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).

>Read more on the Advanced Photon Source (APS) website

Image: Schematic representation of second harmonic generation as light passes through bent HgI2 molecules adopting a non-random orientation within mesoscopic domains of sulfide glass.