Researchers at the University of Ottawa have demonstrated that under certain conditions, a laser beam can act like an opaque object and cast a visible shadow. The finding challenges the traditional understanding of shadows and opens possibilities for technologies that could use a laser beam to control another laser beam. 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 a phenomenon called 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. 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. Courtesy of Abrahao et al. “What's particularly fascinating is how closely this laser shadow behaves like a traditional shadow,” said associate physics professor Jeff 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 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% — comparable to a typical shadow on a sunny day. The laser shadow effect is a consequence of optical nonlinear absorption in the ruby. The effect occurs because the green laser increases the optical absorption of the blue illuminating laser beam, creating a matching region in the illuminating light with lower optical intensity. The result is a darker area that appears as a shadow of the green laser beam. Researchers showed that a laser beam can sometimes act like a solid object and cast a shadow that is visible to the naked eye. The shadow appears as the horizontal line traversing the blue background. Courtesy of Abrahao et al. “Our understanding of shadows has developed hand-in-hand with our understanding of light and optics,” said Raphael Abrahao of Brookhaven National Laboratory, formerly of the University of Ottawa. “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 researchers say that from a technological perspective, the effect they demonstrated shows that the intensity of a transmitted laser beam can be controlled by applying another laser. Next, they plan to investigate other materials and other laser wavelengths that can produce similar effects. The research was published in Optica (www.doi.org/10.1364/OPTICA.534596).