Jul 7 2020
Spatial occupancy presents design limitations in conventional tunable lenses containing complex lenses with a manipulation system, ultimately limiting the applications of these lenses in next-generation pixel-based devices such as flat panel displays.
(a) An Illustration of applying the ETF-USSL in a display. The USSL enables multi-focusing, allowing the implementation of glassless 3D and multi-view displays, and the ETF characteristics enable a variable viewing angle. (b) the spot intensity depends on the distance along the Z-axis and the distance in the lateral direction between the focal spots of peaks 1 and 2. At a fixed focal length position, the maximum intensity of the focal spot decreases as the focal length of the USSL becomes longer owing to the driving voltage. (c) Schematic of the tunable focal length when a DC voltage bias is applied to graphene in the in-plane direction. In the ribbon made of graphene, the center area (C) absorbs the light, and the carriers are concentrated in the left side (L) and right side (R) due to the DC bias; thus, the Fermi level is far from the Dirac point, and light is not absorbed and transmitted. Consequently, the change in the nanoribbon width via an external electric field effectively modulates the FZP topology, thereby changing the focal length of the lens. Image Credit: Sehong Park, Gilho Lee, Byeongho Park, Youngho Seo, Chae bin Park, Young Tea Chun, Chulmin Joo, Junsuk Rho, Jong Min Kim, James Hone, and Seong Chan Jun.
Since it is possible to pattern the graphene, an FZP lens made of graphene can provide a perfect combination of far and near optical fields. This is because graphene has optical conductivity that can be tweaked by adjusting the geometry or by varying the Fermi level.
Earlier, the parallax barrier and lenticular lens employed in multi-view autostereoscopic displays were believed to be impractical for displays, because of their high aberration, low resolution, low transmittance, and thickness. Hence, there was a constant demand for an original device that has beneficial physical characteristics and high optical performances.
A team of researchers, headed by Professor Seong Chan Jun at Yonsei University in Korea, along with fellow scientists from POSTECH, the University of Cambridge in the United Kingdom, and Columbia University in the United States, has created graphene-based ultrathin subpixel square lens that functions by regulating the distribution of carriers inside the Fermi level and accordingly modifying the absorbance properties.
The study was published in the Light: Science & Applications journal in June 2020.
Depending on the Fermi level position, the graphene-based Fresnel lens allows electrically tunable focusing based on the variation in the absorption properties. When an arc ribbon pattern is designed, the effective spacing present in the arc ribbons is managed by the variation in the carrier distribution based on the electric field position.
Consequently, a difference in the diffraction properties of the slit is realized in such a way that the focal length can be modified in the visible regime without altering the design.
The lens can also be customized as per the wavelength of every subpixel in the display device without using any extra device or light source. Therefore, a multifunctional display using an ultrathin square subpixel lens with high resolution and high transmittance can be achieved.
A graphene ultrathin lens like this is exclusively developed for users’ field of view (FOV) in multi-view autostereoscopic displays. The electrically focus-tunable ultrathin device is made up of five graphene layers and has a transmittance of 82% with a focusing efficiency of more than 60%. The device also exhibits a 19.42% shift in focal length that achieves multi-focusing property in accordance with the FOV of the observer.
Thus, such an ultrathin focusing device helps achieve multi-view autostereoscopic display without using any extra calibration system. The researchers have summed up the working principle of the ultrathin focusing device as follows:
“The electric field normal to the plane due to the DC bias concentrates the carrier density at the edges of the arc ribbon. The arc ribbon absorbs light in the central area (C), but the Fermi level on the left (L) and right (R) sides shifts away from the Dirac point due to the increase in the carriers. This results in a longer focal length because the decrease in the size of the arc ribbon increases the linearity of the diffraction by the arc ribbon.”
This subpixel lens can be uniquely designed according to the wavelength of each RGB subpixels. Therefore, the chromatic aberration that frequently occurs in conventional lenticular devices can be eliminated, and each individual wavelength of light can be focused into a single focal spot.
Researchers, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences
“The device’s structural advantage within the subpixel scale can be embedded into each individual pixel in glassless 3D displays, privacy displays, and multiview displays for display applications. In addition, this design can be customised for 3D hologram displays, acoustic applications, and optical devices comprising metasurfaces, as expected by the researchers,” concluded the researchers.
Journal Reference:
Park, S., et al. (2020) Electrically focus-tuneable ultrathin lens for high-resolution square subpixels. Light: Science & Applications. doi.org/10.1038/s41377-020-0329-5.