Disclosing transverse spin angular momentum of surface plasmon polaritons through independent spatiotemporal imaging of its in-plane and out-of-plane electric field components

Abstract

The comprehensive capture of near-field spatiotemporal information of surface plasmon polaritons (SPPs) is a prerequisite for revealing their physical nature. In this study, we first performed an independent, spatiotemporal imaging of the out-of-plane and in-plane components of SPP near-fields in a femtosecond light-excited trench using an obliquely incident time-resolved photoemission electron microscopy (TR-PEEM). We did the capture by imaging of the interference patterns induced by a superposition of the p - or s -polarized probe light, with the out-plane or in-plane components of SPP near-fields, under the noncollinear excitation mode. The method may be used to reconstruct a 3D SPP spatiotemporal field. Moreover, we demonstrated that the fringe shift of the interference patterns between the captured in-plane and out-of-plane components of the SPP field in PEEM images corresponds to the 1/4 fringe period, which is attributed to π / 2 out of phase of the out-of-plane and in-plane near-field components of SPP. The resulting TR-PEEM images are supported by a classical wave mode and FDTD simulations. Essentially, the measured π / 2 phase difference between the in-plane and out-of-plane components of the SPP indicated a rotating field component in the propagation plane, i.e., that the SPP exhibits an elliptically polarized electric field in the propagation plane. The experimental results presented herein provide direct evidence of SPP having the inherent attributes of transverse spin angular momentum.