Special Issue  _____  Next-Generation Hologram Technology Based on Phase-Change Material

Share  

 

Imminent Era to Realize Mobile 3D Holograms

ETRI Maintains Reputation with New Hologram Technology

The next-generation hologram technology recently developed by ETRI researchers was published in Scientific Reports of Nature Publishing Group, demonstrating ETRI’s reputation as a globally renowned ICT research institution. The new technology uses a PCM (Phase-Change Material) to create hologram images that have a pixel pitch of 1m and a panel size of 3cm × 3cm. By using the PCM that can be fabricated, this next-generation hologram panel can ensure high-resolution pixels.
Currently, the LCoS (Liquid-Crystal-on-Silicon) platform is most commonly used as a SLM (Spatial Light Modulator) to display hologram images. However, since LCoS uses a liquid crystal, the pixel pitch can be reduced only to a limited size, which is insufficient to provide hologram images with an ultimately large viewing angle. To address this issue, researchers have tried to produce a SLM having a pixel pitch of 1μm with various materials.
With the changes in reflexibility resulting from the phase-change of a general GST (Germanium antimony tellurium alloy, Ge₂Sb₂Te₅) thin film, however, it is quite difficult to create a light diffraction phenomenon sufficient to observe hologram images with the naked eye. To solve this problem, ETRI researchers designed a multilayered thin-film structure with layers of ITO (Indium-Tin-Oxide) and GST. Using the phase-change of the GST layer inserted into the multilayered thin film, they successfully observed clear hologram images with commercial LED light sources.

Next-Generation Hologram Display Developed with GST Film

The core element of ETRI’s new hologram technology is Ge₂Sb₂Te₅ (GST), a chalcogenide PCM that is recently gaining attention. GST has been applied to optical storage devices such as DVD as well as PRAM (Phase-Change Memory) devices. Depending on different temperatures, this material undergoes a phase change between the amorphous state (with no regularity in the arrangement of atoms or molecules) and a crystalline state (most atoms constituting minerals being arranged regularly). The changing states significantly alter the electric conductivity and optical properties. The DVD is an example of an optical storage device that undergoes changes in reflexibility resulting from a material's phase change.
GST has long been used for semiconductor memory devices, and can be fabricated in a size of tens or hundreds of nanometers. It therefore has a potential to realize micro-sized pixels, the key issue and challenge in developing SLM devices for hologram displays. To complete the proposed SLM device using the GST thin-film structure, however, it was necessary to verify whether the light modulation of GST would be possible when electric signals were applied, as in the case of the existing display panels.
For verification, ETRI researchers produced a GST thin film-based unit pixel. When electric signals were applied to the unit pixel structure designed for an active region (1μm × 4μm), the researchers successfully observed changes in optical properties of the GST-based multilayered structure.
The researchers also designed the technology on the basis of the resonance condition of the multilayered thin film and verified the changes in resonance wavelength by altering the thickness of the ITO layers while maintaining that of the GST layer. This shows that the hologram panel causes diffraction of a specific color’s light source, thereby creating color hologram images with a single panel without any color-filtering process.

Research Continues to Develop New Devices for Optical Modulation

ETRI is conducting research with an aim to design and operate the structure of PCM-based active pixels and thereafter develop high-resolution SLM devices. Unlike the existing liquid crystal-based panels, the PCM used for the recently developed technology can be readily processed for flexible materials. ETRI expects that this new technology also can be applied to flexible hologram display panels.
As the latest achievement of ETRI researchers, the next-generation hologram technology is anticipated to be used for mobile 3D display panels for hologram videos, mobile hologram devices, and VR/AR technologies.