Very Cold Neutron Optics and Interferometry
Principal Investigators
Martin Fally and Jürgen Klepp
We explore optics and interferometry with very cold neutrons (VCN), whose wavelengths are on the order of a few nanometers (λ ≈ 2–10 nm). Traditional approaches for thermal neutrons use perfect crystals and Bragg diffraction. However, at VCN wavelengths these crystals become ineffective, necessitating artificial diffractive elements with larger lattice constants and tailored efficiencies.
Background and Motivation
Our solution brings together materials physics, optical holography, and nonlinear optics: using holography in nonlinear nanoparticle–polymer composites, we fabricate thin (50 μm) diffraction gratings engineered for high VCN diffraction efficiency.
Holographic grating in a nanodiamond polymer composite as a beamsplitter for VCN.
Research Approach and Computational Methods
These artificial gratings, with periods of a few hundred nanometers, act as 2-port and 3-port beam splitters and as mirrors, enabling flexible neutron beam control. Their design is guided by the Bragg condition. A key advantage of our platform is tunability: by selecting the nanoparticle species, we control the neutron-matter interaction via the scattering length density. Nanodiamond-based composites provide strong coherent diffraction and large refractive index modulation for slow neutrons [1,2], while hyperbranched polymer nanocomposites can achieve even higher modulation amplitudes [3]. The latter, however, introduce significant incoherent scattering, which is unfavorable for thicker elements - an important consideration when optimizing device geometry and performance. We also functionalize periodic structures with (superpara)magnetic nanoparticles so that the gratings couple to the neutron spin in addition to the nuclear potential, opening routes to spin-dependent optics. Beyond beam splitting and steering, these neutron-optics components, together with appropriate modeling, allow us to characterize key beam parameters such as the wavelength distribution.
Diffraction efficiency for a 50 μm thick nanodiamond polymer composite grating using VCN at an average wavelength 4.5 nm.
Top: 2D-Detector image (false colors) of VCN diffraction: ~50% efficiency at the first order Bragg angle is observed, i.e., a 2-port beamsplitter.
Bottom: Angular dependence of the diffraction efficiency for seven orders.
Key Results and Applications
Recent advances in grating efficiency and uniformity allow us to cascade three (or more) gratings into a practical VCN interferometer. This architecture aims at precision tests of fundamental interactions at long wavelengths, where phase shifts accumulate strongly. Commissioning at neutron beamlines is technically demanding – alignment tolerances are stringent and environmental stability is crucial – but is well underway [4].
As a complementary line of research, we use cold-neutron diffraction to probe self-organization phenomena in magnetic soft matter. In particular, we study the emergence of stripe patterns and chiral lane formation in ferromagnetic ferrofluids driven by oscillating external magnetic fields, revealing rich non-equilibrium dynamics tied to magnetization textures and interparticle forces [5].
Our neutron experiments are carried out at leading facilities in Europe, including the Institut Laue-Langevin (France) and the Paul Scherrer Institut (Switzerland).
Image of a very cold neutron interferometer at the beamline PF2-VCN.
- E. Hadden et al., “Holographic nanodiamond-polymer composite grating with unprecedented slow-neutron refractive index modulation amplitude,” Appl. Phys. Lett. 124(7), 071901 (2024) [doi:10.1063/5.0186753].
- Y. Tomita et al., “Fabrication of nanodiamond-dispersed composite holographic gratings and their light and slow-neutron diffraction properties,” Phys. Rev. Appl. 14, 044056 (2020) [doi:10.1103/PhysRevApplied.14.044056].
- E. Hadden et al., “Holographic hyperbranched polymer nanocomposite grating with exceptionally large neutron scattering length density modulation amplitudes,” Sci. Rep. 15, 31512 (2025) [doi:10.1038/s41598-025-16998-z].
- R. H. Ackermann et al., “Commissioning measurements for a very cold neutron interferometer based on nanodiamond-polymer composite gratings,” in Photosensitive Materials and their Applications IV 14102, R. R. McLeod, Y. Tomita, and I. Pascual Villalobos, Eds., p. 1410207, SPIE (2026) [doi:10.1117/12.3107915].
- M. Vilfan et al., "Spontaneous Chiral Symmetry Breaking and Lane Formation in Ferromagnetic Ferrofluids" Small, 2304387 (2023) [doi:10.1002/smll.202304387]