NIST shows off one-way photon-passing metamaterial

A light take on a diode could be just the thing for photonic networks

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If photonics is ever to displace or supplement electronics in microprocessors, the operations we perform on electrons need to be replicated on photons. A group at NIST is the latest to demonstrate a photonic diode – a device that passes light in one direction only, similarly to how diodes pass electrons in one direction only.

The NIST work, which has been published in Nature Communications (abstract here), differs from previous one-way-light-devices because it works with visible light, not the microwave or infrared wavelengths blocked by other techniques.

“Until now, no one had achieved one-way transmission of visible light, because existing devices could not be fabricated at scales small enough to manipulate visible light's short wavelengths”, the researchers write.

The NIST researchers, Ting Xu and Henri Lezec created what they call “a multi-layered block of alternating silver and glass sheets and metal grates with very narrow spacings”. This, they say, creates a structure which is opaque to visible light from the outside, but allows light to “propagate inside the material within a narrow range of angles”.

In other words, in this hyperbolic metamaterial, with layers tens of nanometers thick (compared to visible light's 400 nm to 700 nm range of wavelengths), the behaviour of light depends on its direction. The "block" direction passes light at one-thirtieth the intensity of the "pass" direction.

NIST's one-way photonic metamaterial

NIST's metamaterial passes visible light in one direction (left beam).

The block has 20 alternating layers of silicon dioxide glass and silver. The outside surfaces then have chromium grates with sub-wavelength spacings. On one side, the grates bend red or green light into the “block”, while on the other side, the grate is designed to let light out of the structure.

“While the second set of grates let light escape the material, their spacing was slightly different from that of the first grates. As a result, the reverse-direction grates bent incoming light either too much or not enough to propagate inside the silver-glass layers,” the NIST statement explains.

As well as possible applications in photonic chips, the researchers say the metamaterial could be used in biosensing applications, detecting small amounts of light from a sample. ®

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