Plasmonic Radiation from Spin-Momentum Locking

Abstract

Chiral light emission plays a key role in sensing, tomography, quantum communication, among others. Whereas, achieving highly pure, tunable chirality emission across a broad spectrum currently presents significant challenges. Free-electron radiation emerges as a promising solution to surpass these barriers, especially in hard-to-reach regimes. Here, chiral free-electron radiation is presented by exploiting the spin-momentum locking (SML) property of spoof surface plasmons (SSPs). When the phase velocity of free electrons matches that of the SSPs, the SSPs can be excited. By implementing wavenumber compensation through perturbations, the confined SSPs are transformed into free-space free-electron radiation. Owing to the law of angular momentum conservation, this process converts the transverse spin angular momentum of SSPs into the longitudinal spin angular momentum of free-electron radiation during the process, producing pure, tunable, and chiral free-electron radiation across a broad spectrum. This method achieves an optimal degree of circular polarization approaching -1. The innovative methodology can be adapted to SML-enabled guided states or silicon photonics platforms, offering new avenues for achieving chiral emission.

Keywords: chirality; free‐electron radiation; spin‐momentum locking; spoof surface plasmons.

Figure 1

Schematic of Chiral Free‐electron Radiation based on SML. As swift electrons skim over the metallic grating, SML enables the generation of chiral SSPs with opposite T‐SAM, which are then transformed into chiral radiation with opposite L‐SAM based on the Brillouin‐folding effect. The left inset illustrates the structural description of the scheme, while the right insets indicate the chirality of the T‐SAM of SSPs and L‐SAM of SPR. The length, period, depth, and gap width of the metallic grating are l_p = 0.80 mm, p = 0.40 mm, h_p = 0.25 mm, and w_p = 0.20 mm. The diameter, period, and depth of the periodically loaded cylinder and its distance away from gratings are d_c = 0.20 mm, p_c = 0.80 mm. h_c = 0.30 mm, g_c = 0.10 mm. The length, width, and thickness of the cathode are denoted as l_s = 0.40 mm, w_s = 0.10 mm, th_s = 0.10 mm.

Figure 2

The theoretic analysis of SSPs. a) Dispersion curve of SSPs. b) Electric field distributions of SSPs. c) Spin momentum locking phenomenon.

Figure 3

Chiral SPR emission. a) Dispersion diagram of the interaction system. The orange and purple lines represent the dispersion curves of the modified structure and the electron beam, respectively. b–d) Electric field distribution in the x–y plane, far‐field pattern, and chirality with cylinders on +y side (b), −y side (c), and both sides (d).

Figure 4

Tunability analysis of modified chiral SPR with normalized beam velocity varying a) theoretical operation frequency; b) simulation frequency and main lobe direction; c) degree of circular polarization analysis.

Figure 5

Experimental verification. a) Conceptual and fabricated antenna in the microwave band. b) Experimental environment and setup for testing the microwave antenna. c,d) S parameters for SML‐based microwave antenna presented in both simulation and experimental results. e) Radiation pattern of the radiation. f,g) Far‐field radiation plot for LH and RH components and the light‐yellow plane stands for the modified structure. h) Measured evolutions of the S₂₁ spectra versus radiation angle φ where φ is remapped into wavenumber space. The dark blue dashed line represents the theoretical dispersion analysis.