Researchers have used liquid crystals to create magical windows that create a hidden image when light shines on them. Credit: Felix Hufnagel, University of Ottawa

Researchers are creating a flat magic window with liquid crystals

For the first time, scientists used liquid crystals to construct a flat magic window – a transparent device that creates a hidden image when light illuminates it. Technology is a new twist on a very old little trick.

Thousands of years ago, artisans in China and Japan made bronze mirrors that looked like ordinary flat mirrors while looking at someone’s reflection, but generated a different image when illuminated by direct sunlight. It took scientists until the early 20th century to understand that these devices work because an image thrown in the back of a mirror creates small variations on the surface that cause the image to form – and so far engineers have had to apply the same principle to liquid crystals. for high-tech displays.

“The magic window we created looks perfectly flat to the naked eye, but there are actually slight variations that create an image in response to light,” said Felix Hufnagel, head of research at the University of Ottawa. “By designing the window to be relatively smooth, the created image can be seen at a wide range of distances from the window.”

IN Optics, the journal of the Optica Publishing Group for high-impact research, Hufnagel and colleagues describe the process they have developed to create transparent liquid crystal magic windows that can produce any desired image. The process can also be used to create magic mirrors that reflect rather than transmit light to create an image.

Magic Windows Create a hidden image

The magic windows created by the researchers look perfectly flat to the naked eye, but actually have slight variations that create an image in response to light. The video shows the intensity distribution, which gradually develops from the profile of the input beam to the desired image model. Credit: Felix Hufnagel, University of Ottawa

“Using liquid crystals to make magic windows or mirrors may one day make it possible to create a reconfigurable version to produce dynamic art magic windows or movies,” Hufnagel said. “The ability to get great depth of focus can also make the approach useful for 3D displays that produce stable 3D images, even when viewed from different distances.

Create magic with liquid crystals

Although scientists have learned for decades that ancient bronze magic mirrors formed images as a result of small variations on the surface, it was not until 2005 that Michael Berry, a mathematical physicist from[{” attribute=””>University of Bristol in the UK, derived the mathematical basis for this effect. He later extended this knowledge to develop a theoretical basis for transparent magic windows in addition to reflective magic mirrors. This work inspired Hufnagel and colleagues to create a magic window based on liquid crystals.

Liquid crystals are materials that can flow like a conventional liquid but have molecules that can be oriented like a solid crystal. In the new work, the researchers used a modified version of a well-known fabrication process that produces a specific liquid crystal pattern that allows a desired image to be created when illuminated.

They used a Pancharatnam-Berry Optical Element (PBOE), which is a liquid crystal device that operates under a well-known principle called the Pancharatnam-Berry phase. By changing the orientation of liquid crystal molecules in this device, the researchers could alter the properties of the light as it travels through the device on a pixel-by-pixel basis.

Stable images over multiple distances

“On a conceptual level, the theory developed by Berry was instrumental in determining how these liquid crystals must be oriented to create an image that is stable over a large distance,” said Hufnagel. “Our use of flat optical elements and a liquid crystal pattern with gentle variations prescribed by Berry’s Laplacian image theory allows the magic windows to appear normal, or flat, when one looks through them.”

After fabricating a magic mirror and window, the researchers used a camera to measure the light intensity patterns produced by both devices. When illuminated with a laser beam, both the mirror and window produced a visible image that remained stable even as the distance between the camera and the mirror or window changed. The researchers also showed that the devices created images when illuminated with an LED light source, which would be more practical to use in real-life applications.

The researchers are now working to use their fabrication approach to create quantum magic plates. For example, two of these plates could create entangled images which one could use to study new quantum imaging protocols. They are also exploring the possibility of fabricating magic windows using approaches other than liquid crystals. For instance, using dielectric metasurfaces to make a magic window device could reduce its footprint while increasing bandwidth.

Reference: “Flat Magic Window” by F. Hufnagel, A. D’Errico, H. Larocque, F. Alsaiari, J. Upham, E. Karimi, 5 May 2022, Optica.
DOI: 10.1364/OPTICA.454293

Inspired by an Ancient Light Trick, “Flat Magic Window” Technology Could Enable a New Type of 3D Display

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