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1 0 A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 2 1 wavelengths and could be useful for nondestructive testing and analysis of materials such as ceramics and paint samples. Researchers demonstrated their proof-of-concept experiment for mid- infrared OCT based on ultrabroadband entangled photon pairs. They showed that this approach can produce high quality 2D and 3D images of scattering samples using a relatively compact, straightforward optical setup. This method eliminates the need for broad- band mid-infrared sources or detectors, which havemade it challenging to devel- op practical OCT systems that work at thesewavelengths. The technique could be useful for many applications includ- ing analyzing the complex paint layers used on airplanes and cars or monitor- ing pharmaceutical coatings. It can also provide detailed 3D images that could be useful for art conservation. For their technique, the research- ers developed and patented a nonlinear crystal that creates broadband photon pairs with varying wavelengths. They tested the setup on a range of real- world samples, including highly scat- tering paint samples. The team is now working with industry partners and other research institutes to develop a compact OCT sensor head and full sys- tem for a pilot commercial application. www.osa.org/en-us. TESTING | CHARACTERIZATION METAL 3D PRINTING Lawrence Livermore National Lab- oratory, Calif., researchers are measur- ing the emission of electrons from the surface of stainless steel during laser processing to improve the reliability of laser-based metal 3D-printing tech- niques. They collected thermionic emission signals from 316L stainless steel under laser powder bed fusion (LPBF) conditions using a custom test- bed system and a current preamplifier that measured the flow of electrons between the metal surface and the chamber. Then, they used the gen- erated thermionic emission to iden- tify dynamics caused by laser-metal interactions. The results illustrate the poten- tial for thermionic emission sensing to detect laser-drivenphenomena that can cause defects in parts, optimize build parameters, and improve knowledge of the LPBF process while com- plementing existing diag- nostic capabilities. The ability to capture thermal emission of electrons will help advance basic under- standing of the laser- material interaction dynam- ics involved in the LPBF process and support the broader technology matur- ation community in build- ing confidence in parts cre- ated using the technique. By observing and ana- lyzing the electrons emitted during laser processing, lab researchers demonstrated they could tie increases in thermionic emission to surface temperature and laser scanning conditions that cause pore formation and part defects. The work represents an important step toward establishing effective in situ monitoring capabilities that can accelerate qualification and certification of LPBF components, the researchers say. llnl.org. NONDESTRUCTIVE IMAGING WITH PHOTONS A collaborative research team from Humboldt-Universität zu Berlin and the Research Center for Non-Destructive Testing GmbH, Austria, showed that entangled photons can be used to improve the penetration depth of opti- cal coherence tomography (OCT) in highly scattering materials. The meth- od, as published in Optica, represents a way to perform OCT with mid-infrared Triangular holes make this material more likel y to crack from left to right. Courtesy of N.R. Brodnik et al./ Phys. Rev. Lett. Katherine Faber, FASM, and her colleagues at the California Institute of Tech- nology, 3D-printed rectangular plates with a regular array of triangular holes. Overall, the holes made the material tougher than it would have been without them. But when pulled from its edges, the material tended to break in a specific pattern—if the triangles pointed right, a crack first formed on the left and propagated rightwards. This cracking sequence revealed that the patterning caused an asymmetrical improvement in toughness. The researchers say their technique could help steer cracks in a prescribed direction and help protect critical components when a failure is inevitabl . nature.co . BRIEF Top: multi-physics simulation of laser-induced melting of stainless steel, showing the electron emission signal primarily produced at the front of the surface depression. Bottom: cross sections of laser tracks produced in stainless steel. Courtesy of Aiden Martin/LLNL. Triangular holes make this material more likely to crack from left to right. Courtesy of N.R. Brodnik et al./ Phys. Rev. Lett.