Hypercrystals for Faster Optoelectronics

This drawing depicts a photonic hypercrystal, which is promising for future Li-Fi technologies that offer major advantages over Wi-Fi and other radiofrequency communications systems. (Source: T. Galfsky, CCNY)

Sources that integrate two arti­ficial optical material concepts may drive ultrafast Li-Fi communi­cations. In many appli­cations, Li-Fi through-the-air optical networks poten­tially offer major advantages over Wi-Fi and other radio­frequency systems. Li-Fi nets can operate at extremely high speeds and exploit an extremely broad spectrum of fre­quencies. They avoid the interference problems that plague radio­frequency systems, which are especially pro­blematic in high-security environ­ments such as airplane cockpits and nuclear power stations. They are less open to hackers. And while their range is relatively limited, they don’t need line-of-sight connections to operate, said Evgenii Narimanov, a Purdue Uni­versity professor of electrical and computer engi­neering. Today’s Li-Fi nets can’t fully achieve all these potential benefits because they lack suitable light sources, he said. But designs that integrate two optical material concepts into photonic hyper­crystals may fill this gap.

Nari­manov first proposed this concept in 2014. This month, he and colleagues at the City College of New York reported demon­strations of photonic hyper­crystals with greatly increased light emission rates and inten­sities. Photonic hyper­crystals combine the properties of meta­materials and photonic crystals, both arti­ficial optical materials with pro­perties that are not usually found in nature, Narimanov said. Meta­materials are created from arti­ficial building blocks that are much smaller than the wave­length of light, while in photonic crystals the size of the unit cell is comparable to this wavelength. While these two types of composite materials generally show very different pro­perties, the photonic hyper­crystals combine them all within the same structure.

Photonic hyper­crystals are based on one type called hyper­bolic meta­materials, which can be built with alter­nating layers of metal and dielectric materials, where the electrical current can only travel along the metallic layers. “Generally, for light, metals and dielec­trics are funda­mentally different: light can travel in dielec­trics, but is reflected back from metals,” Narimanov said. “But a hyper­bolic meta­material behaves as metal along the layers and as a dielec­tric in the direction perpen­dicular to the layers, at the same time. For light, hyper­bolic media is, therefore, the third estate of matter, entirely different from the usual metals and dielec­trics.”

Among the interes­ting pro­perties that this structure produces, the meta­material accom­modates a large number of phot­onic states, allowing spon­taneous light emis­sion at extremely high rates. “For a light source, the problem is that this light in the hyper­bolic meta­material can’t get out,” said Nari­manov. But photonic crystals can mani­pulate optical inter­ference to opti­mize light trans­mission. In the integrated photonic hypercrystals, the hyper­bolic meta­material consists of alter­nating layers of silver and aluminum oxide. Hexa­gonal arrays of holes milled into the layers create the photonic crystal. In the design, the visible light is emitted by quantum dots embedded in one of the layers that form the hype­rbolic meta­material. The result: extremely high levels of control and enhance­ment of the emitted light.

“These photonic hyper­crystals were fabri­cated at the City Uni­versity of New York’s Advanced Science Research Center using standard nano- and micro-fabri­cation tech­niques such as thin film eva­poration and focused ion beam milling,” said Tal Galfsky, a CCNY graduate student. “These techniques are scalable with modern industry capa­bilities.”  The work demonstrates that “on a funda­mental level, the problem of designing photonic hyper­crystals has been solved,” said Narimanov. He cautions, however, that signi­ficant engi­neering challenges must be overcome before these devices can be commer­cialized. Among these barriers, the demon­stration devices are pumped optically by a laser, but commercial versions will need to be driven elec­trically and incor­porate either semi­conductor or organic LEDs, he said. As they mature, photonic hyper­crystals also may fill many other demanding roles in ultra­fast opto­electronics. One of the most promising avenues of research, Nari­manov suggested, is to create more efficient versions of the single-photon guns employed in quantum infor­mation processing. (Source: Purdue U.)

Reference: T. Galfsky et al.: Photonic hypercrystals for control of light– matter interactions, Proc. Nat. Ac. Sc. 114, 5125(2017). DOI: 10.1073/pnas.1702683114

Link: Birck Nanotechnology Center, School of Computer and Electrical Engineering, Purdue University, West Lafayette, USA • Dept. of Physics, City College, City University of New York, New York, USA

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