Observation of Ultrabroadband Striped Space-Time Surface Plasmon Polaritons

Abstract

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 spatiotemporal 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 toward 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 toward realization of the full potential of ST-SPPs for plasmonic sensing and imaging.

Transverse spin angular momentum of space-time surface plasmon polariton wave packet

“…For v g v sp , the spectral domain in (k x , ω) plane shows a slight curvature but almost constant |k x |, resulting in very small ∆k x . The spatial distribution of the wave packet is similar to that of a striped ST-SPP wave packet whose spectral domain is defined by iso-k x plane [70], and w x is almost infinite. In contrast, for ST-SPP wave packets with large v diff , the difference between k y and k sp is larger for larger Ω = ω o − ω, resulting in large ∆k x .…”

“…Although the calculations up to this point only considered the case t = 0 and propagation loss was not taken into account, the actual propagation of an ST-SPP wave packet on the dispersive metal surface has a frequencydependent attenuation determined by the imaginary part of the dielectric function. Attenuating the overall intensity is inevitable, and there will be a slight deformation where the ST-SPP wave packet has an ultrabroadband spectral width [66,70]. To estimate the spin texture stability of the ST-SPP wave packet, calculations of the ST-SPP wave packet before and after propagation were performed with an attenuation model that introduced propagation loss into Eq.…”

“…ST-SPP wave packets have been theoretically shown to possess attractive properties such as diffraction-free and dispersion-free propagation invariance and arbitrary group velocities. Efforts to experimentally excite and observe them are ongoing [70,71].…”