Eager: Demonstration of Space-Time Surface Plasmon Polaritons

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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.