Science for all newsletter Scientists cause one beam of light to cast a shadow against another

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When some object stands in the path of light, the object casts a shadow behind it. It’s the place the light can’t reach because of the object. The question is if some light can block some other light, thus casting a shadow of its own. Sometimes light can interfere with other light, but that’s not really the same thing: the dark patches in an interference pattern aren’t shadows so much as darkness created by the elimination of light (waves).

In a new study, a team from universities in the U.S. and Canada has reported creating a shadow as a result of one beam of light blocking another. The team members placed three items in a straight line with some distance between them: a blue laser source, a small cubic crystal, and a camera with a blue laser. The blue light illuminated the cube before hitting the camera lens. Then the team hit the cube with a green laser in a perpendicular direction. As the green light cut across the path of the blue light through the cube, a slender dark shadow appeared in the camera’s view. When the researchers changed the position of the green laser source, the shadow moved as well. It was also visible to the naked eye when the team placed a sheet of paper in front of the camera lens.

This result isn’t as straightforward as a lamppost casting a shadow on the ground. It was possible because the crystal, inside which the two light beams intersected, was made of ruby — a mineral consisting of aluminium oxide and a small population of chromium atoms. When the green light struck the chromium nuclei, they absorbed the energy and jumped to a higher energy level. Then they shed only a part of this energy to slide down to an intermediate level.

At this point the nuclei could absorb photons from the blue light and jump to a new higher energy level. This absorption effectively blocked the blue light and created a shadow in the camera’s image.

The green and blue beams have to have just the right colour or they’d carry too much or too little energy to result in these energy transitions.

Even if the shadow was real and true to its name, it was also peculiar. First, it wasn’t due to the ruby crystal. Instead the green light changed the crystal’s response to blue light. Without the green light also illuminating the cube, the chromium atoms respond differently to the blue. Second, photons — the particles of light — don’t have mass or a physical size and thus can’t cast shadows. Yet there is a shadow thanks to polaritons: packets of energy consisting of photons strongly coupled to the atomic excitations. Polaritons can have mass and thus cast shadows.

Playful though the experiment seems to be, the research team already foresees value and important applications of their findings. As the team’s paper, published in the journal Optica, concludes: “Potential applications can be envisioned in areas such as optical switching, controllable shade or transmission, control of the opaqueness of light with light, and lithography” — the last an image-making technique that uses flat surfaces.

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