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Crystals, Lasers, Glasses, and Bent Molecules: Adventures in Nonlinear Optics

Light is an indispensable scientific tool. For example, laser-based optical sensors can detect oxygen in the environment, proteins in biomedical samples, and process markers in industrial settings, among other applications. However, not all wavelengths of light can be generated by a single laser, which limits what chemicals can be detected with optical sensing. That's where nonlinear optical crystals can help. By shining multiple lasers with different wavelengths through such crystals, researchers can tune laser beams via frequency conversion and cover more of the optical spectra. This has been a successful approach for wavelengths from ultraviolet to near-infrared(IR), but the mid-IR region has lacked practical nonlinear optical crystals. However, crystals may not be the only game in town. A multi-institution international research team is exploring a possible solution to the crystal problem: cutting-edge glasses containing mercuric iodide. The idea is that these glasses could behave like nonlinear optical crystals, offering an enticing approach to the generation of novel amorphous optical materials. But first, the researchers needed to figure out what these glasses look like at the atomic scale. For that, they went to the U.S. Department of Energy’s (DOE’s) Advanced Photon Source (APS) to collect high-energy x-ray diffraction data. By combining the diffraction data with other structural data and computer modeling, the team uncovered the secrets behind how a glass can act like a crystal.

Nonlinear optical crystals are widely used in photonics applications, but can be challenging to synthesize. To sidestep the need for crystals, scientists have been pursuing glassy materials that can tune lasers. One challenge is that glassy materials with isotropic internal structures aren't able to perform the frequency conversion necessary to tune lasers. However, glasses with chiral asymmetric properties have nonlinear optical potential. This research team is investigating hybrid molecular/network glasses with non-centrosymmetric mercuric iodide (HgI2).

To explore this material, the researchers probed the atomic structure of liquid mercury iodide as it transitions to a solid form at low temperatures, as well as its transition to molecular vapor at high temperatures. They employed several techniques to get at this molecular architecture, including high-energy x-ray diffraction at the X-ray Science Division 6-ID-D beamline at the APS, and at the BL04B2 beamline at SPring-8 (Japan); time-of-flight neutron diffraction at the ISIS Neutron and Muon Source (Rutherford-Appleton Laboratory, UK); and Raman spectroscopy. Furthermore, they fed this data into a density functional theory-modeling program to come up with a working structural model for this glassy material in different states of matter.

At higher temperatures, above 400 K, HgI2 exists as a linear triatomic molecule. However, the experimental data and structural modeling suggests that in a stable liquid and related glassy solid states, HgI2 becomes bent, with a bond angle of around 156° degrees. That's important, because that bend creates asymmetry in the material, in turn generating intrinsic optical non-linearity. To bolster these structural insights, the researchers checked the material's second harmonic generation. This property is one of the main avenues for frequency conversion in nonlinear optical crystals. Indeed, the bent HgI2-containing sulfide glass showed strong second-harmonic generation.

The team developed a hypothesis to help explain how the bent HgI2 containing materials generated second harmonic generation in a sulfide glass. At one extreme, the bent HgI2 molecules could be completely randomly oriented, conditions under which the second harmonic generation effect would be eliminated. At the other extreme, the molecules could arrange themselves non-randomly in preferred orientations within mesoscopic domains, the result of which would be the observed second harmonic generation effects. 

In future research, the team plans to perform small-angle neutron scattering and anomalous small-angle x-ray scattering on the glass. This data could provide verification for their hypothesis that nonrandom orientation within mesoscopic domains is responsible for the second harmonic generation effects.

A full understanding of this phenomena may help explain more generally the manifestation of nonlinear optical effects in isotropic media, as well provide a basis for the development of new materials with improved nonlinear optical properties. ― Erika Gebel Berg

See: Mohammad Kassem1, Maria Bokova1, Andrey S. Tverjanovich2, Daniele Fontanari1, David Le Coq3, Anton Sokolov1, Pascal Masselin1, Shinji Kohara4, Takeshi Usuki5, Alex C. Hannon6, Chris J. Benmore7, and Eugene Bychkov1*, “Bent HgI2 Molecules in the Melt and Sulfide Glasses: Implications for Nonlinear Optics,” Chem. Mater. 31, 4103 (2019). DOI: 10.1021/acs.chemmater.9b00860

Author affiliations: 1Université du Littoral Côte d’Opale, 2St. Petersburg State University, 3Institut des Sciences Chimiques de Rennes, 4Research Center for Advanced Measurement and Characterization, 5Yamagata University, 6Rutherford Appleton Laboratory, 7Argonne National Laboratory

Correspondence: *

This work was partly supported by Agence Nationale de la Recherche (ANR, France) under Grant No. ANR-15-ASTR-0016-01. The experiments at the SPring-8 were approved by the Japan Synchrotron Radiation Research Institute (proposal No. 2014B1197) and supported by the Centre for Advanced Science and Technology (Japan). A.T. is grateful to Saint-Petersburg State University grant No. 12.40.1342.2017. Work at the Advanced Photon Source, Argonne National Laboratory, was supported in part by the Basic Energy Sciences, Office of Science, U.S. Department of Energy, under Contract No. DE-AC02-06CH1135.

The APS is one of the world’s most productive x-ray light source facilities. Each year, the APS provides high-brightness x-ray beams to a diverse community of more than 5,000 researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. Researchers using the APS produce over 2,000 publications each year detailing impactful discoveries, and solve more vital biological protein structures than users of any other x-ray light source research facility. APS x-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being.

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