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Physics World

Laser beam casts a shadow in a ruby crystal

Isabelle Dumé

Published Feb 1, 2025• 0 citations
Publisher

Abstract

Researchers have discovered that photons can cast a shadow, when a laser beam is illuminated by another light source as it passes through a highly nonlinear medium.
DOI
10.1088/2058-7058/38/02/14

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arXiv:1112.0095•Physics-Uspekhi•Aug 31, 2004•6 citations
Light, being massless, casts no shadow; under ordinary circumstances, photons pass right through each other unimpeded. Here, we demonstrate a laser beam acting like an object — the beam casts a shadow upon a surface when the beam is illuminated by another light source. We observe 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, we use a nonlinear optical process involving four atomic levels of ruby. We are able to control the intensity of a transmitted laser beam by applying another perpendicular laser beam. We experimentally measure 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. We provide a theoretical model that predicts the contrast of the shadow. This work opens new possibilities for fabrication, imaging, and illumination.
Light, being massless, casts no shadow; under ordinary circumstances, photons pass right through each other unimpeded. Here, we demonstrate a laser beam acting like an object - the beam casts a shadow upon a surface when the beam is illuminated by another light source. We observe 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, we use a nonlinear optical process involving four atomic levels of ruby. We are able to control the intensity of a transmitted laser beam by applying another perpendicular laser beam. We experimentally measure the dependence of the contrast of the shadow on the power of the object laser beam, finding a maximum of approximately of approximately 22 percent, similar to that of a shadow of a tree on a sunny day. We provide a theoretical model that predicts the contrast of the shadow. This work opens new possibilities for fabrication, imaging, and illumination.
On speed of a photon in a dispersing medium Ogluzdin Valeriy E. Abstract Based on the author of the experimental results and their treatment, a model that demonstrates that the real dispersive medium in spectral regions where the refractive index greater than unity, the velocity of propagation of photons in this environment corresponds to the phase velocity of light. These portions of the spectrum (for example, atomic potassium vapor or other alkali metal) are from low-frequency side of the main lines of the doublet and are available for research using tunable lasers. In the calculations took into account the fact that the atomic vapor of alkali metal used as a dispersing medium, in accordance with the theory of dispersion in the above sections of the spectrum, on the periphery of the cross section of laser beam with Gaussian intensity distribution across the beam - in the shadows refractive index n (ν) is greater than unity, and the axis of the beam, where the radiation intensity is maximum - for radiation following the leading edge of the light pulse, the refractive index is unity, which ensures smooth on the beam axis, without any delay "superluminal "propagation of light with velocity c. This fact causes the appearance of the shadow cone of Vavilov - Cherenkov radiation, indicating a slow propagation of photons from the source of the cone with phase velocity c / n (ν).