Cornell University researchers have developed nanostructures that enable record-breaking conversion of laser pulses into high-harmonic generation. The work paves the way for new scientific tools for high-resolution imaging as well as the study of physical processes that occur at the scale of an attosecond. High harmonic generation is used to merge photons from a pulsed laser into one ultrashort photon with higher energy, producing extreme ultraviolet (EUV) light and x-rays used for a variety of scientific purposes. Gases have traditionally been used as sources of harmonics, but a research team led by Gennady Shvets in the College of Engineering found that engineered nanostructures constitute an easier solution. The nanostructures that the team created make up an ultrathin resonant gallium phosphide metasurface that overcomes many of the usual problems associated with high harmonic generation in gases and other solids. The gallium phosphide materials permit harmonics of all orders without re-absorbing them, and the specialized structure is able to interact with the laser pulse’s full spectrum. “Achieving this required engineering of the metasurface’s structure using full-wave simulations,” said Maxim Shcherbakov, who was a postdoctoral associate at Cornell. “We carefully selected the parameters of the gallium-phosphide particles to fulfill this condition, and then it took a custom nanofabrication flow to bring it to light.” Illustration of an infrared laser hitting a gallium phosphide metsurface, which efficiently produces even and odd high-harmonic generation. Courtesy of Cornell via Daniil Shilkin. Those efforts yielded nanostructures capable of generating both even and odd harmonics — a limitation of most other harmonic materials — covering a wide range of photon energies between 1.3 and 3 electron volts. The level of conversion efficiency enables scientists to observe the molecular and electronic dynamics within a material with just one laser shot, which helps preserve samples that may otherwise be degraded by multiple high-powered shots. To the researchers’ knowledge, the study is the first to observe high-harmonic generated radiation from a single laser pulse, which allowed the metasurface to withstand high powers — five to 10 times higher than previously shown in other metasurfaces. “It opens up new opportunities to study matter at ultrahigh fields, a regime not readily accessible before,” said Shcherbakov. “With our method, we envision that people can study materials beyond metasurfaces, including but not limited to crystals, 2D materials, single atoms, artificial atomic lattices, and other quantum systems.” With the preliminary work of demonstrating the advantages of nanostructures for high harmonic generation, the team now seeks to improve high-harmonic devices and facilities by stacking the nanostructures together to replace a solid-state source, such as crystals. The work was funded by the Office of Naval Research, the Cornell Center for Materials Research through the National Science Foundation’s Materials Research Science and Engineering Centers program, and the Air Force Office of Scientific Research. The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-24450-9).