【作者】 霍燕燕；

【导师】 贾天卿；

【作者基本信息】 华东师范大学 ， 光学， 2014， 博士

【摘要】 随着纳米加工技术的迅速发展,金属纳米结构的尺寸可以达到纳米量级。金属纳米结构的表面等离激元共振有很强的局域电场增强特性,对金属结构参数和环境的变化特别敏感,使其在化学生物传感器件、表面增强拉曼散射、二次谐波产生等方面有着广泛的应用。本论文主要研究金属纳米结构的表面等离激元特性。首先设计了棒-同心矩形环盘结构,对其所支持的Fano共振及基于Fano共振的表面等离激元受激辐射放大(Spaser)进行了研究；再次,研究了飞秒激光诱导不锈钢表面周期纳米条纹的机制,主要讨论了表面等离激元所起的作用。主要研究结果如下：(1) Fano共振是减小共振线宽和增强光谱灵敏度的重要方法之一,实现窄线宽和高对比度的Fano共振是一个重要的研究目标。我们用Comsol软件设计了一个棒-同心矩形环盘金属纳米结构,实现了四极Fano共振现象。通过调节矩形环盘中环盘的间隙,或者是棒与环的间隙来操控Fano共振的线宽和对比度。当环盘间隙为5nm,棒环间隙为20nm时,四极Fano共振的线宽只有0.025eV,对比度能高达80%,此时的探测灵敏度FOM也能达到15。这种金属纳米结构在生物化学传感中有广泛的应用前景。(2)我们提出了基于棒-同心环盘结构Fano共振的等离激元激射,它是利用Fano共振中对同心矩形环盘结构的暗态四极共振的激发,并通过增益介质所提供的能量对其进行损耗补偿和放大来实现的。基于此暗态等离激元放大的激射具有很高的Purcell因子3.24×107,很高的信噪比(SNR)为4.4×106和较低的阂值0.02086。这些良好的光学特性都归因于暗态四极共振模式的局域电场增强和被抑制的辐射损耗。在以后的发展中,如何减小金属的损耗是表面等离激元激光器的一个研究热点和重点,而暗态模式的表面等离激元的激发为此提供了一个新思路。(3)飞秒激光在金属材料表面诱导周期纳米条纹结构时,表面等离激元起着非常重要的作用。我们研究了800nm飞秒激光在不锈钢表面诱导形成的周期纳米条纹结构和其形成的物理过程。在不锈钢表面发现了LSFRs(A>0.45λ)、 MSFRs (0.2A<A<0.45λ)和HSFRs(A<0.2λ)三种周期的条纹结构。其中LSFRs就是入射光与表面等离激元相互作用的结果,而HSFRs是LSFRs两次分裂的结果,其周期只有100-130nm,入射光波长的1/6-1/8。我们理论模拟的结果表明,在LSFRs条纹不断加深的过程中,由于局域表面等离激元使得电场先局域到条纹凹槽的内部,使得条纹进一步加深。当深度大于形状共振深度时,就会破坏条纹的形状共振,电场会溢出凹槽而局域到条纹的突起上,这就使得条纹会发生了分裂。条纹的分裂过程会受到脉冲数、功率密度、样品所处的周围的介电环境以及样品的热学参数的影响。我们制备的HSFRs条纹周期,在超材料、数据存储以及纳米光电子学等领域具有广泛的应用前景。更多还原

【Abstract】 With the development of science and technology, the metallic structures can be reduced to the scale of nanometers. The surface plasmon of the metallic nanostructures supports a very high electric field and extraordinary sensitive to surface conditions and the the nanostructure. This properties lead the surface plasmon to be used in chemical and biological sensors, surface enhanced Raman scattering, second harmonic generation (SHG), etc. In the project, we focused on the studies of the characteristics of surface plasmon resonances. We propose plasmonic nanostructure consisting of a rod and concentric square ring-disk (RCSRD) structure, and study the quadrupolar Fano resonance of the RCSRD structure. We also study the spaser based on Fano resonance of the RCSRD. In the last, we introduce the periodic ripples induced by the femtosecond laser and the role of surface plasmon in formation of the ripples. The innovative results are as follow:(1) We propose a plasmonic nanostructure consisting of a rod and concentric square ring-disk (RCSRD) structure, which can support a Fano resonance. Fano interference is an important way to decrease the resonance line-width and enhance the spectral detection resolution, but realizing a Fano line-shape with both a narrow line-width and high spectral contrast-ratio is still a challenge. Fano line-width and spectral contrast-ratio of the RCSRD structure can be actively manipulated by adjusting the gap between the nanorod and CSRD, and by adjusting the gap between the ring and disk in CSRD. When the gap size in CSRD is reduced to5nm, the quadrupolar Fano line-width is of0.025eV, with a contrast ratio of80%, and the figure of merit reaches15. So the RCSRD structure has a very extensive application in the biological chemical sensing.(2) We reports a spaser based on Fano resonance of a plasmonic nanostructure consisting of a rod and concentric square ring-disk (RCSRD) structure coated with a layer of gain media. The amplification of the dark quadrupolar mode at the Fano resonance wavelength causes the spaser with a high Purcell factor of3.24×107, a high signal to noise ratio (SNR) of4.4×106and a lower threshold of0.02086. These significant optical properties are attributed to the greatly enhanced spontaneous emission and depressed radiation loss supported by the strong localized dark mode at the Fano resonance wavelength. Reducing the loss of the metal is an important focus in the study of the Spaser. The amplification of the dark mode provide a new way to reduce the loss of the Spaser.(3) Surface plasmon play an important role in the formation of the ripples induced by the femtosecond laser. Three kind of periodic ripples formed in the ablation spots in sequence:LSFRs (A>0.45λ), MSFRs (0.2A<A<0.45λ) and HSFRs(A<0.2A). The formation of high spatial frequency ripples (HSFRs) in stainless steel irradiated by50fs,800nm,1kHz femtosecond laser pulses in water. We further conduct theoretical calculation to study the ultrafast dynamics, and find that double split of LSFRs plays the decisive role in the formation of HSFRs. Theoretical calculations show that as the groove depths reach to a certain value, the incident laser localizes on the protuberance, which induces the split of ripples. Similar experiments are conducted in copper, however, no split of LSFRs is observed. The different experimental results on stainless steel and copper conducted in water and in air are also discussed. The split process is affected by the laser pulse numbers, fluences, surrounding medium and thermal parameters of samples. These nanostructures are with features of1/6-1/8laser wavelength, which has potential applications in nanophotonics, metamaterials, data storage, and nano-optoelectronics.更多还原