Under very specific conditions, a laser beam can cast a shadow, challenging our traditional understanding of light. This unusual phenomenon, revealed by a team of researchers, paves the way for surprising applications in manipulating light with light.
A visible "laser shadow" appears as a dark line against the blue background. Credit: R. A. Abrahao et al.
The scientists relied on an optical property called nonlinear absorption to demonstrate this effect. In their experiment, a powerful green laser beam passed through a ruby crystal, locally altering its response to lateral blue light. The result: a visibly darker region, akin to the shadow of a physical object.
Traditionally, light is understood to pass through other light without interacting. This discovery, carried out at the Brookhaven National Laboratory, challenges that principle. For Raphael A. Abrahao, head of the team, it illustrates the complexity of light-matter interactions and prompts a reevaluation of the very concept of shadows.
The initial idea emerged during an informal discussion among researchers. Inspired by 3D models suggesting laser shadows, they decided to test the possibility in the lab. By modifying the optical properties of ruby, the green laser acted like a physical object, while the blue laser served as illumination.
The team observed that the laser shadow shared many characteristics with conventional shadows: visible to the naked eye, true to the shape of the beam, and adhering to geometric principles. This experiment shows that lasers can control other lasers, a promising prospect for advanced technologies.
A green laser beam modifies the optical absorption of the blue laser, creating a visible dark region. Credit: R. A. Abrahao et al.
The researchers also modeled the contrast of this shadow as a function of the laser beam's power. They achieved a maximum contrast of 22%, comparable to the shadow of a tree on a sunny day. These experimental results confirm their theoretical model.
Beyond its scientific significance, this phenomenon could revolutionize fields like optical switches or high-power laser systems. The team plans to study other materials and wavelengths to explore similar effects.