Eager: Demonstration of Space-Time Surface Plasmon Polaritons

基于纳米技术的光学传感器一直是一个具有重要技术意义的领域，因为这些设备对周围环境的变化极其敏感。然而，这项技术的有效性仅限于在非常有限的范围内感知这些干扰。PI将领导对一种新型现象的研究，这种现象可能导致显著增强的光学传感器可以在显著延长的距离内工作。研究活动将提供一个机会，培训和指导至少一名研究生，他们属于代表人数不足的群体，从事实验纳米光学和相关数据分析。此外，学生将以各种学术渠道，包括技术同行评议出版物、会议报告和研讨会的形式，帮助广泛传播所有结果。涉及奇异光束的纳米光学的概念将被纳入一门新的入门、动手操作的光学显微镜课程，目标是来自不同工程学科的本科生大四学生和研究生。传统的表面等离子体激元(SPP)存在于金属-电介质界面上，基于光学平面波或高斯光束的激发。因此，表面等离子体激元在近红外区域沿该界面传播达几百微米(在可见光区域传播的长度要短得多)，并逐渐从表面衰减。本文研究的是时空表面等离子体激元(ST-SPP)的存在，它有望沿着金属-电介质界面呈现扩展传播。ST-SPP利用新开发的时空(ST)波包，实验表明，这种波包可以在任何介质中沿一个横向维度无衍射和无色散传播。ST波包是光场沿着(通常)两个可分离的自由度--在这种情况下是空间频率和波长--“经典纠缠”的表现。传统的表面等离子体波只局限在沿其传播的表面横面上的一个方向上，而ST-SPP则被限制在两个横向维度上。因此，ST-SPP将保持受限，并继续沿着基本上无衍射的表面传播，而不是SPP所经历的一维衍射扩散。在吸收损耗存在的情况下，ST-SPP的强度有预期的降低，但衍射对ST-SPP的影响保持不变。因此，与同时受金属损耗和衍射限制的SPP不同，ST-SPP可以传播仅受金属损耗限制的长距离。此外，与基于高斯的SPP波包相比，ST-SPP波包不会经历任何时间扩展，即色散。此外，与艾里等离子激元不同的是，所提出的ST-SPP是直线传播的。因此，ST-SPP的特征类似于等离子子弹。除了确认ST-SPP的存在外，这里进行的基础研究还旨在证明ST-SPP可以比SPP传播更远的距离(~300倍无损失，有损失和严格限制时高达~1000倍)，以及在飞秒时间尺度上表面上ST-SP的波束转向。这一裁决反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Optical sensors based on nanotechnology have been an area of technological importance because of the extreme sensitivity of these devices to changes in the surrounding environment. However, the effectiveness of this technology is limited to sensing these disturbances over a very limited range. The PI will lead an investigation of a new type of phenomenon that could lead to significantly enhanced optical sensors that can work over significantly extended distances. The research activity will provide an opportunity to train and mentor at least one graduate student belonging to an underrepresented group in experimental nano-optics and associated data analysis. Moreover, the student will assist in the broad dissemination of all results in the form of various academic channels including technical peer-reviewed publications, conference presentations, and seminars. Concepts in nano-optics involving exotic optical beams will be incorporated into a new introductory, hands-on optical microscopy course that targets undergraduate seniors and graduate students from various engineering disciplines. Traditional surface plasmon polaritons (SPPs), which exist at a metal-dielectric interface, are based on excitation by either an optical plane wave or Gaussian beam. As a result, SPPs propagate along this interface up to a few hundred microns in the NIR regime (and much shorter lengths in the visible) and evanescently decay away from the surface. The research carried out here investigates the existence of space-time surface plasmon polaritons (ST-SPPs), which are expected to exhibit extended propagation along a metal-dielectric interface. ST-SPPs utilize newly developed space-time (ST) wave packets, which have been shown experimentally to travel diffraction-free and dispersion-free in any medium along one transverse dimension. ST wave packets are a manifestation of ‘classically entangling’ an optical field along (typically) two separable degrees-of-freedom, which in this case are spatial frequency and wavelength. While traditional SPPs are confined along only one direction in the transverse plane on the surface, which they propagate along, ST-SPPs are expected to be confined in two transverse dimensions. Thus, in lieu of the 1-D diffractive spreading that SPPs undergo, ST-SPPs will remain confined and continue to propagate along the surface essentially diffraction-free. In the presence of absorptive loss, there is the expected reduction in intensity of the ST-SPP, but the effect of diffraction on the ST-SPP remains unchanged. Thus, an ST-SPP could propagate long distances limited by the metal loss only as opposed to an SPP which will be limited by both metal loss and diffraction. Moreover, in contrast to a Gaussian-based SPP wave packet, a ST-SPP would not undergo any temporal spread, i.e., dispersion. In addition, unlike Airy plasmons, the proposed ST-SPPs travel in a straight line. Thus, the characteristics of ST-SPPs would be akin to a plasmonic bullet. In addition to confirming the existence of ST-SPPs, the fundamental studies carried out here aims to demonstrate that ST-SPPs can travel at much greater distances (~300x without loss, and up to ~1000x with loss and tight confinement) than SPPs, and beam steering of ST-SPPs on a surface at femtosecond time scales.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.