太赫兹散射式扫描近场光学显微镜中探针与样品互作用及其影响探究

Tip-sample interactions in terahertz scattering scanning nearfield optical microscopy and its influences

太赫兹散射式扫描近场光学显微镜(scattering-type scanning near-field optical microscopy,s-SNOM)在生物纳米成像、太赫兹纳米光谱学、纳米材料成像以及极化激元的研究中有着广泛的应用前景.原子力显微镜探针作为太赫兹s-SNOM的重要组成部分,起着近场激发、探测、增强等关键作用.但是在测量过程中,探针与样品的相互作用会影响测量结果.本文通过仿真和实验,分别揭示了太赫兹s-SNOM中探针与样品相互作用对近场激发、近场探测以及太赫兹近场频谱的影响.首先,研究了探针激发的近场的波矢权重分布,发现波矢主要集中在10^(5) cm^(-1)量级,与一般的太赫兹激元的波矢相差2-3个数量级,这表明太赫兹近场很难激发太赫兹激元.其次,通过理论和实验研究,发现金属针尖会干扰石墨烯圆盘结构的表面近场,这表明太赫兹近场系统在探测结构的近场分布具有局限性;最后研究了探针对近场频谱的影响,发现探针长度和悬臂长度是影响近场频谱的重要参数,可以通过增大探针长度或者悬臂长度的方法来减小探针对近场频谱的影响.

Terahertz scattering scanning near-field optical microscopy(s-SNOM),as an important means to break through the limits of conventional optical diffraction,can achieve super-resolution imaging on a nanoscale and has a wide range of applications in biological nano-imaging,terahertz nano-spectroscopy,nanomaterials imaging,and the study of polarized excitations.As an important component of the terahertz s-SNOM,the atomic force microscope tip plays a key role in implementing the near-field excitation,detection,and enhancement.However,the tip-sample interaction can greatly affect the results.In this paper,the effects of tip-sample interaction on near-field excitation,near-field detection,and terahertz near-field spectrum in terahertz s-SNOM are revealed through simulations and experiments.First,the wave vector coupling weight of the near field excited by the tip is investigated,and it is found that the wave vector is concentrated mainly on the order of 10^(5) cm^(-1),which differs from that of the general terahertz excitations by 2 to 3 orders of magnitude,indicating that the terahertz near field is difficult to excite terahertz excitations.Secondly,through theoretical and experimental studies,it is found that the metal tip interferes with the surface near-field of the graphene disk structure,which indicates the limitations of the terahertz s-SNOM in probing the near-field distribution of the structure.Finally,the influence of the tip on the near-field spectrum is studied.It is found that the tip length and cantilever length are important parameters affecting the near-field spectrum,and the influence of the tip on the near-field spectrum can be reduced by increasing the tip length or cantilever length.