Can light truly cast a shadow? While this may initially appear to be a philosophical question, cutting-edge research reveals that under specific conditions, a laser beam can indeed behave like an opaque object, producing a shadow. This groundbreaking discovery challenges our conventional understanding of shadows and opens up exciting possibilities for innovative technologies that could allow one laser beam to control another.
“Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting,” said research team leader Raphael A. Abrahao from Brookhaven National Laboratory, previously at the University of Ottawa. “Our demonstration of a very counter-intuitive optical effect invites us to reconsider our notion of shadow.”
Researchers used a ruby crystal and specific laser wavelengths to demonstrate that a laser beam could obstruct light and produce a visible shadow through a nonlinear optical effect. This phenomenon occurs when light interacts with a material in a manner that depends on its intensity and can affect another optical field.
“Our understanding of shadows has developed hand-in-hand with our understanding of light and optics,” said Abrahao. “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.”
The research delves into the fascinating interplay between two light beams under unique conditions and nonlinear optical processes. What began as a lighthearted discussion during lunch about experimental schematics illustrating a laser beam’s shadow — incorrectly visualized as a simple cylinder — sparked an intriguing question among scientists: Could this phenomenon be replicated in a lab?
“What started as a funny discussion over lunch led to a conversation on the physics of lasers and the nonlinear optical response of materials,” said Abrahao. “From there, we decided to conduct an experiment to demonstrate the shadow of a laser beam.”
To bring this concept to life, the researchers directed a high-power green laser through a standard ruby crystal cube while introducing a side illumination with a blue laser. As the green laser penetrated the ruby, it altered the material’s response to the blue wavelength. In this setup, the green laser functioned as a solid object, while the blue laser served as the source of illumination. The interaction of these light sources resulted in a discernible shadow on a screen — a distinct dark area where the green laser obstructed the blue light.
This shadow met all conventional criteria: it was visible to the naked eye, conformed to the contours of its surface, and mirrored the shape and position of the green laser beam, acting as the object. This striking laser shadow effect arises from optical nonlinear absorption within the ruby. Essentially, the green laser enhances the optical absorption of the blue illuminating beam, creating a region of reduced intensity in the illuminating light. Consequently, the result is a pronounced dark area that visually represents the shadow of the green laser beam.
“This discovery expands our understanding of light-matter interactions and opens up new possibilities for utilizing light in ways we hadn’t considered before,” said Abrahao.
The researchers conducted experiments to evaluate how the contrast of the shadow varies with the power of the laser beam, discovering a peak contrast of around 22%, comparable to the contrast of a tree’s shadow on a bright day. They also created a theoretical model that successfully predicts the shadow contrast.
According to the researchers, this phenomenon indicates that the intensity of a laser beam being transmitted can be modulated by the application of another laser. Moving forward, they intend to explore different materials and laser wavelengths that might yield similar results.
Journal reference:
- Raphael A. Abrahao, Henri P. N. Morin, Jordan T. R. Pagé, Akbar Safari, Robert W. Boyd, Jeff S. Lundeen. Shadow of a laser beam. Optica, 2024; DOI: 10.1364/OPTICA.534596