In a new paper published today in the journal Optica, physicists describe how a laser beam can be made to cast a shadow that behaves as any other ordinary shadow.
Photographic images of the shadow of a laser beam; a high-power green laser beam (the object), traveling through a cube of ruby, is illuminated from the side by blue light: (A) a photograph of the shadow cast by the object laser beam on a piece of white paper, image magnified approximately by a factor of four using a simple lens; the ruby cube length is about 1.2 cm, and the magnified image is about 4.8 cm; hence, regardless of magnified or not, it portrays what can be seen in person; (B) a photographic image showing the surrounding for reference of scale; a white plastic marker (i.e., a broad tip pen) is placed in the path of the shadow, between the object beam and the paper, and the camera focus is fixed on (C) the paper or (D) the marker, thereby showing that the shadow follows the contours of the surface the shadow falls on. All images were taken with a regular consumer digital camera in a darkened room. Image credit: Abrahao et al., doi: 10.1364/optica.534596.
“Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting,” said Dr. Raphael Abrahao, a researcher at Brookhaven National Laboratory.
“Our demonstration of a very counter-intuitive optical effect invites us to reconsider our notion of shadow.”
To show that a laser beam could block light and create a visible shadow due to a nonlinear optical process, Dr. Abrahao and colleagues used a ruby crystal and specific laser wavelengths.
This effect occurs when light interacts with a material in an intensity-dependent way and can influence another optical field.
“Our understanding of shadows has developed hand-in-hand with our understanding of light and optics,” Dr. Abrahao said.
“This new finding could prove useful in various applications such as optical switching, devices in which light controls the presence of another light, or technologies that require precise control of light transmission, like high-power lasers.”
In their experiments, the researchers directed a high-power green laser through a cube made of standard ruby crystal and illuminated it with a blue laser from the side.
When the green laser enters the ruby, it locally changes the material response to the blue wavelength.
The green laser acts like an ordinary object while the blue laser acts like illumination.
The interaction between the two light sources created a shadow on a screen that was visible as a dark area where the green laser blocked the blue light.
It met all the criteria for a shadow because it was visible to the naked eye, followed the contours of the surface it fell on and followed the position and shape of the laser beam, which acted as an object.
The laser shadow effect is a consequence of optical nonlinear absorption in the ruby.
The effect occurs because the green laser increases the optical absorption of the blue illuminating laser beam, creating a matching region in the illuminating light with lower optical intensity.
The result is a darker area that appears as a shadow of the green laser beam.
“This discovery expands our understanding of light-matter interactions and opens up new possibilities for utilizing light in ways we hadn’t considered before,” Dr. Abrahao said.
The researchers experimentally measured the dependence of the shadow’s contrast on the laser beam’s power, finding a maximum contrast of approximately 22%, similar to the contrast of a tree’s shadow on a sunny day.
They also developed a theoretical model and showed that it could accurately predict the shadow contrast.
“From a technological perspective, the effect we demonstrated shows that the intensity of a transmitted laser beam can be controlled by applying another laser,” the scientists said.
“Next, we plan to investigate other materials and other laser wavelengths that can produce similar effects.”
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Raphael A. Abrahao et al. 2024. Shadow of a laser beam. Optica 11 (11): 1549-1555; doi: 10.1364/optica.534596
