Plasmonic Spin-Hall Effect in Surface Plasmon Polariton Focusing

Abstract

Surface plasmon polaritons (SPPs) are fundamental collective charge density excitations at surfaces of solid-state plasmas. They can energize physical and chemical processes at the nanoscale, but control of the spatiotemporal evolution of their energy, momentum, and spin is necessary to effectively transfer their quantum properties to molecules and other nanoscopic objects. Thus, to design the coupling of SPPs into other modes of an electronic system, it is important to describe and control their nanofemto spatiotemporal distributions. We investigate the spatial SPP field and spin distributions when launched by illuminating a converging lens coupling structure in an Ag film with obliquely incident linearly or circularly polarized femtosecond light pulses. The propagation and focusing of SPPs are recorded as movies by scanning the pump−probe pulse delay and recording the evolving interference patterns between the SPP and optical fields by ultrafast interferometric time-resolved two-photon photoemission electron microscopy. The observed PEEM images of SPPs for p-polarized excitation light are symmetric with a distinct focal spot, but with circularly polarized excitation the coupling structure also acts as a plasmonic spin-Hall device exhibiting light helicity dependent asymmetric launching and focusing with respect to the optical plane. We show that the anisotropy arises because the SPP excitation efficiency depends on matching of the spin angular momenta between the incident vacuum light and the SPP field. Because the coupling structure generates distinct plasmonic field texture, the generated chiral density of the SPP field is spatially dependent on the focusing conditions and helicity of the excitation light. We demonstrate that the SPP fields excited by circularly polarized light (CPL) converge to a focal point of high chiral density with the location and sign defined by the light-helicity, which can be utilized to design and enhance quantum interactions with chiral objects, such as molecules.

Plasmonic Spin-Hall Effect of propagating Surface Plasmon Polaritons in Ni$_{80}$Fe$_{20}$ microstructures

“…In the latter images, the same wavelength of the interference patterns as in figure 5(a) is observed but without any significant phase shift. This observation is consistent with the theoretical considerations explained above as well as previous experiments on silver [31]. The interference in the linear dichroic signal originates dominantly from s-polarization, because p-polarized light has no wave vector component perpendicular to the edge of the structure.…”

“…It has been recently discovered that the evanescent electric field creates an additional spin angular momentum (SAM) perpendicular to the propagation direction of the SPP [26,27,28]. Experimentally, this additional SAM allows for selective excitation of SPPs at different edges of a microstructure as shown in figure 1 [29,30], and can be related to the plasmonic spin-Hall effect [31]. Since this effect relies on the presence of an additional SAM to which the SAM of the incident light can be coupled, it is not present in a plane wave propagating in the bulk [27].…”

Ultrafast Photoemission Electron Microscopy: Imaging Plasmons in Space and Time

“…The vectorial properties of the optical and SPP SAMs affect the light coupling into SPPs. As we will discuss further, the matching of SAM of elliptical light with that of SPPs can be used to control the amplitude of SPP propagation, as well as related properties, such as the local chirality density. ,− …”

The 2021 ultrafast spectroscopic probes of condensed matter roadmap

“…By combining this functional interface with a plasmonic lens, the SPP focus can be controlled and used as a plasmonic switch (figure 16(b)). Alternatively, a switching of the focus spot can also be achieved by utilizing the plasmonic spin-Hall effect that leads to different spot positions depending on the pumping light's helicity [153].…”