Phase Matching Could Be on Its Way Out as Color Gains Momentum
Martina Scapin posted on March 12, 2018 |

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. 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(SEAS), along 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.

“For any wavelength conversion process to be efficient, it has to be carefully designed to phase match, and it only works at a single wavelength,” said Marko Loncar, SEAS Tiantsai Lin professor of electrical engineering and senior author of the paper. “The devices shown in this work, in contrast, do not need to satisfy the phase-matching requirement and can convert light in a broad color range.”

Scanning electron microscope (SEM) image of the fabricated device. (Image courtesy of Loncar Lab/Harvard SEAS.)
Scanning electron microscope (SEM) image of the fabricated device. (Image courtesy of Loncar Lab/Harvard SEAS.)

How Does It Work?

The research found that the converter relies on a metasurface, consisting of an array of silicon nanostructures, integrated into a lithium niobate waveguide. The light passes through the 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.

“Unlike most metasurfaces, where light travels perpendicularly to the metasurface, here light interacts with the metasurface while being confined inside a waveguide,” said Cheng Wang, co-first author of the paper and SEAS postdoctoral fellow. “In this way, we take advantage of both the momentum control from the metasurface and a long interaction distance.”

The researchers demonstrated that they could double the frequency of a wavelength, converting near infrared colors to red, with high efficiency over a broad bandwidth.

“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, Columbia assistant professor of applied physics 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.”

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