"Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting," explained Raphael A. Abrahao, leader of the research team at Brookhaven National Laboratory and former University of Ottawa researcher. "Our demonstration of a very counter-intuitive optical effect invites us to reconsider our notion of shadow."
The researchers outlined their findings in 'Optica', the journal of the Optica Publishing Group. They detailed their use of a ruby crystal and specific laser wavelengths to create conditions where a laser beam could block light and form a visible shadow through a nonlinear optical process. This phenomenon occurs when light interacts with a material in a way 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," Abrahao said. "This new finding could prove useful in various applications such as optical switching, devices where light controls the presence of another light, or technologies that require exact control of light transmission, like high-power lasers."
Origins of the Discovery
The concept emerged during a lunch conversation about 3D visualization software. The scientists noticed that some experimental schematics in the software depicted the shadow of a laser beam as if it were a physical cylinder, ignoring the physics of laser beams. This sparked the question: could such an effect be created in a real laboratory setting?
"What started as a funny discussion over lunch led to a conversation on the physics of lasers and the nonlinear optical response of materials," Abrahao noted. "From there, we decided to conduct an experiment to demonstrate the shadow of a laser beam."
The team directed a high-power green laser through a ruby crystal cube while illuminating it from the side with a blue laser. The interaction between these light sources caused the green laser to change the ruby's response to the blue wavelength. The green laser then behaved like a tangible object, while the blue laser provided the illumination.
This interaction resulted in a shadow on a screen - a dark area where the green laser blocked the blue light. The shadow met all classic criteria: it was visible to the naked eye, adapted to the surface it fell on, and followed the green laser's position and shape.
Underlying Mechanism
The shadow effect stemmed from nonlinear optical absorption in the ruby. The green laser increased the optical absorption of the blue illuminating beam, creating an area with reduced intensity. This appeared as a shadow of the green laser beam.
Shadow Testing and Future Plans
"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 said.
Experiments measured the shadow's contrast as a function of the laser's power, reaching a peak contrast of approximately 22%, similar to a tree's shadow on a sunny day. The team also developed a theoretical model that accurately predicted this contrast.
Looking ahead, the researchers aim to explore other materials and laser wavelengths that might produce similar effects, offering further potential in advanced optical control applications.
Research Report:The shadow of a laser beam
Related Links
Brookhaven National Laboratory,
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