"The most beautiful things in the world cannot be seen or touched; they are felt with the heart."
― Antoine de Saint-Exupéry, The Little Prince
There are many types of light – some visible to the human eye and others invisible. Red and blue light, and all the colors in between, are visible, but our eyes cannot process UV or infrared light, which remains invisible to us. Then there is another kind of light that is invisible because it simply never reaches our eyes. When light reflects on certain surfaces, a part of it sticks to the surface. This is called near-field light, and it is quite elusive. Today it is used for ultra-high resolution microscopy. But the near-field light has great potential for applications in particle manipulation, molecular detection, and optical communication. For example, near-field light can detect and separate molecules with subtle differences, which is vital in the development of new drugs. It can also increase the capacity of optical connections in data centers. To make these and other applications possible, it is essential to design the near-field light as desired.
To manipulate near-field light on a surface, the researchers have now developed a component in which light moves through a waveguide. In this component, the light bounces back and forth. After each bounce, the light changes shape and propagates with a different spatial pattern. "When all the different patterns of the near-field light are superimposed on top of each other, a specific shape is created," said Marco Piccardo, a research associate at SEAS and co-author of the paper.
"We can pre-program that specific form by adjusting the amplitude of the phase of the bouncing light," says Ginis. "It's a bit like music. The music you hear consists of a row of many notes that have been put together in patterns by a composer. The sound of one note alone is very flat, but with many notes together you can generate beautiful music. While music works in time, our light component works in a three-dimensional space. The extra intriguing aspect of our technique is that one note generates the other."
To demonstrate the design, the researchers structured the near-field light in the shape of an elephant. Or, more specifically, an elephant inside a boa-constrictor, or a hat. It's a tribute to the classic Le Petit Prince by Antoine de Saint-Exupéry.
"Over the years, our group has developed new powerful techniques to structure propagating light using subwavelength-patterned metasurfaces," says Federico Capasso, the Robert Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, and senior author of the paper. "With this work, we show how to structure the near field remotely, opening exciting opportunities in science and technology."
"We already have a lot of tools and techniques to manipulate free rays of light," Ginis says. "There are lenses, telescopes, prisms, and holograms. We can now start supplementing this rich toolbox with tools that shape near-field light."
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