Surface Plasmon Mediated Giant Goos-Hanchen and Imbert-Fedorov Shifts on a Corrugated Metal Surface

TL;DR

This paper investigates how surface plasmon resonance at a corrugated metal–dielectric interface modulates spin–orbit interactions of light to amplify Goos–Hänchen and Imbert–Fedorov beam shifts. A vectorial, nonparaxial angular-spectrum framework is used to model structured beams (Gaussian and LG with OAM) reflecting from a corrugated Ag surface, capturing both in-plane and conical diffraction geometries. The authors demonstrate giant SPR-enhanced GH shifts for in-plane p-polarized illumination, and present two strategies to amplify IF shifts: polarization-post-selection (diagonal projections) and weak-value amplification with nearly orthogonal pre- and post-selection, plus vortex-induced coupling that modifies shift behavior. In conical diffraction, cross-polarization coupling enables SPR access for s polarization and can even annihilate the conventional spin Hall effect, underscoring the rich interplay of geometry, polarization, and plasmonic modes. These findings advance opportunities for nanoscale light control, plasmonic sensing, and metrology by exploiting SOI and weak measurement concepts at structured plasmonic interfaces.

Abstract

Enhanced beam shifts mediated by surface plasmon resonance (SPR) at metal-dielectric interfaces have been widely investigated. However, research on the associated Imbert-Fedorov or spin Hall shifts, driven by the spin-orbit interaction of structured light in structured interfaces, has been comparatively scarce and limited. We explore the reflection characteristics of generic polarized, non-paraxial light beams from a corrugated silver (Ag) interface, since surface corrugation can naturally couple the incident radiation modes to the surface excitations. In the vicinity of SPR, we report a significant enhancement in the beam shifts, attributed to the rapid variation of the specular reflection coefficient near its minima, resulting in amplified weak values. By carefully selecting the incident and projected polarization states of the beam, we achieve a pronounced spatial spin Hall effect. We also investigate vortex-induced beam shifts within this resonant regime, revealing distinctive signatures of the angular momentum of the beam. Furthermore, a comprehensive analysis is also presented for the conical diffraction geometry, wherein polarization conversions between p and s states are fully incorporated. Our work establishes the interplay of the spin-orbit interaction of light and the weak measurement approach as an important methodology in amplifying SPR effects, which may have important connotations in applications involving light at nanoscales.

Figures (4)

• Figure 1: Schematics showing (a) in-plane and (b) conical diffraction on a corrugated metallic surface, where the grating vector $\mathbf{K}$ (blue arrow) lies within or outside the plane of incidence, respectively. Only the central wave vector $\mathbf{k_c}$ (red arrow) of the incident beam spectrum is shown, oriented at an azimuthal angle $\Phi$ and an angle of incidence $\vartheta_i$. Angular dependence of (c, e) zero-order and (d, f) +1-order reflected intensities for p- and s-polarized plane-wave illuminations for the in-plane diffraction geometry. (g–j) Corresponding intensities for the conical diffraction with an azimuthal angle $\Phi=35^{\circ}$ for an incident $p$ polarized plane wave. Giant spatial GH shifts near the SPR angle for a p-polarized incident Gaussian beam at three different incident angles: (k) $41.75^{\circ}$, (l) $44.2^{\circ}$, and (m) $45.5^{\circ}$, respectively. Dashed (solid) crosshair represents the reflected (incident) beam centroid.
• Figure 2: Reflected beam intensity profiles for incident (a) LCP and (b) RCP Gaussian beams at an angle of incidence $\vartheta_i = 44.6^\circ$. (c) Transverse IF shifts as a function of the angle of incidence for the incident LCP (blue line) and RCP (red line) beams. Reflected beam intensities for the (d) $+45^{\circ}$ ($I^{p}_{+45}$) and (e) $-45^{\circ}$ ($I^{p}_{-45}$) diagonal polarization projections correspond to an incident p-polarized Gaussian beam at an angle of incidence $\vartheta_{i} = 45^{\circ}$. (f) IF shifts as a function of the angle of incidence for the $\pm45^{\circ}$ diagonally projected states. IF shifts of the reflected beams projected onto the nearly orthogonal states with (g) offset $\epsilon = +0.01$ and (h) offset $\epsilon = -0.01$. The incident beam is p-polarized and impinges at an angle of incidence $\vartheta_{i} = 40^{\circ}$. (i) Magnitude of the weak value $|A_w|$ of the spatial IF shift as a function of offset $\epsilon$ for nearly orthogonal pre- and post-selected states. Here also, the dashed (solid) crosshair represents the reflected (incident) beam centroid.
• Figure 3: Intensity profiles of the reflected beams for the incident (a) LCP Gaussian beam ($\ell=0$) and (b) LCP LG beam ($\ell=2$) at an incidence angle of $43.2^{\circ}$ ($\theta<\theta_p$). The transverse IF shift for the LG beam is more pronounced compared to the Gaussian beam. (c,d) Similar intensity profiles of the reflected beams for an angle of incidence of $45.2^{\circ}$ ($\theta>\theta_p$). Variation of the spatial (e) GH and (f) IF shifts with the angle of incidence for the incident LCP Gaussian and LCP LG beams. As in Fig. \ref{['fig1']}, the white dashed (solid) crosshair denotes the reflected (incident) beam centroid.
• Figure 4: Intensity profiles of the reflected beams for the incident (a) p-polarized, (b) s-polarized, (c) left circularly polarized, and (d) right circularly polarized Gaussian beams. In all the cases, the angle of incidence is $47.7^\circ$ and the azimuthal angle $\Phi=35^\circ$ (conical diffraction). The dashed and solid crosshairs correspond to the centroids of the reflected and incident beams, respectively.