An illustration depicting the NIST neutron holography experiment. [Image: D. Sarenac et al. doi: 10.1364/OE.24.022528] [Enlarge image]
A team of researchers from several institutions in the United States and Canada report using neutron beams and a Mach-Zehnder interferometer to create a hologram of a macro-scale spiral phase plate (SPP; Opt. Express, doi: 10.1364/OE.24.022528). The hologram—a fork dislocation image—is created using a method similar to Emmett Leith and Juris Upatnieks’ off-axis dual-beam technique for optical holography from 1962, and could provide researchers with information about the internal structure of solid objects that can’t be penetrated by light.
In the past, neutron holograms have been used to determine repetitive crystalline structures on an atomic scale. However, this, according to the authors, is the first demonstration of a neutron hologram created from a macroscopic object.
Doing the splits
The neutron holography experiment began with one neutron beam that splits into an “object” beam and a “reference” beam, after entering a neutron interferometer. The object beam passes through the interferometer arm containing an SPP (q = 2). The SPP twists the beam, which experiences a phase shift. The reference beam then travels through a second arm of the interferometer.
In the second arm, a prism introduces a linear gradient that tilts the wavefronts (see diagram) so both beams reflect at a second beamsplitter (BS). The beams then coherently combine at a third BS. Upon exiting the third BS, one beam is directed into a digital camera while the other is directed into an integrating counter to act as an intensity monitor.
Furthering neutron-optics
The resulting neutron hologram is a composite image, created from the incoherent superposition of several individual neutron’s trips through the interferometer. The digital reconstruction, which looks a bit like an overhead view of water rivulets flowing around a rock, gives the researchers information about the phase generated by the SPP. This information could, the authors say, be a new neutron-optics tool for interferometric testing and could also offer “an extension of coherent phase-control techniques to applications in neutron radiography and imaging,” which are used to analyze specific points inside macro-scale solid objects.
The research team included scientists from the University of Waterloo, Canada; National Institutes of Standards and Technology, USA; North Carolina State University, USA; Triangle Universities Nuclear Laboratory, USA; Joint Quantum Institute, USA; Perimeter Institute of Theoretical Physics, Canada; Canadian Institute for Advanced Research; and Tulane University, USA.