To ask if a beam of light can make a shadow sounds like a sort of Zen paradox, but physicists have demonstrated the possibility. They used a laser beam to block another laser beam, albeit with some crystalline assistance. They say the work has the potential to allow one laser to control another, but if they’re not also trying to make lightsabers with it we want our tax dollars back.

If you point two beams of light so the paths cross, they will pass through each other undisturbed. Anyone with a view of the object illuminated by one will not even know whether the other beam is on or off, unless it is also in their field of view. This has long been considered one of the fundamental aspects of light, a consequence of the masslessness of photons, but a type of exception has been found.

“Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting,” said Dr Raphael Abrahao, previously of the University of Ottawa, in a statement. “Our demonstration of a very counter-intuitive optical effect invites us to reconsider our notion of shadow.”

As part of a broader study of how light beams can interact under unusual conditions, Abrahao and colleagues shone green and blue lasers at right angles, meeting in a cube of ruby crystal.

The technique works because the green laser (wavelength 532 nanometers) changes the way the crystal responds to some other wavelengths of light, including the 450 nm of the blue laser. Consequently, the crystal now serves to block the blue laser’s path, creating a shadow on a nearby screen. 

Specifically, the green laser drives some of the crystal’s electrons into an excited state, and when they decay, they enter a state where they absorb blue light that would otherwise pass through. It only works because the absorption cross-section of the transition produced by the green laser is greater than that of the laser in ruby, something that is not the case for most crystals.

The shadow created is real, but one might quibble about whether to attribute it to the green laser or the ruby crystal. A paper describing the work gets somewhat philosophical on the matter. 

By adjusting the power of the green laser, the authors were able to change the induced resistance to blue light, at maximum efficiency reducing transmission of the blue laser by 22 percent. That might not sound like much, but they note it is similar to the shadow cast by leaves on a sunny day. The authors claim the shadow is hard to distinguish by eye alone from one cast by a black hair.

Although the work was not inspired by the way light is given physical form in the Star Wars universe, the idea did come out of a different sort of fiction: 3D visualization software. Members of the team noted that such software sometimes depicts laser beams as if they were opaque cylinders, creating a shadow in light that crosses them.

“What started as a funny discussion over lunch led to a conversation on the physics of lasers and the nonlinear optical response of materials,” said Abrahao. “From there, we decided to conduct an experiment to demonstrate the shadow of a laser beam.”

“This discovery expands our understanding of light-matter interactions and opens up new possibilities for utilizing light in ways we hadn’t considered before.”

“Our understanding of shadows has developed hand-in-hand with our understanding of light and optics,” Abrahao noted. As the authors observe; “Throughout...history, humans saw that shadows were cast by material objects like trees, clouds, or the Moon.” An understanding of how shadows formed was crucial to the development of perspective in Renaissance art, and Eratosthenes’ calculation of the circumference of the Earth.

“This new finding could prove useful in various applications such as optical switching, devices in which light controls the presence of another light, or technologies that require precise control of light transmission, like high-power lasers,” Abrahao said.

The study is open access in Optica.