Less Losses in Nanophotonic Devices

Directly measured polaritons propagating through a flake of hexagonal boron nitride (hBN). This material has been identified as an ideal substrate for two-dimensional materials research while also recently being demonstrated as an exciting optical material for infrared nanophotonics. (Source: NRL)

A team of physicists, headed by the U.S. Naval Research Labora­tory NRL, have demonstrated the means to improve the optical loss charac­teristics and trans­mission effi­ciency of hexagonal boron nitride devices, enabling very small lasers and nanoscale optics. “The appli­cations for this research are consi­derably broad,” said Alexander J. Giles, research physicist, NRL Electronics Science and Tech­nology Division. “By confining light to very small dimensions, nano­photonic devices have direct appli­cations for use in ultra-high reso­lution micro­scopes, solar energy har­vesting, optical computing and targeted medical therapies.”

Hexagonal boron nitride (hBN) forms an atomi­cally thin lattice consisting of boron and nitrogen atoms. This material has recently been demonstrated as an exciting optical material for infrared nano­photonics and is considered an ideal substrate for two-dimen­sional materials. While previous work demonstrated that natural hBN supports deeply sub-diffrac­tional hyper­bolic phonon polaritons desired for appli­cations, such as, sub-diffrac­tional optical imaging, energy conversion, chemical sensing, and quantum nano­photonics, limited tran­smission effi­ciencies continue to persist.

“We have demonstrated that the inherent effi­ciency limi­tations of nano­photonics can be overcome through the careful engi­neering of isotopes in polar semi­conductors and dielec­tric materials,” Giles said. Naturally occurring boron is comprised of two isotopes, boron-10 and boron-11, lending a 10 percent difference in atomic masses. This dif­ference results in substan­tial losses due to phonon scattering, limiting the potential appli­cations of this material. The research team has engi­neered greater than 99 percent isotopically pure samples of hBN, meaning they consist almost entirely of either boron-10 or boron-11 isotopes.

This approach results in a dramatic reduction in optical losses, resulting in optical modes that travel up to three times farther and persist for up to three times longer than natural hBN. These long-lived vibra­tional modes not only enable immediate advances specific to hBN – near field optics and chemical sensing – but also provide a strategic approach for other materials systems to exploit and build upon. “Control­ling and mani­pulating light at nano­scale, sub-diffrac­tional dimensions is noto­riously diffi­cult and ineffi­cient,” said Giles. “Our work represents a new path forward for the next gene­ration of materials and devices.” (Source: NRL)

Reference: A. J. Giles et al.: Ultralow-loss polaritons in isotopically pure boron nitride, Nat. Mat., online 11 December 2017; DOI: 10.1038/nmat5047

Link: Electronics Science and Technology, United States Naval Research Laboratory NRL, Washington DC, USA

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