A blue laser was used to illuminate the ruby cube from the side while a high-power green laser was focused through it for the experiment. In order to produce a matched region in the illuminating light and a darker area that looks as a shadow of the green laser beam, the green laser increases the optical absorption of the blue illuminating laser beam.
Is it possible for light to cast a shadow? Researchers have discovered that, in some circumstances, a laser beam can behave like an opaque object and cast a shadow, which may sound like a philosophical conundrum. The finding casts doubt on the conventional wisdom regarding shadows and creates new opportunities for devices that could manipulate another laser beam with a laser beam.
“Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting,” said Raphael A. Abrahao, the leader of the study team from Brookhaven National Laboratory, formerly at the University of Ottawa. “Our demonstration of a very counter-intuitive optical effect invites us to reconsider our notion of shadow.”
In Optica, scientists explain how they demonstrated that a laser beam could block light and produce a visible shadow because of a nonlinear optical process using a ruby crystal and particular laser wavelengths. Light can affect another optical field when it interacts with a material in an intensity-dependent manner.
“Our understanding of shadows has developed hand-in-hand with our understanding of light and optics,” Abrahao stated. “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 new study is a part of a broader investigation of nonlinear optical processes and the interaction of two light beams under specific conditions.
Some experimental drawings created with 3D visualization software portray the shadow of a laser beam because they interpret it as a cylinder without taking into account the physics of a laser beam. This idea was first raised during a lunch chat. Could this be done in a lab? asked some of the scientists.
“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 remarked. “From there, we decided to conduct an experiment to demonstrate the shadow of a laser beam.”
The researchers accomplished this by shining a blue laser sideways on a cube of regular ruby crystal and passing a high-power green laser through it. The material’s reaction to the blue wavelength is locally altered when the green laser enters the ruby. Whereas the blue laser behaves like illumination, the green laser behaves like a regular object.
On a screen, the interaction of the two light sources produced a shadow that could be seen as a dark spot where the blue light was blocked by the green laser. Because it was visible to the unaided eye, followed the outlines of the surface it fell on, and followed the location and form of the laser beam, which functioned as an object, it satisfied all the requirements for a shadow.
The ruby’s optical nonlinear absorption is what causes the laser shadow effect. The green laser produces a corresponding region in the illuminating light with reduced optical intensity by increasing the blue illuminating laser beam’s optical absorption. As a result, the green laser beam appears as a shadow in a darker area.