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Changing the Color of Light

Metasurface-assisted phase-matching-free second harmonic generation in lithium niobate waveguides. (source: Loncar Lab/Harvard SEAS)

Image of a fabricated device showing four phased antenna arrays consisting of silicon nano-rods of different lengths patterned on the top surface of a LiNbO3 waveguide (source: Loncar Lab/Harvard SEAS)

August 13, 2018 | Source: Harvard Paulson School, seas.harvard.edu, 31 Jan 2018, Leah Burrows

One of the biggest challenges in developing integrated photonic circuits — which use light rather than electrons to transport information — is to control the momentum of light.  Colors of light travel at different speeds through a material but in order for light to be converted between colors, it needs to have the same momentum or phase.

Many devices have been designed to momentum-match or phase-match light at various points throughout an integrated circuit but what if the phase-matching process could be circumvented all together in certain cases?

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences, together with collaborators from the Fu Foundation School of Engineering and Applied Science at Columbia University, have developed a system to convert one wavelength of light into another without the need to phase match.

The converter relies on a metasurface, consisting of an array of silicon nanostructures, integrated into a lithium niobate waveguide. The light passes through waveguide, interacting with the nanostructures along the way. The array of nanostructures act like a TV antenna — receiving the optical signal, manipulating its momentum and re-emitting it back into the waveguide.

“The integrated metasurface is distinct from other phase-matching mechanisms in that it provides a unidirectional optical momentum to couple optical energy from one to another color components — while inhibiting the inverse process — which is critical for realizing broadband nonlinear conversion,” said Nanfang Yu, assistant professor of applied physics at Columbia and a co-senior author of the paper. “Future work will demonstrate broadband integrated photonic devices based on metasurfaces for realizing other functions such as optical modulation.”