Can lasers cast shadows? Answer redefines understanding of light, researchers say

Physicists have made a groundbreaking discovery that challenges the traditional understanding of light by demonstrating that laser beams can cast shadows. In an experiment led by Dr. Raphael A. Abrahao from Brookhaven National Laboratory, researchers observed that under specific conditions, one laser beam can block another, effectively creating a visible shadow.

"Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting," said Dr. Abrahao, according to The Debrief. This counterintuitive finding opens new doors in optics and photonics, prompting scientists to reconsider foundational concepts in physics.

The idea for the experiment emerged from an informal conversation among scientists. Members of the team noticed that 3D visualization software depicted laser beams casting shadows, sometimes showing them as opaque cylinders that create a shadow in light. This unusual observation sparked a discussion that led to designing an experiment to see if they could create a visible shadow with a laser beam, a phenomenon that challenges foundational concepts in physics.

"What began as a playful reflection transformed into a fascinating discovery, expanding the understanding of interactions between light and matter," as reported by SciencePost. Inspired by numerical simulations where lasers were represented as solid cylinders, the team wanted to test the hypothesis in the laboratory, leading to the creation of a visible shadow with a laser beam.

To demonstrate this phenomenon, the researchers conducted an experiment using a ruby crystal cube. They shone a green laser beam through the cube while illuminating it perpendicularly with blue light. When the green laser passed through the ruby crystal, it created a region that absorbed more blue light, effectively casting a "shadow" shaped exactly like the green laser beam. This shadow was visible to the naked eye, followed the contours of the laser beam, and moved with its source.

The key to this effect lies in ruby's unusual optical properties. Materials like ruby exhibit nonlinear responses, meaning their interaction with light changes based on the light's intensity. Specifically, the phenomenon relies on a property known as reverse saturation of absorption, where the material absorbs more light at higher intensities rather than becoming more transparent. This counterintuitive behavior is critical to producing the shadow.

When the green laser passes through the ruby, its photons excite specific energy levels within the crystal, driving some of the ruby crystal's electrons into an excited state. This excitation alters the crystal's absorption characteristics, selectively blocking portions of the blue illumination beam. As a result, a darker region appears where the green laser had passed, mimicking the shadow of an opaque object.

By adjusting the power of the green laser, the researchers were able to change the induced resistance to blue light and vary the intensity of the shadow produced. They achieved a maximum contrast of about 22%, comparable to the shadow cast by leaves on a sunny day. "We experimentally measure the dependence of the contrast of the shadow on the power of the laser beam, finding a maximum of approximately 22%, similar to that of a shadow of a tree on a sunny day," the researchers explained, as reported by The Debrief.

The researchers developed a theoretical model and showed that it could accurately predict the shadow contrast, quantifying the effect using mathematical models and advanced imaging techniques. Their findings aligned closely with theoretical predictions, solidifying the experimental results and highlighting the robustness of the laser shadow phenomenon. "Our demonstration of a very counterintuitive optical effect invites us to reconsider our notion of shadow," Dr. Abrahao added.

This discovery has potential real-world applications in the field of optics and laser technologies. According to Dr. Abrahao, "This discovery expands our understanding of light-matter interactions and opens up new possibilities for utilizing light in ways we hadn't considered before." One potential use would be the creation of systems where one laser beam controls another beam. In optical switches or devices requiring precise control of light, such as high-power lasers, this effect could be used to modulate light transmission with great precision.

By controlling light with light, researchers foresee uses in optical computing, precision imaging, and advanced fabrication techniques. The ability to cast shadows with laser beams might revolutionize fields like lithography and 3D imaging. "Potential applications can be envisioned in areas such as optical switching, controllable shade or transmission, control of the opaqueness of light with light, and lithography," the researchers added.

The research team plans to investigate other materials, such as alexandrite, and other laser wavelengths that might produce similar effects, aiming to explore the potential scalability of this phenomenon for industrial applications. Each new material could offer unique properties and unprecedented technological possibilities. They also aim to refine their theoretical models to better understand the underlying physics.

This breakthrough challenges long-standing assumptions about the nature of light and shadows. While photons—the particles of light—are massless and typically pass through one another without interaction, this experiment shows that light can behave as if it has mass, casting a shadow under specific conditions. As researchers continue pushing the boundaries of optical science, the possibilities for innovation could be limitless.

"This experiment redefines our understanding of what a shadow is—under the right conditions, laser beams can cast a shadow," the researchers concluded. It shows that a phenomenon as familiar as a shadow can still reveal fascinating secrets. This result is also a reminder of the importance of scientific curiosity: what initially seemed to be a simple theoretical question led to a major advance.

The research has been accepted for publication in Optica and can be found on the pre-publish site arXiv, representing an important contribution to research in the field of optics and light physics.

This article was written in collaboration with generative AI company Alchemiq