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.Comment: 9 pages, 8 figure