Researchers have discovered that, in specific situations, a laser beam can behave like an opaque object and cast a shadow. This revelation opens new avenues for technology that could utilize one laser beam to manage another.
Is it possible for light to create a shadow? While this might sound like a baffling question, scientists have uncovered that, under certain circumstances, a laser beam can indeed act like a solid object and produce a shadow. This finding challenges the conventional views on shadows and presents exciting new potential for using lasers in innovative technologies.
“It was previously considered impossible for laser light to cast a shadow, as light typically goes through other light without any interaction,” stated Raphael A. Abrahao, the research team leader from Brookhaven National Laboratory, formerly at the University of Ottawa. “Our demonstration of this unusual optical phenomenon encourages us to rethink what we know about shadows.”
In the journal Optica, published by Optica Publishing Group, the researchers explain how they demonstrated that a laser beam can block light and cast a visible shadow by utilizing a ruby crystal and specific laser wavelengths. This phenomenon is a result of a nonlinear optical process, which occurs when light interacts with materials based on the intensity of the light, affecting another optical field.
“Our comprehension of shadows has evolved alongside our understanding of light and optics,” remarked Abrahao. “This recent discovery may have valuable applications in areas such as optical switching, where one light controls another, or in technologies that require accurate regulation of light transmission, particularly in high-power lasers.”
Idea sparked by a lunch discussion
This new research is part of a wider investigation into how light beams influence each other under unique conditions and nonlinear optical effects. The notion originated during a lunch conversation when scientists noted that some experimental models created using 3D visualization software depicted a shadow of a laser beam, treating it like a cylinder without considering the physics involved. This spurred curiosity: Was it feasible to replicate this in a laboratory?
“What began as a lighthearted discussion over lunch evolved into a serious examination of the physics of lasers and the nonlinear optical responses of materials,” said Abrahao. “We then resolved to perform an experiment to showcase the shadow of a laser beam.”
In their experiment, the researchers directed a powerful green laser through a cube made of ruby crystal while illuminating it with a blue laser from the side. The entry of the green laser modified how the ruby crystal reacted to the blue wavelength. In this scenario, the green laser behaved like an actual object, while the blue laser served as the light source.
The interaction between the two lasers created a shadow on a screen, clearly visible as a dark area where the green laser obstructed the blue light. It met all the criteria for a shadow—it was observable, conformed to the shape of the surface it landed on, and mirrored the position and shape of the green laser beam, functioning as an object.
This laser shadow effect arises from the nonlinear optical absorption in the ruby crystal. The phenomenon occurs because the green laser enhances the optical absorption of the blue laser, creating a region within the illuminating light where the optical intensity is lower. The end result is a darker area that resembles the shadow of the green laser beam.
Measuring shadows
“This discovery broadens our comprehension of how light interacts with matter and introduces new possibilities for employing light in ways we have not contemplated before,” commented Abrahao.
The researchers experimentally evaluated how the contrast of the shadow was influenced by the power of the laser beam, observing a maximum contrast of about 22%, similar to the shadow cast by a tree on a sunny day. They also developed a theoretical model that accurately predicted the shadow’s contrast.
From a technological standpoint, the team notes that their findings suggest the intensity of a transmitted laser can be regulated by another laser. Their future work will explore different materials and laser wavelengths that could yield similar effects.
