TECH

Scientists manipulate light that never reaches your eyes

Physicists have finally succeeded in developing a system for shaping "near-field light", paving the way for unprecedented control over this powerful, but largely unexplored, type of light.
There are many types of light, some visible and others invisible to the human eye - for example, our eyes and brain do not have the tools to process ultraviolet or infrared light, making them invisible to us.
But there is another type of light that is invisible simply because it never reaches our eyes. When light hits certain surfaces, part of it "sticks" and stays close to the surface, rather than being reflected or scattered. This type of light is called near field light.
Today, near-field light is used primarily for ultra-high resolution microscopy in near-field scanning optical microscopes.
However, near-field light also has an untapped potential for particle manipulation - optical tweezers or tractor rays -, sensing and optical communication. But, because that light doesn't reach our eyes like distant field light, the researchers haven't developed a comprehensive toolkit to manipulate and take advantage of it.
Now, Vincent Ginis and colleagues at Harvard University, USA, have developed a system to control near-field light, achieving unprecedented three-dimensional control of this very special type of light - a control that allows its practical use.



Light shades
To manipulate near-field light, Ginis developed a device in which the light confined in a waveguide oscillates between two mirrors. Each reflection causes it to change modes, which means that the light starts to propagate with a different spatial pattern. After several reflections, these patterns add up, generating a complex profile of light intensity along the waveguide.
The near-field light near the waveguide surface also changes: When all the different light patterns are superimposed, a specific waveform is born, which can be programmed by adapting the amplitude of the reflected light modes.
"It's more or less like music," compares Ginis. "The music you are listening to is the superposition of many notes, or modes, gathered in patterns designed by the composer. One note alone is not much, but, taken together, you can generate any type of music. As long as the music operates in time, our near-field generator operates in three-dimensional space, and the most intriguing aspect of our device is that one note generates the other ".
An important detail for practical applications is that this light shaping process takes place remotely, which means that no part of the device interacts directly with the near-field light. This reduces interference, which is important for applications such as particle manipulation, in addition to representing a radical change from the methods currently used in microscopy, in which light generally needs to fall on metal tips and nanoparticles.
To demonstrate the apparatus, Ginis molded the light from the nearby field into the shape of an elephant. Or, more specifically, an elephant inside a boa constrictor, an homage to Antoine de Saint-Exupery's classic, The Little Prince.
He also shaped light into simpler geometric shapes, including curves, straight lines and a plateau.

Remote structuring of near-field landscapes
Authors: Vincent Ginis, Marco Piccardo, Michele Tamagnone, Jinsheng Lu, Min Qiu, Simon Kheifets, Federico Capasso