Observation of space-time surface plasmon polaritons.

Area of Science:

• Nanophotonics and Plasmonics
• Ultrafast Optics
• Surface Physics

Background:

• Surface plasmon polaritons (SPPs) offer nanoscale field confinement at metal-dielectric interfaces, crucial for sensing and imaging.
• Diffraction limits SPP spatial delocalization, hindering applications.
• Existing methods to counteract diffraction, like Airy or cosine plasmons, are limited or follow curved paths.

Purpose of the Study:

• To demonstrate diffraction-free, rectilinear propagation of surface plasmon polaritons.
• To introduce space-time surface plasmon polaritons (ST-SPPs) as a solution to SPP delocalization.
• To explore novel applications in nanophotonics by combining structured light with SPPs.

Main Methods:

• Synthesizing a spatiotemporally structured light field in free space.
• Coupling the structured field to an axially invariant ST-SPP at a metal-dielectric surface.
• Utilizing time-resolved two-photon fluorescence microscopy to reconstruct the surface-bound field.

Main Results:

• Demonstrated ultrashort (16-fs) ST-SPPs that propagate rectilinearly without diffraction.
• Precisely controlled ST-SPP group velocity and propagation characteristics via spectral sculpting.
• Verified diffraction-free propagation and spatiotemporal wavefront through experimental reconstruction.

Conclusions:

• ST-SPPs overcome the diffraction limitations of conventional SPPs.
• This technique enables the combination of spatiotemporally structured light with nanophotonic field localization.
• Potential for new applications in surface-enhanced sensing and nonlinear optical interactions.