【作者】 吴志勇；

【导师】 徐政基；

【作者基本信息】 中山大学 ， 电子科学与技术， 2025， 博士

【摘要】 红外光子共振与有机分子的振动跃迁之间的强耦合被称为振动强耦合（Vibrational Strong Coupling,VSC）效应。截至目前,VSC效应已被证明可以显著改变分子的化学性质和化学反应性。此外,研究表明石墨烯等离激元可以在中红外至太赫兹波段实现显著的光学束缚和近场增强。因此,借助石墨烯等离激元的优异特性,有望实现纳米级有机分子的VSC效应。本论文对石墨烯等离激元的传播和束缚特性及其增强的VSC效应进行了较为系统的理论研究,主要研究成果如下:（1）本文设计了一种渐变折射率波导阵列透镜（Graded-Index Waveguide Array Lens,GIWAL）,实现了传播的石墨烯等离激元极化子（Graphene Plasmon Polaritons,GPPs）模式的片上消色差聚焦。通过研究GIWAL所支持的GPPs模式的传播特性,建立了一个完整的解析模型,该模型不仅能够解释片上聚焦的物理机制,还可以阐明高斯光束的自聚焦、自准直、光摆效应以及数字光学信号的空间翻转等复杂的光束演化行为。通过调控GIWAL中电介质光栅的占空比,可以灵活设计透镜的有效折射率,从而获得与Mikaelian透镜相一致的渐变折射率分布。此时,在GIWAL中传播的GPPs模式会在透镜的中轴线上实现周期性的自聚焦。计算结果表明,在10～20 THz频段内,GPPs模式能够实现消色差的片上聚焦,焦点尺寸可小至约0.02λ（λ为自由空间中的工作波长）。此外,也证实了GIWAL可用作宽带的数字光学信号的空间翻转器件。（2）本文提出了一种石墨烯-电介质-金属混合结构（Graphene-Dielectric-Metal Hybrid-Structure,GDMHS）,用以实现局域的石墨烯等离激元共振（Localized Graphene Plasmon Resonances,LGPRs）模式的远场激发。通过分析LGPRs模式的形成过程,构建了一个预测LGPR波长的解析模型,以指导实际的实验观测。借助该模型,既能直观地理解偶阶LGPRs模式无法通过远场激发的原因,也能理解为何损耗的增大会导致本征模式消逝。此外,由数值仿真结果验证了的解析模型表明,通过追踪第一阶LGPR波长的变化,在监测电介质隔层的厚度变化时所能获得的分辨率可达亚飞米量级。当第一阶LGPR模式被用于探测纳米间隙中的材料变化时,可检测到的最小折射率变化达到10^-6RIU。此外,通过减小石墨烯的费米能级或增大共振腔的长度,检测灵敏度可以进一步增大。值得注意的是,这种基于LGPRs模式的感测应用可以通过主动地调谐石墨烯的费米能级而应用于其他感兴趣的光谱范围。（3）本文研究了传播的GPPs模式与有机分子之间的VSC效应。首先从理论上证明了在完美吸收器的帮助下,远场光谱的峰值可以显著提高,从而增强了VSC效应的可观测性。当厚度仅为5 nm的有机分子与GPPs模式发生强耦合时,可以在耦合的分子系统的吸收光谱中观察到明显的抗交叉行为和高达约60 cm^-1的拉比劈裂。而且,增加分子层的厚度或降低石墨烯的费米能级可以进一步增强VSC模式的耦合强度,这将显著提高VSC效应在纳米尺度上的灵敏度。通过电调石墨烯的费米能级,能够在几何形状不变的同一结构上观测VSC光谱的演化。此外,推导了一个与数值仿真吻合的解析模型,用以理解VSC模式的形成机制和激发条件。重要的是,GPPs模式可在红外至太赫兹的宽光谱范围内被支持,因此可以与各种有机分子进行强耦合作用。（4）本文提出了一个石墨烯等离激元纳米空腔（Graphene Plasmon Nano-Cavity,GPNC）结构,用以增强纳米级有机分子的VSC效应。通过与几类等离激元纳米结构进行比较,首先证明了由石墨烯-石墨烯构成的GPNC是研究VSC效应的更优配置。在将5 nm厚的有机材料嵌入GPNC后,可在强耦合的分子系统的光谱演化中观察到明显的抗交叉特性,其拉比劈裂超过80 cm^-1。为了进一步提高VSC效应的可观测性,在GPNC的底部添加了足够厚的金属层,从而组装了一个完美吸收器结构,这可以显著增强VSC光谱的峰值。此外,推导了一个与数值仿真相一致的解析模型,将LGPRs模式和VSC模式纳入一个统一的理论框架中,从而为理解这些模式的形成过程和激发条件提供了清晰的物理图像。值得注意的是,LGPRs模式的频带与大多数分子的振动跃迁相互重叠,因此所提出的解析模型为今后设计此类结构提供了一种更简洁、更快速的方法。更多还原

【Abstract】 The strong coupling between infrared photonic resonances and vibrational transitions of organic molecules is called the vibrational strong coupling（VSC）effect.So far,the VSC effect has been shown to significantly change the chemical properties and chemical reactivity of molecules.Moreover,graphene plasmons have been shown to achieve significant optical confinement and near-field enhancement within the mid-infrared to terahertz band.Therefore,leveraging the outstanding properties of graphene plasmons,it is expected to achieve the VSC effect of nanoscale organic molecules.This dissertation conducts a relatively systematic theoretical study on the propagation and confinement characteristics of graphene plasmons and their enhanced VSC effect.The main research results are as follows:1.A graded-index waveguide array lens（GIWAL）is designed to achieve achromatic on-chip focusing of propagating graphene plasmon polaritons（GPPs）.By studying the propagation characteristics of the GPP modes supported by the GIWAL,a complete analytical model is established,which can not only explain the physical mechanism of on-chip focusing,but also clarify the complex beam evolution behaviors such as self-focusing,self-collimation,and optical pendulum effect of Gaussian beams,as well as spatial inversion of digital optical signals.By adjusting the duty cycle of the dielectric grating in the GIWAL,the effective index of the lens can be flexibly designed to obtain a graded index distribution consistent with that of a Mikaelian lens.At this time,the GPP modes propagating in the GIWAL will achieve periodic self-focusing on the central axis of the lens.The calculation results show that the GPP modes can achieve achromatic on-chip focusing within the 10-20 THz frequency band,and the focus size can be as small as about 0.02λ（λis the operating wavelength in free space）.Moreover,it is also demonstrated that the GIWAL can be used as a spatial inversion device for broadband digital optical signals.2.A graphene-dielectric-metal hybrid-structure（GDMHS）is proposed to achieve the far-field excitation of localized graphene plasmon resonances（LGPRs）.By analyzing the formation process of the LGPR modes,an analytical model for predicting the LGPR wavelength is constructed to guide actual experimental observations.With the help of this model,we can intuitively understand why the even-order LGPR modes cannot be excited by the far-field and why the increase in loss leads to the extinction of the intrinsic modes.Moreover,the analytical model verified by numerical simulation results shows that by tracking the change of the first-order LGPR wavelength,sub-femtometer resolution can be achieved in monitoring variations in the dielectric spacer thickness.When the first-order LGPR mode is used to detect material changes within the nanogap,a minimum detectable refractive index change as low as 10^-6 RIU can be realized.Furthermore,the detection sensitivity can be further increased by reducing the Fermi level of graphene or increasing the length of the resonant cavity.Notably,this sensing approach based on the LGPR modes can be applied to other spectral ranges of interest by actively tuning the Fermi level of graphene.3.The VSC effect between the propagating GPP modes and organic molecules is studied.First,it is theoretically proved that with the help of a perfect absorber,the peaks of the far-field spectra can be significantly increased,thereby enhancing the observability of the VSC effect.When organic molecules with a thickness of only 5 nm are strongly coupled to the GPP modes,obvious anti-crossing behavior and Rabi splitting up to about 60 cm^-1 can be observed from the absorption spectra of the coupled molecular system.Moreover,increasing the thickness of the molecular layer or lowering the Fermi level of graphene can further enhance the coupling strength of the VSC mode,which substantially improves the sensitivity of the VSC effect at the nanoscale.By electrically tuning the Fermi level of graphene,the evolution of the VSC spectra can be observed on the same structure with unchanged geometry.Moreover,an analytical model consistent with the numerical simulations is derived to understand the formation mechanism and excitation conditions of the VSC mode.Importantly,the GPP modes can be supported within a wide spectral range from infrared to terahertz,enabling strong coupling with a wide variety of organic molecules.4.A graphene plasmon nano-cavity（GPNC）structure is proposed to enhance the VSC effect of nanoscale organic molecules.By comparing several types of plasmonic nanostructures,it is first demonstrated that the GPNC composed of graphene-graphene represents a more favorable configuration for studying VSC phenomena.When a 5-nm-thick organic layer is embedded into the GPNC,a pronounced anti-crossing feature is observed from the spectral evolution of the strongly coupled molecular system,with a Rabi splitting exceeding 80 cm^-1.To further improve the observability of the VSC effect,a sufficiently thick metallic layer is introduced beneath the GPNC to form a perfect absorber structure,which significantly enhances the peaks of the VSC spectra.Additionally,an analytical model consistent with numerical simulations is developed,integrating both LGPRs and VSC modes into a unified theoretical framework.This model provides a clear physical picture for understanding the formation mechanisms and excitation conditions of these modes.Notably,the spectral range of the LGPR modes overlaps with the vibrational transitions of most molecules,making the proposed analytical model a more concise and efficient approach for the future design of such structures.更多还原