The shadow of a laser beam

TL;DR

This paper demonstrates that a laser beam can cast a visible shadow when illuminated by another light source, using nonlinear optical processes in ruby, revealing new ways to control light for imaging and fabrication.

Contribution

It introduces a novel method where a laser beam acts as an object casting a shadow, enabled by nonlinear optics, which is a significant departure from traditional light behavior.

Findings

Maximum shadow contrast of ~22% observed

Shadow follows surface contours and object shape

Theoretical model accurately predicts shadow contrast

Abstract

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.