Lasers That Can Block Light Itself

or 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. Credit: R. A. Abrahao, H. P. N. Morin, J. T. R. Pagé, A. Safari, R. W. Boyd, J. S. Lundeen

Light, being massless, casts no shadow; under ordinary circumstances, photons pass right through each other unimpeded.

Researchers, in the Department of Physics at uOttawa, along with the Boyd Research Group, have demonstrated a laser beam acting like an object — the beam casts a shadow upon a surface when the beam is illuminated by another light source.

They observed a regular shadow in the sense it can be seen by the naked eye, it follows the contours of the surface it falls on, and it follows the position and shape of the object (the laser beam). Specifically, the team used a nonlinear optical process involving four atomic levels of ruby. The researchers were able to control the intensity of a transmitted laser beam by applying another perpendicular laser beam.

They experimentally measured the dependence of the contrast of the shadow on the power of the laser beam, finding a maximum of approximately 22%, similar to that of a shadow of a tree on a sunny day.

They provide a theoretical model that predicts the contrast of the shadow. This work opens new possibilities for fabrication, imaging, and illumination.

Reference: “Shadow of a laser beam” by Henri P. N. Morin, Jordan T. R. Pagé, Raphael A. Abrahao, Akbar Safari, Jeff S. Lundeen and Robert W. Boyd, 19 November 2024, Optica.
DOI: doi:10.1364/OPTICA.534596

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