Observation of space-time surface plasmon polaritons

TL;DR

This paper reports the first observation of space-time surface plasmon polaritons (ST-SPPs), ultrashort, diffraction-free surface waves with controlled propagation, achieved through spatiotemporal spectral sculpting and nanoslit coupling.

Contribution

It introduces the first realization of space-time SPPs with ultrashort duration and diffraction-free propagation, using precise spatiotemporal spectral shaping and broadband nanoslit coupling.

Findings

Demonstrated 16 fs diffraction-free SPPs propagating in a straight line.

Verified phase-tilted spatiotemporal wave-fronts through microscopy.

Controlled group velocity and propagation characteristics of ST-SPPs.

Abstract

Surface plasmon polaritons (SPPs) are surface-bound waves at metal-dielectric interfaces that exhibit strong out-of-plane field confinement, a key feature for applications is nano-scale sensing and imaging. However, this advantage is offset by diffractive spreading during in-plane propagation, leading to transverse spatial delocalization. Conventional strategies to combat diffraction through spatial structuring are not applicable for dimensionally restricted SPPs -- except for cosine plasmons that are not localized or Airy plasmons that propagate along a curved trajectory. Here, we report the first realization of space-time SPPs (ST-SPPs), ultrashort (16 fs) diffraction-free SPPs that propagate in a straight line, whose unique propagation characteristics stem from precise sculpting of their spatiotemporal spectra. By first synthesizing a spatiotemporally structured field in free space, we couple the field to an axially invariant ST-SPP at a metal-dielectric surface via an ultra-broadband nanoslit coupling mechanism, further enabling control over the ST-SPP group velocity and propagation characteristics. Time-resolved two-photon fluorescence interference microscopy enables reconstructing the surface-bound field in space and time, thereby verifying their predicted phase-tilted spatiotemporal wave-front and diffraction-free propagation. Our work opens new avenues for combining spatiotemporally structured light with the field-localization associated with nanophotonics, and may thus enable novel applications in surface-enhanced sensing and nonlinear optical interactions.