An ancient optical illusion has been updated using the flat optics of today to create a device that reveals a hidden image when light is shined on it. Developed at the University of Ottawa, the magic window is a liquid crystal (LC)-based device that can produce any image desired — an effect that could potentially be used in 3D displays. The process used to create the magic window can also be used to create a magic mirror that creates an image by reflecting light, rather than by transmitting it. The magic window and mirror consist of optical devices with a surface deformation or thickness distribution arranged so as to form a specific image. The image intensity is related to the Laplacian of the height of the surface relief. “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,” research team leader Felix Hufnagel said. Researchers have used liquid crystals to create a magic window that produces a hidden image when light shines on it. Courtesy of Felix Hufnagel/University of Ottawa. Although scientists have understood for decades how the magic mirrors created thousands of years ago formed images when hit by direct sunlight, it was not until 2005 that Michael Berry, a physicist at the University of Bristol, discovered the mathematical basis for the effect. Inspired by Berry’s work, the Ottawa team used LC-based optical devices, a reflective spatial light modulator, and a Pancharatnam-Berry optical phase element to construct a magic mirror and magic window. After using the Laplacian theory to calculate the needed phase pattern, the researchers developed both devices with flat optics using optical polarization-wavefront coupling. They implemented the desired pattern and experimental specifications for designing the flat optics with a reconfigurable spatial light modulator, which acted as the magic mirror. A flat plate, which served as an optical polarization-wavefront coupler, was then fabricated by spatially structuring nematic liquid crystals. The researchers used the plate to demonstrate the concept of a polarization-switchable magic window, where either the desired image or the image resulting from the negative of the window’s phase can be displayed, depending on whether the input polarization is circularly left- or right-handed. The researchers’ fabrication process allowed them to produce a specific pattern with the LCs, which created the desired image when it was illuminated. “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,” Hufnagel said. “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 they fabricated the magic mirror and magic window, the researchers used a camera to measure the light intensity patterns produced by both devices. When illuminated with a laser beam, both the window and the mirror produced a visible image that remained stable even as the distance between the camera and the window, or the camera and the mirror, changed. “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,” Hufnagel said. The researchers also showed that both devices can create images when illuminated with an LED light source, which could be the most practical light source for some applications. The flat magic plate can be tuned to operate at different wavelengths. Although the researchers used the device under monochromatic illumination, they said that in principle the device can work equally well under broadband illumination. The researchers envision many possible uses for their flat magic window and mirror. “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,” Hufnagel said. “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.” The team is currently investigating how to create quantum magic plates. Two of these plates could be used, for example, to create entangled images, which in turn could be used to study new quantum imaging protocols. The researchers are also exploring the possibility of using different approaches that do not involve LCs to fabricate magic windows. For example, dielectric metasurfaces could potentially be used to make a magic window device, reducing the device’s footprint while increasing its bandwidth. The research was published in Optica (www.doi.org/10.1364/OPTICA.454293).
