Nanophotonics for pair production

Abstract

The transformation of electromagnetic energy into matter represents a fascinating prediction of relativistic quantum electrodynamics that is paradigmatically exemplified by the creation of electron-positron pairs out of light. However, this phenomenon has a very low probability, so positron sources rely instead on beta decay, which demands elaborate monochromatization and trapping schemes to achieve high-quality beams. Here, we propose to use intense, strongly confined optical near fields supported by a nanostructured material in combination with high-energy photons to create electron-positron pairs. Specifically, we show that the interaction between near-threshold γ-rays and polaritons yields higher pair-production cross sections, largely exceeding those associated with free-space photons. Our work opens an unexplored avenue toward generating tunable pulsed positrons from nanoscale regions at the intersection between particle physics and nanophotonics.

Fig. 1. Pair production by interaction of polaritons and γ-photons.

a We consider polaritons supported by a material structure. Energetic γ-rays interact with the polaritons, giving rise to electron–positron pairs. b Direct and time-reversed Feynman diagrams contributing to the investigated pair production. We indicate the energies and wave vectors of the polariton, the γ-photon, and the fermions by color-coordinated labels. Both polariton emission and absorption processes contribute to pair production, as indicated by orange arrows pointing toward the vertex or away from it, respectively.

Fig. 2. Pair-production assisted by surface polaritons.

a We consider surface modes excited in a 2D material by a coupling tip illuminated by laser pulses (red), while the γ-rays (dark gray) normally impinge on the surface. The positron emission direction (θ, φ) (purple) determines the electron direction (blue) by conservation of energy and in-plane momentum. b Comparison between the regions allowed by energy–momentum conservation in either Bethe–Wheeler (BW) photon–photon scattering (yellow) and polariton–photon scattering under the configuration of Fig. 1a (purple) as a function of polariton/photon energies. The BW threshold ${\hslash }^2{\omega }_p{\omega }_{\gamma }=2m_{\mathrm{e}}^2{c}^4/\left(1-\cos {\theta }_{p\gamma }\right)$ is indicated for a relative photon-photon angle θ_pγ of π (absolute threshold) and π/2. c Pair-production cross sections for polariton-photon scattering (σ^pol, purple curves), BW scattering (σ^BW for θ_pγ = π/2, black curves; see Supplementary Note 5.3), and Bethe–Heitler (BH) scattering by a carbon atom (σ^BH, green curve; see Supplementary Note 5.1). We consider different polariton energies (see legend) with a fixed k_p = 0.05 nm⁻¹ in all cases. Solid vertical lines indicate the γ-photon BW threshold energies taken from the θ_pγ = π/2 curve in (a).

Fig. 3. Pair-production from a gap polariton.

a Sketch of the geometry under consideration, in which pairs are produced by γ-photons traversing a gap polariton. The latter can be excited by a laser pulse and is taken to have frequency ω_p and uniform field ${\overrightarrow{\mathcal{E}}}_p$ confined to a spherical region of radius R_p (flanked by a polaritonic material). b Differential cross-section as a function of polar angle for polariton-assisted positron emission under the configuration in (a) (colored curves for different values of R_p and ω_p, as indicated by labels), compared with the BH cross section for a gold atom (see Supplementary Note 5.1). We consider 1.17 MeV γ-photons in all cases.