超透镜对倏逝波重聚焦的共振表面等离子体激元激发
Excitation of resonant surface plasmons for evanescent waves refocusing by a superlens
摘要：
平面超透镜对倏逝波的放大需要近共振耦合，这与共振表面等离子体激元有关。一个微妙的相互作用已被提出之间存在的激发明确的共振表面等离子体和聚焦能力的超透镜。为了深入了解这些谐振模式及其对倏逝波的放大和恢复的贡献，我们对因果负折射率洛伦兹模型进行了简单但强大的全波FDTD模拟。我们发现，当发散的透射光束和会聚的折射光束在第二界面处发生耦合时，会激发出一对定义明确的共振表面等离子体。这些模式在界面处的共振耦合导致理论预测的单个界面共振的激发。这种共振的物理后果是入射波能量被泵入负折射率材料介质中并超越它。这些现象对像平面近场的放大和恢复有很大贡献。值得注意的是，为了在平坦超透镜中实现倏逝波重聚焦，应当优化其厚度和源到超透镜的距离。这种优化是至关重要的，因为最佳厚度以及最佳源到超透镜距离将允许倏逝波重新聚焦以支配材料损失。FDTD模拟结果显示了超透镜厚度优化后，点源在超透镜内部和外部的图像。超透镜之外的图像的分辨率为$\sim.58λ$，并且该超透镜表现得像一个近乎完美的透镜。总的来说，这些数值模拟的结果可以作为有用的近似的共振放大和近场的重新聚焦的程度，可以实现在近场实验成像装置中的平面超透镜。
英文摘要：
The amplification of evanescent waves by flat superlens requires a near-resonance coupling which has been linked to resonant surface plasmons. A subtle interplay has been proposed to exist between the excitation of well-defined resonant surface plasmons and the focusing capability of a superlens. To gain insights into these resonant modes and their contributions to the amplification and recovery of evanescent waves, we performed simple but robust full-wave FDTD simulations on causal negative index Lorentz models. We found that well-defined pair of resonant surface plasmons is excited whenever a coupling of a diverging transmitted beam and a converging refracted beam occurs at the second interface. The resonant coupling of these modes at the interface led to the excitation of a single interface resonance predicted by the theory. The physical consequence of this resonance is that incident wave energy is pumped into the negative-index material medium and beyond it. These phenomena contribute substantially to the amplification and recovery of the near-fields in the image plane. It is noteworthy that, for evanescent wave refocusing to be achieved in a flat superlens, its thickness, and the source-to-superlens distance should be optimized. This optimization is critical in that an optimal thickness alongside optimal source-to-superlens distance will allow evanescent wave refocusing to dominate material lost. The FDTD simulated result showed an image of the point source inside the superlens and beyond it when its thickness was optimized. The resolution of the image beyond the superlens was $\sim.58λ$ and this superlens behaves like a near-perfect lens. Overall, these numerically simulated results could serve as useful approximations to the degree of resonant amplification and refocusing of near-fields that can be achieved by flat superlens in near-fields experimental imaging setups.
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