Multimode structured neutron beams

TL;DR

This work addresses the limited information capacity of single-mode neutron beams by introducing a multimode approach that stacks phase-grating arrays to access multiple OAM modes and Airy states simultaneously, yielding a discretized OAM spectrum with content described by $\ell = m q$. The authors experimentally generate and characterize mixtures of OAM states $\ell = 3$ and $\ell = 7$, as well as hybrid Airy–OAM configurations, using SANS measurements and Fresnel–Kirchhoff diffraction simulations to validate the observed far-field patterns. The results demonstrate controllable, parallel access to multiple structured neutron states in a single measurement, enabling neutron OAM spectroscopy and 2D multiplexing (OAM and energy) at pulsed sources. This multimode capability promises enhanced information retrieval in material scattering and fundamental neutron interactions, with practical benefits for time-resolved and symmetry-resolved neutron scattering experiments.

Abstract

The experimental realization of neutron orbital angular momentum (OAM) states and neutron Airy beams has opened new avenues for structured neutron science in both materials characterization and fundamental physics. These additional degrees of freedom in scattering experiments enable the exploration of selection rules for neutrons, the analysis of scattering properties in topological materials, and the generation of auto-focusing neutron beams. In the effort to enhance the amount of spatial and angular-momentum information retrievable from a single measurement, and to overcome current phase-grating efficiency limits, here we demonstrate multimode structured neutron beams that enable simultaneous access to multiple, well-defined OAM modes, and to hybrid combinations of OAM and Airy states. This multimode approach, analogous to wavelength- or OAM-multiplexing in optics, facilitates the efficient investigation of material scattering properties and nuclear interactions with a neutron source composed of a discretized OAM spectrum.

Figures (3)

• Figure 1: Schematic illustration of multimode structured neutron beam generation. An incident neutron beam passes through a series of phase-grating arrays, spatially separated along the beam (z) axis at distances $d_1$, $d_2$, etc. In this illustration, each grating array contains fork dislocations with topological charges $q=3,~5,$ or $7$, diffracting neutrons into well-defined OAM states in the first diffraction orders. After the final grating (Grating 3), the beam consists of a mixture of three distinct OAM states, which co-propagate to the detector plane. The resulting intensity profile exhibits a broadened doughnut-shaped structure, as shown in the simulated diffraction pattern. All gratings share the same periodicity and the separation between the gratings is on the order of a few mm. The size of the grating arrays within the mounts is exaggerated for visual clarity.
• Figure 2: a) The measured and simulated diffraction spectra at $z = 15.4$ m after a combination of a $q=3$ and a $q=7$ fork dislocation phase-grating. The smaller $q = 3$ doughnut-shaped profile is clearly observed and partially surrounded by the larger, less intense $q = 7$ profile. The $q=7$ grating is rotated 4 degrees with respect to the $q = 3$ phase-grating as seen in the experiment and simulation. b) The azimuthally integrated profile of the $m=+1$ diffraction order from the $q = 3$ and $q=7$ diffraction gratings in a). To compare, we measured a reference $q = 3$, $m = +1$ profile (analogous to Ref. sarenac2022experimental) to distinguish the contribution from the $q = 7$ grating. Notably, there is increased intensity at larger radial distances $r$ due to the larger $q = 7$ doughnut profile. The two measurements were normalized to the same measurement time. In the experiments a beam trap was placed at the center of the detector in the direct beam path to better emphasize neutron counts of the nonzero diffraction orders at the detector.
• Figure 3: a) The measured and simulated diffraction spectra at $z = 15.4$ m after a cubic phase-grating and a $q=7$ fork dislocation phase-grating. The $m = \pm 1$ diffraction orders show a superposition of the Airy beam profile and the $q=7$ doughnut-shaped profile. b) The measured and simulated diffraction spectra at $z = 15.4$ m after a cubic phase-grating and a $q = 3$ fork dislocation phase-grating. Here we plot the measured intensity in log scale to better emphasize the Airy profile as well as the second diffraction order. In the experiments a beam trap was placed at the center of the detector in the direct beam path to better emphasize neutron counts of the nonzero diffraction orders at the detector.