The invention of the laser turned optical holography into a practical imaging tool. But the utility of recording both the amplitude and phase of a scattered wave as an interference pattern is not exclusive to light. Holographic techniques find use in electron microscopy, acoustics, and now neutron interferometry. Dmitry Pushin and his colleagues at the University of Waterloo, together with collaborators from NIST, the Joint Quantum Institute at the University of Maryland, and North Carolina State University, have made the first neutron holograms of a macroscopic object. The interferometer’s first blade splits each incident neutron wavepacket into a reference beam and an object beam, as illustrated in the schematic; then a second blade directs the beams to the third blade where they interfere. The wavepacket is again split and sent to a digital neutron camera and a beam monitor. For the experiment, the researchers placed a 224-μm-thick aluminum disk with a spiral staircase structure carved into it in the object beam’s path. That structure imparted a helical wavefront, or orbital angular momentum, to the beam. By slightly tilting the reference beam’s wavefronts with a prism, the researchers obtained the pitchfork interference pattern seen in the figure. Neutrons, unlike electrons and photons, easily penetrate most materials, so neutron holography should lend itself to imaging structures in the interiors of material. And because appropriately illuminating a hologram reproduces the object beam that created it, the demonstration by Pushin and his colleagues suggests that holograms could be employed to custom shape neutron beams. (D. Sarenac et al., Opt. Express 24, 22528, 2016.)
