‘Impossible’ finding: laser light can cast its own shadow

Nov. 14 () –

Under certain conditions, a laser beam can act as an opaque object and cast a shadow, an ‘impossible’ discovery that opens the way to using a laser beam to control another laser beam.

“It was previously believed that it was impossible for laser light to cast a shadow, since light usually passes through other light without interacting“said research team leader Raphael A. Abrahao of Brookhaven National Laboratory. “Our demonstration of a very counterintuitive optical effect invites us to reconsider our notion of shadowing.”

In Optics magazinethe researchers describe how they used a ruby ​​crystal and specific laser wavelengths to demonstrate that a laser beam could block light and create a visible shadow due to a nonlinear optical process. This effect occurs when light interacts with a material in an intensity-dependent manner and can influence another optical field.

“Our understanding of shadows has developed hand in hand with our understanding of light and optics,” Abrahao said. “This new finding could be useful in various applications, such as optical switching, devices in which light controls the presence of other light, or technologies that require precise control of light transmission. like high-powered lasers.”

The new research is part of a broader exploration of how a ray of light interacts with another ray of light under special conditions and nonlinear optical processes.

The idea came about during a lunch conversation, when it was pointed out that some experimental schemes made with 3D visualization software represent the shadow of a laser beam because they treat it as a cylinder without taking into account the physics of a laser beam. Some of the scientists wondered: could this be done in a laboratory?

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

To do this, the researchers directed a high-power green laser through a cube made of standard ruby ​​crystal and illuminated it with a blue laser from the side. When the green laser enters the ruby, it locally changes the material’s response to the blue wavelength. The green laser acts as an ordinary object, while the blue laser acts as illumination.

The interaction between the two light sources created a shadow on a screen that was visible as a dark area where the green laser blocked the blue light. It met all the criteria for a shadow because it was visible to the naked eye, followed the contours of the surface it hit, and followed the position and shape of the laser beam, which acted as an object.

Laser shadowing is a consequence of nonlinear optical absorption in ruby. The effect occurs because the green laser increases the optical absorption of the blue illuminating laser beam, creating a coincident region in the illuminating light with lower optical intensity. The result is a darker area that appears as a shadow of the green laser beam.

“This discovery expands our understanding of light-matter interactions and opens up new possibilities for using light in ways we had not considered before,” Abrahao said.

The researchers experimentally measured the dependence of shadow contrast on laser beam power, finding a maximum contrast of about 22%, similar to the contrast of a tree shadow on a sunny day. They also developed a theoretical model and showed that it could accurately predict shadow contrast.

The researchers say that, from a technological point of view, the effect they have demonstrated demonstrates that the intensity of a transmitted laser beam can be controlled by applying another laser. Next, they plan to investigate other materials and other laser wavelengths that can produce similar effects.