Metasurface Converts Colors of Light

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)

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. “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, the Tiantsai Lin Professor of Elec­trical Engineering at SEAS. “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.”

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. “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, post­doctoral fellow at SEAS. “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. In previous research, the team demon­strated that they could also control and convert the polari­zation and mode of a guided wave using a similar structure. “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. “Future work will demon­strate broadband inte­grated photonic devices based on meta­surfaces for realizing other functions such as optical modu­lation.” (Source: SEAS)

Reference: C. Wang et al.: Metasurface-assisted phase-matching-free second harmonic generation in lithium niobate waveguides, Nat. Commun. 82098 (2017); DOI: 10.1038/s41467-017-02189-6

Link: Nanoscale Optics, John A. Paulson School of Engineering and Applied Sciences SEAS, Harvard University, Cambridge, USA

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