If shadows are created by light, how can light cast a shadow?

Can light itself, perhaps from a laser, cast a shadow? It might seem like a trick, but under certain conditions, the answer to this paradoxical question is “yes.”

Researchers have discovered that a laser beam can act like an opaque object, blocking another beam of light and creating a visible shadow.

This finding challenges our traditional understanding of how light behaves.

“Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting,” said Raphael A. Abrahao, the research team leader from Brookhaven National Laboratory and the University of Ottawa.

His team has opened the door to new ways of thinking about light and its interactions, pushing the boundaries of optics and could lead to innovative technologies that harness light in unexpected ways.

Lasers, lights, and lunch

The idea started over a lunch conversation.

Some scientists noticed that experimental schematics made with 3D visualization software depicted the shadow of a laser beam, treating it as a solid cylinder rather than accounting for the physics of light.

This sparked a lively discussion.

“What started as a funny discussion over lunch led to a conversation on the physics of lasers and the nonlinear optical response of materials,” remembered Abrahao.

“From there, we decided to conduct an experiment to demonstrate the shadow of a laser beam.”

Making laser light cast a shadow

To explore this curious idea, the team set up an experiment using a high-power green laser directed through a cube made of standard ruby crystal.

At the same time, they illuminated the cube with a blue laser from the side. When the green laser entered the ruby, it changed the material’s response to the blue wavelength.

The interaction between the two light sources created a shadow on a screen — a dark area where the green laser blocked the blue light.

The effect occurs due to a nonlinear optical process. Nonlinear optics involves light interacting with a material in an intensity-dependent way, allowing one light beam to influence another.

“Our demonstration of a very counter-intuitive optical effect invites us to reconsider our notion of shadow,” Abrahao explained.

In this case, the green laser increased the optical absorption of the blue laser in the ruby crystal, creating a region where less blue light passed through — essentially, a shadow.

Very cool, but why does it matter?

Nonlinear optics is a field that studies how light behaves in materials under high-intensity conditions.

When light is intense enough, it can change the properties of the material it passes through, affecting how other light waves move through that material.

This is different from linear optics, where light beams pass through each other without interaction.

“Our understanding of shadows has developed hand-in-hand with our understanding of light and optics,” Abrahao noted.

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.

It could also impact the development of optical computing, where light is used instead of electricity to process information.

Measuring laser light’s shadow

The researchers measured the shadow’s contrast based on the laser beam’s power, finding a maximum contrast of approximately 22%.

That’s similar to the contrast of a tree’s shadow on a sunny day. They also developed a theoretical model that accurately predicted the shadow contrast.

This alignment between experiment and theory strengthens the validity of their findings.

The ability to control one light beam with another could lead to advances in optical communication and signal processing.

For instance, optical switches that use light instead of electronic signals could operate at much higher speeds and with less energy loss.

This discovery might also contribute to the development of new types of lasers with adjustable properties for use in medicine, manufacturing, or scientific research.

What happens next?

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

The team plans to investigate other materials and laser wavelengths that can produce similar effects. They are curious to see how different combinations might enhance or alter the shadow effect.

The notion that light can cast a shadow on light challenges fundamental ideas in physics. It invites scientists and engineers to rethink how we can manipulate light for advanced technologies.

This discovery is a reminder that even well-established concepts can be revisited and that curiosity can lead to surprising insights.

From a casual conversation to a laboratory breakthrough, the journey of this discovery highlights the importance of questioning assumptions. The shadow of a laser beam is just one example of how much more there is to learn.

The full study was published in the journal Optica.

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