Laser beam casts visible shadow

Led by Jeff Lundeen, Associate Professor in the Department of Physics at uOttawa, in collaboration with the Boyd Research Group, this discovery marks the first time such an effect has been observed and challenges understanding of how light interacts with itself.

Typically, photons pass through each other without interacting, but the Ottawa experiment revealed an effect in which the shadow of a laser beam shows the same behaviour of shadows cast by solid objects.

"We've shown that under certain conditions, light can actually block other light, creating a shadow," Professor Lundeen said in a statement. "This opens up exciting new possibilities for controlling and manipulating light in ways we never thought possible before."

The team's experimental setup involved shining a green laser beam through a ruby crystal while illuminating it from the side with blue light. This arrangement created a shadow on a surface, visible to the naked eye. The effect occurs due to reverse saturation of absorption in the ruby crystal, which allows the green laser to block the passage of blue light, resulting in a dark region that follows 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 to predict the shadow's contrast, which is said to have closely matched their experimental data. They found that the shadow's darkness increased proportionally with the power of the green laser beam, reaching a maximum contrast of 22 per cent, which is comparable to a typical shadow on a sunny day.

"We're excited about the possibilities this opens up in fields like optical switching, fabrication, and imaging technologies," said Prof. Lundeen.

The full study, titled "Shadow of a laser beam," is available in Optica.