Getting a Grip on Near-Field Light

Today, near-field light is mostly used for ultra-high-reso­lution micro­scopy, the near-field scanning optical microscopes (NSOM). However, near-field light also has untapped potential for particle mani­pulation, sensing, and optical communi­cations. But since near-field light doesn’t reach our eyes like far-field light does, researchers haven’t developed a compre­hensive toolkit to harness and manipulate the near field. “Today, we have a lot of tools and techniques to design what far-field light looks like,” said Vincent Ginis, a professor at the Vrije Uni­versity of Brussel. “We have lenses, telescopes, prisms and holo­grams. All these things enable us to sculpt freely propa­gating light in space.”

Designer landscape of localized light in the shape of an elephant. Guided light is molded by bouncing back and forth between two mode converters. (Source: Second Bay Studios / Harvard SEAS)

Now, researchers at Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a system to mold near-field light – opening the door to unpre­cedented control over this powerful, largely unexplored type of light. “Over the years, our group has developed new powerful techniques to structure propa­gating light using subwave­length-patterned meta­surfaces,” said Federico Capasso. “With this work, we show how to structure the near field at a distance, opening exciting oppor­tunities in science and technology.”

In order to manipulate near-field light, the researchers developed a device in which light confined to a waveguide bounces back and forth between two reflectors. After each bounce it changes mode, meaning it propa­gates with a dif­ferent spatial pattern. With multiple bounces, these patterns add up to generate a complex light intensity profile along the wave­guide. The near field light near the surface of the waveguide also changes. When all the different patterns of the near-field light are super­imposed on each other, a specific shape is created. The researchers can pre-program that shape by tailoring the amplitude of the modes of the bouncing light.

“The coexistence of all these modes can be designed to create near-field landscapes at will on the surface of the device,” said Marco Piccardo, a research associate at SEAS. “The shape of the landscape is determined by the combined proper­ties of the cascading light.” “It’s a bit like music,” said Ginis. “The music that you are hearing is the super­position of many notes or modes assembled in patterns conceived by the composer. One note alone isn’t much but taken together you can generate any type of music. While music operates in time, our near-field generator operates in three-dimen­sional space and the extra intriguing aspect of our device is that one note generates the other.”

Importantly, this molding process happens remotely, meaning no part of the device directly interacts with the near-field light. This reduces interference, which is important for appli­cations such as particle mani­pulation, and is a major departure from current local methods of sculpting near fields such as shining light on metallic tips and nano­particles. To demons­trate their design, the researchers molded near-field light into the shape of an elephant. Or, more speci­fically, an elephant inside a boa con­strictor, an homage to the play on dimensions in Antoine de Saint-Exupéry’s classic The Little Prince.

The researchers also shaped the light into a curve, a plateau and a straight line. “This research provides a new path towards unpre­cedented three-dimen­sional control of near-field light,” said Capasso. “It is a portent of the exciting disco­veries and techno­logy developments I expect to come out of this work in the future.” (Source: Harvard SEAS)

Reference: V. Ginis et al.: Remote structuring of near-field landscapes, Science 369, 436 (2020); DOI: 10.1126/science.abb6406

Link: Capasso Group, Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Mass., USA

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