超宽带条纹时空表面等离激元极化子的观测
Observation of ultrabroadband striped space-time surface plasmon polaritons
AI总结：
本文首次实验观测到超宽带条纹时空表面等离激元极化子，通过自由空间时空波包与金-介质界面耦合并利用双光子荧光轴向拍频进行识别，为实现其在传感与成像中的潜力迈出关键一步。
摘要：
由于表面等离激元极化子（SPPs）是具有一个自由横向维度的表面波，SPPs 唯一的单色无衍射空间轮廓是余弦波和 Airy 波。最近已经构建了脉冲 SPP 波包，这些波包凭借其空间频率与时间频率之间紧密的光谱关联，在传播方向上保持不变并在面内维度上局域化，因此被称为“时空”（ST）SPPs。由于 ST-SPPs 特有的时空光谱结构，这种新型等离激元场构型的最优激发策略仍然是一个悬而未决的问题。我们在此通过报告超宽带条纹 ST-SPPs 的观测结果，向实现 ST-SPPs 迈出了关键一步。在这些 SPPs 中，每个波长都以相对于传播轴的规定角度传播，从而产生周期性的（条纹状）横向空间轮廓，并且该轮廓是无衍射的。我们从一个自由空间 ST 波包出发，将其耦合到金-介质界面处的 ST-SPP，并通过在入射 ST 波包与激发的表面束缚 ST-SPP 叠加产生的双光子荧光中检测到的轴向拍频，明确地识别出 ST-SPP。这些结果突显了一种高效且可靠地耦合到 ST-SPPs 的可行方法，因此代表了朝着实现 ST-SPPs 在等离激元传感和成像方面全部潜力的第一个关键步骤。
英文摘要：
Because surface plasmon polaritons (SPPs) are surface waves characterized by one free transverse dimension, the only monochromatic diffraction-free spatial profiles for SPPs are cosine and Airy waves. Pulsed SPP wave packets have been recently formulated that are propagation-invariant and localized in the in-plane dimensions by virtue of a tight spectral association between their spatial and temporal frequencies, which have thus been dubbed `space-time' (ST) SPPs. Because of the spatio-temporal spectral structure unique to ST-SPPs, the optimal launching strategy of such novel plasmonic field configurations remains an open question. We present here a critical step towards realizing ST-SPPs by reporting observations of ultrabroadband striped ST-SPPs. These are SPPs in which each wavelength travels at a prescribed angle with respect to the propagation axis to produce a periodic (striped) transverse spatial profile that is diffraction-free. We start with a free-space ST wave packet that is coupled to a ST-SPP at a gold-dielectric interface, and unambiguously identify the ST-SPP via an axial beating detected in two-photon fluorescence produced by the superposition of incident ST wave packet and the excited surface-bound ST-SPP. These results highlight a viable approach for efficient and reliable coupling to ST-SPPs, and thus represent the first crucial step towards realization of the full potential of ST-SPPs for plasmonic sensing and imaging.
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