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University of Ottawa creates flat magic window

10 May 2022

New way to conjure an image from flat optical surface could lead to novel displays.

The ability to make both transparent and mirrored surfaces switch between their natural optical behavior and the display of some other form of image has been a fixture in fiction and a challenge in the real world for some time.

Back in 2003 Philips developed a "magic mirror" TV screen, which integrated a liquid crystal (LC) display with a wall-mounted mirror, switching functions through a polarized coating that controlled light transmission.

A project at the University of Ottawa has now used LCs to create a flat and polarization-switchable "magic window" - a transparent device that produces a hidden image when light shines on it, modernizing an old optical illusion.

As reported in Optica, the principle behind the device could lead to new forms of 3D display, or be miniaturized to operate on sub-wavelength scales.

"The magic window we created appears perfectly flat to the naked eye but, in fact, has slight variations that create an image in response to light," said team leader Felix Hufnagel. "By designing the window to be relatively smooth, the image that is created can be seen over a large range of distances from the window."

A traditional "magic mirror" optical illusion involves mirrors which display a new image under direct sunlight, thanks to patterning on the back side of the structure and subtle changes in the height of the surface, although the mathematical basis for the effect was only determined by Michael Berry at the University of Bristol in 2005.

Berry later extended this knowledge to develop a theoretical basis for transparent magic windows in addition to reflective magic mirrors. The Ottawa project brought this problem into the realm of flat optical devices, with the team combining liquid crystals, a reflective spatial light modulator, and a Pancharatnam-Berry optical phase element (PBOE) to make the two types of optical device.

A PBOE is a liquid crystal device that modulates the phase of light through a varying spatial anisotropy, rather than changes in optical path length. If its LC orientation pattern produces the same phase profile as the one induced by the thickness variations of a transparent window, then a LC PBOE can effectively act as a magic window.

Stable 3D images from different 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 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 project also showed that the devices could create images when illuminated with a LED light source, which would be more practical to use in real-life applications.

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

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