Bending Light Around Tight Corners

The central part of the new photonic crystal topological insulator waveguide, with the path of a photon’s path highlighted in green. The experiment showed that each turn resulted in backscattering losses of only a few percent. (Source: N. Litchinitser, Duke U.)

Engineers at Duke Uni­versity have demon­strated a device that can direct photons of light around sharp corners with virtually no losses due to back­scattering, a key property that will be needed if electronics are ever to be replaced with light-based devices. The result was achieved with photonic crystals built on the concept of topo­logical insulators, which won its disco­verers a Nobel Prize in 2016. By carefully controlling the geometry of a crystal lattice, researchers can prevent light traveling through its interior while trans­mitting it perfectly along its surface.

Through these concepts, the device accom­plishes its near-perfect trans­mittance around corners despite being much smaller than previous designs. The Semi­conductor Industry Asso­ciation estimates that the number of electronic devices is increasing so rapidly that by the year 2040, there won’t be enough power in the entire world to run them all. One potential solution is to turn to massless photons to replace the electrons currently used for transmitting data. Besides saving energy, photonic systems also promise to be faster and have higher bandwidth.

Photons are already in use in some appli­cations such as on-chip photonic communi­cation. One drawback of the current technology, however, is that such systems cannot turn or bend light effi­ciently. But for photons to ever replace electrons in microchips, travel­ling around corners in micro­scopic spaces is a necessity. “The smaller the device the better, but of course we’re trying to minimize losses as well,” said Wiktor Walasik, a post­doctoral associate in electrical and computer engineering at Duke. “There are a lot of people working to make an all-optical computing system possible. We’re not there yet, but I think that’s the direction we’re going.”

Previous demon­strations have also shown small losses while guiding photons around corners, but the new Duke research does it on a rectangular device just 35 micrometers long and 5.5 micrometers wide – 100 times smaller than previously demon­strated ring-resonator based devices. In the new study, researchers fabri­cated topo­logical insu­lators using electron beam lithography and measured the light trans­mittance through a series of sharp turns. The results showed that each turn only resulted in the loss of a few percent.

“Guiding light around sharp corners in conven­tional photonic crystals was possible before but only through a long laborious process tailored to a specific set of para­meters,” said Natasha Litchi­nitser, professor of electrical and computer engi­neering at Duke. “And if you made even the tiniest mistake in its fabri­cation, it lost a lot of the properties you were trying to optimize.”

“But our device will work no matter its dimensions or geometry of the photons’ path and photon transport is topo­logically protected,” added Mikhail Shalaev, a doctoral student in Litchi­nitser’s labora­tory. “This means that even if there are minor defects in the photonic crystalline structure, the wave­guide still works very well. It is not so sensitive to fabrication errors.” The researchers point out that their device also has a large operating bandwidth, is compatible with modern semi­conductor fabri­cation tech­nologies, and works at wave­lengths currently used in telecommu­nications.

The researchers are next attempting to make their waveguide dynami­cally tunable to shift the bandwidth of its operation. This would allow the wave­guide to be turned on and off at will – another important feature for all-optical photon-based tech­nologies to ever become a reality. (Source: Duke Univ.)

Reference: M. I. Shalaev et al.: Robust topologically protected transport in photonic crystals at telecommunication wavelengths, Nat. Nano., online 12 November 2018; DOI: 10.1038/s41565-018-0297-6

Link: Dept. of Electrical and Computer Engineering, Duke University, Durham, USA

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