical light so the NIM based on LC has the advantage of high transmittance. A group of LC devices with different molecular orientation direction are tested to prove the imapct of molecular direction on the negative refraction. Besides, the biggest negative refraction angle is akin to in linear relation with Δn of LC. TM wave with wavelength of 532nm can realize negative refraction in LC and the negative refraction angle is about -14°for the LC with Δn=0.42, which is twice of the value reported. The critical angle is about -24°.Becaus the polarize direction of LC dipole is restricted along molecule axis, the wave only in limited incidence angle scale permits negative refracion in LC. Though the molecule density of LC is larger than that of metal line arrays, due to the finite delocalized distance of the π electrons the negative refrction angle is small. The free electrons in metal enable plasma to be redued by electromagnetic fields in any directions. So metal nano-structures can be employed to realize all-angle negative refraction and we only need to keep down the energy loss. The relation beteween the specific surface area of silver nanoline and energy loss is analyzed, and the PA/Ag material with nanoline radius of 25nm and the distance between adjacent lines of 100nm is prepared. The negative refraction waveband of PA/Ag is 600nm-800nm. The experiment shows the transmittance of the PA/Ag material with the thickness of 20μm is still almost zero.To overcome the strong energy loss of PA/Ag, silver nanowires array with large specific surface area based on reverse hexagonal lyotropic liquid crystal template(LC/Ag) is proposed. The radius of nanoline is only 10nm and the distance of the array is about 40nm. Theory turns out the image permittivity value of silver nanolines array is the one sixth of the value of PA/Ag, which means the multiple enhancement of transmittance. Simultaneously, the waveband of negative refrction is 400nm-800nm. The LC/Ag composite structrue materials have been obtained in experiment.The experiment of negative refraction test is ongoing.
语种	中文
文献类型	学位论文
条目标识符	http://ir.ciomp.ac.cn/handle/181722/41415
专题	中科院长春光机所知识产出
推荐引用方式 GB/T 7714	贾丹. 可见光波段负折射材料的研究[D]. 中国科学院大学,2014.

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