In Optica, Brookhaven National Laboratory researchers revealed how they employed a ruby crystal and precise laser wavelengths to demonstrate that a laser beam could block light and cast a visible shadow due to a nonlinear optical process.
For the experiment, a high-power green laser was directed through ruby cube and illuminated with a blue laser from the side. The green laser increases the optical absorption of the blue illuminating laser beam, creating a matching region in the illuminating light and creating a darker area that appears as a shadow of the green laser beam. Image Credit: Abrahao et al.
Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting. Our demonstration of a very counter-intuitive optical effect invites us to reconsider our notion of shadow.
Raphael A. Abrahao, Research Team Leader, Brookhaven National Laboratory
This phenomenon occurs when light interacts with a substance in an intensity-dependent manner and influences another optical field.
Abrahao added, “Our understanding of shadows has developed hand-in-hand with our understanding of light and optics. 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.”
Lunch Talk Sparks Idea
The new study is part of a more extensive investigation into how one light beam interacts with another under specific conditions and nonlinear optical processes. The idea arose during a lunch chat when it was pointed out that specific experimental drawings created with 3D visualization tools show the shadow of a laser beam because they consider it a cylinder without considering the physics of a laser beam. Some of the experts asked if this could be done in a laboratory.
“What started as a funny discussion over lunch led to a conversation on the physics of lasers and the nonlinear optical response of materials. From there, we decided to conduct an experiment to demonstrate the shadow of a laser beam,” Abrahao stated.
To do this, the researchers shone a high-power green laser into a ruby crystal cube before illuminating it with a blue laser from one side. When the green laser enters the ruby, it causes a local modification in the material’s reaction to the blue wavelength. The green laser behaves like an ordinary object, whereas the blue laser provides illumination.
The interaction of the two light sources produced a shadow on the screen, which appeared as a dark region where the green laser obscured the blue light. It met all of the requirements for a shadow because it was visible to the human eye, followed the contours of the surface it fell on, and matched the position and form of the laser beam, which served as an object.
The laser shadow effect is the result of optical nonlinear absorption in ruby. The effect arises because the green laser enhances the optical absorption of the blue illuminating laser beam, resulting in a matched patch in the illuminating light with decreased optical intensity. The result is a darker area that looks to be a shadow cast by the green laser beam.
Shadow Measurements
“This discovery expands our understanding of light-matter interactions and opens up new possibilities for utilizing light in ways we hadn’t considered before,” added Abrahao.
The researchers analyzed the shadow's contrast in relation to the power of the laser beam. They discovered a maximum contrast of about 22%, comparable to the contrast of a tree's shadow on a sunny day. They also created a theoretical model and demonstrated that it effectively predicted shadow contrast.
According to the researchers, their result indicates that, from a technological standpoint, using another laser can alter the strength of a transmitted laser beam. They intend to investigate different materials and laser wavelengths that can have comparable effects.
Journal Reference:
Abrahao, R. A. et. al. (2024) Shadow of a laser beam. Optica. doi.org/10.1364/OPTICA.534596