Researchers discover that laser light can cast a shadow

Shadows have been an integral part of human exploration of light and its properties. From their appearance in ancient shadow puppetry to their role in groundbreaking scientific discoveries, shadows have helped unlock the mysteries of our world.

However, a recent experiment has redefined this seemingly simple phenomenon by showing that even light can cast a shadow.

Traditionally, shadows occur when a physical object blocks light, creating a dark outline on a surface. Artists like Leonardo da Vinci and scientists like Eratosthenes have used shadows to develop techniques in art, calculate the Earth’s dimensions, and understand celestial mechanics.

But the discovery of a shadow cast by a laser beam adds a new layer to this narrative and challenges long-standing assumptions about the behavior of light.

This phenomenon emerged from an innovative experiment led by researchers at Brookhaven National Laboratory. Using a green laser beam, a ruby ​​crystal, and blue light illumination, they demonstrated that under certain conditions, light can block another light source and create a visible shadow.

“Our demonstration of this counterintuitive optical effect invites us to rethink our conception of shadows,” said Raphael A. Abrahao, the team leader. It used to be believed that light waves passed through each other without interaction.

The team’s results, published in Optica, show that under intense optical conditions, nonlinear processes cause light to act like a material object.

The researchers shined a powerful green laser beam through a ruby ​​crystal while simultaneously shining a blue laser from the side. This setup caused the green beam to change the optical properties of the crystal and increase its absorption of blue light.

This caused a shadow to form on a screen that resembled that of a physical object. The effect was visible to the naked eye, following the contours of the surface it fell on and changing shape and position with the laser beam.

The idea came about during a casual conversation over lunch about how experimental schematics in 3D visualization software often depict laser beams as solid cylinders. The team asked themselves: Could a laser beam behave similarly in reality?

“What started as a fun discussion over lunch led to a conversation about the physics of lasers and the nonlinear optical response,” Abrahao explained. Inspired by this, the team designed an experiment to test the hypothesis.

The shadow effect is a result of nonlinear optics, a field that studies how materials respond to intense light. Here, the green laser beam triggered a local change in the ruby ​​crystal’s response to the blue light, effectively “blocking” some of the illumination and creating a shadow.

This phenomenon is consistent with existing criteria for shadows, including visibility, adherence to surface contours, and three-dimensionality.

Under normal circumstances, photons – the particles of light – rarely interact. However, extreme optical intensities or special materials can facilitate such interactions.

Nonlinear absorption played a key role. As the green laser beam passed through the ruby ​​crystal, an area was created where the blue light was absorbed more strongly, creating a dark outline.

This shadow effect has similarities to phenomena such as photon-photon interactions in extreme environments such as vacuum polarization or Rydberg gases. These processes have long interested physicists because they illustrate the complexity of the interactions between light and matter.

“Our understanding of shadows has evolved hand in hand with our understanding of light and optics,” Abrahao said. “This new insight could prove useful in various applications, such as optical switching or devices that require precise control of light transmission.”

The team also quantified the shadow’s properties. They found that contrast depends on the intensity of the laser, with a maximum contrast of 22% – comparable to the shadow of a tree on a sunny day. They developed a theoretical model to predict this effect and confirmed its accuracy with experimental data.

The discovery not only advances fundamental research, but also offers practical implications. For example, the ability to control one laser beam using another could lead to innovations in high-power laser systems, optical communications, and imaging technologies.

The researchers now want to investigate whether other materials and laser wavelengths can replicate this effect. By understanding the underlying mechanisms, they hope to develop new tools for manipulating light.

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

This breakthrough exemplifies how curiosity and interdisciplinary collaboration can lead to unexpected progress. As researchers continue to push the boundaries of optics, light’s ability to cast a shadow could pave the way for a new era of optical technologies.