Shadows are a natural consequence of opaque walls in an illuminated universe. The light shines; photons travel until they hit an object through which they cannot pass; This blocking creates a shadow, which is a small patch of darkness through which light enters its path. But physicists have just discovered something very strange. A laser passing through a desired transparent material can cause its surroundings to become opaque, as if casting its own shadow.

When two beams of laser light intersect correctly, the primary beam cannot pass through the secondary beam, creating a shadowed line of light hitting the opposing surface.

“Our demonstration of a very controversial optical effect prompts us to reconsider our understanding of the shadow,” says physicist Rafael Abrahão from Brookhaven National Laboratory in the US.

While intersecting waves pass without being exposed to any obstacles, photons do not interact with each other except in very special cases. If you pass over two rays, they will pass through each other as if nothing were happening; just like one flashlight beam passing through another.

Almost as a side project, Abrachao and his colleagues decided to investigate whether a ray of light could cast a shadow. They studied how light rays interact when nonlinear materials are added. These are materials whose interaction with light does not scale linearly, leading to effects such as amplification, absorption, self-focusing, and harmonics (or frequency repetition).

In their experiments, they used 3D modeling software to create simple circuits. In this software, a beam of light was represented as a solid cylinder that cast a shadow, and physicists found this quite funny… until they found it interesting.

“What started as a humorous discussion over lunch led to a conversation about the physics of lasers and the nonlinear optical response of materials,” says Abrachao. “From there we decided to do an experiment to demonstrate the shadow of the laser beam.”

Since ruby ​​is a popular material for studying nonlinear optics, the researchers used it as a meeting point for two lasers, one blue and one green. Blue laser light was directed to one side of the ruby, passing through it and creating a cold glow on the screen, while from the other side a narrow beam of green laser light traveled perpendicular to the first.

Where the thin line of green light fell on the ruby ​​molecules, there was a complex dance of electrons going up and down. As a result, the slightly shorter wavelength of blue light was entangled by the transition electrons and its path through the translucent material was blocked.

Therefore, the green laser beam acts like an object, creating a dark line in the blue light hitting the screen from the other side of the ruby. This dark line met all the criteria to be classified as a shadow. It was visible to the naked eye; it conformed to the contours of the screen on which it was cast; and when the laser source was moved, it moved with the green laser beam.

“This discovery expands our understanding of the interaction between light and matter and opens new possibilities for using light in ways we had not considered before,” says Abrahão.

“Our understanding of shadow developed hand in hand with our understanding of light and optics. “This new discovery could be useful in a variety of applications, such as optical switching, devices where light controls the presence of other light, or technologies that require precise control of light transmission, such as high-power lasers.” The study has been accepted for publication. Opticaland the previous publication can be found on the arXiv site.