Researchers at the University of Ottawa have observed a new phenomenon where a laser beam can cast a shadow, challenging conventional understanding of light interactions. The study was led by Jeff Lundeen, an associate professor in the Department of Physics at uOttawa, in collaboration with the Boyd Research Group.
Traditionally, photons are known to pass through each other without interaction. However, this experiment demonstrated that under specific conditions, light can block other light and create a shadow. "We've shown that under certain conditions, light can actually block other light, creating a shadow," explained Professor Lundeen. "This opens up exciting new possibilities for controlling and manipulating light in ways we never thought possible before."
The experimental setup involved directing a green laser beam through a ruby crystal while illuminating it from the side with blue light. This arrangement resulted in a visible shadow on a surface due to reverse saturation of absorption in the ruby crystal. This phenomenon allows the green laser to obstruct blue light passage, forming a dark region following the contours of the laser beam.
"What's particularly fascinating is how closely this laser shadow behaves like a traditional shadow," said Prof. Lundeen. "It follows the shape of the 'object' - in this case, our laser beam - and even conforms to the contours of surfaces it falls on, just like the shadow of a tree branch would."
The researchers developed a theoretical model predicting the contrast of this laser-induced shadow, which aligned closely with their experimental findings. They discovered that as they increased the power of the green laser beam, so did the darkness of its shadow—reaching up to 22% contrast similar to shadows seen on sunny days.
This discovery not only expands knowledge about light-matter interactions but also suggests potential applications in optical switching, fabrication processes, and imaging technologies. "We're excited about the possibilities this opens up in fields like optical switching, fabrication, and imaging technologies," added Prof. Lundeen.