Key technologies for implementing spatial light modulation devices (SLMs)

The Electronics and Telecommunications Research Institute of Korea has implemented a holographic image measuring 3 centimeters in width and height with 1 micrometer pixels using a phase-transition material, and the results were published in Scientific Reports on January 24.

A Korean research team has successfully developed a hologram capable of ultra-high-resolution pixels using phase-transition materials. This development promises to enable the development of display panels capable of playing holographic videos in the future.

The phase-transition material used in this study is germanium antimony telluride (Ge2Sb2Te5, GST), a chalcogenide compound that has recently been receiving renewed attention. It has been applied to DVDs and phase-change memory devices (PRAM). Currently, holographic image displays mainly use a spatial light modulator method using liquid crystals. By applying voltage to the liquid crystals, the phase and polarization of light are effectively changed to create a holographic image. However, liquid crystal devices have limitations in producing micrometer-level pixel sizes, which are necessary to improve the image quality and viewing angle of holographic images.

To address these issues, ETRI utilized phase transition materials (GSTs), previously studied as semiconductor memory devices. Phase transition materials can exist in both amorphous and crystalline states, with varying transmittance and refractive index. They can be manufactured with pixel sizes sub-micrometers and can control the phase of light, enabling the creation of holographic images. The significance of this result lies in the fact that it achieved a pixel size close to the wavelength of light while making the pixels about 1/4 smaller than those using existing liquid crystals.

The research team used a holographic device utilizing a phase-change material, using indium tin oxide (ITO) on both sides and a semiconductor material, GST, in between. By using this multi-layer thin film structure, they were able to maintain the thickness of the phase-change material layer while controlling the thickness of the transparent electrode layer, creating a device that can maximize phase modulation in specific colors. By controlling the thin film thickness without a separate color filter process, they made it possible to generate holographic images of various colors depending on the thickness of the transparent electrode layer.

In this research, a 3-centimeter-sized hologram composed of 1-micrometer-sized pixels was created using laser-assisted phase transitions. The increased resolution allows the hologram to appear clearly even with standard LED light. In the prototype, when illuminated by a smartphone flashlight (LED), the green lettering "NANO" appeared as a hologram on a checkered background. Various color expressions are also possible.

ETRI has revealed that it is possible to implement ultra-small pixels of less than 1 micrometer, which has been one of the biggest topics and obstacles in developing spatial light modulation devices for holographic displays. To verify this, the research team created a unit pixel based on a thin film of a phase-change material. When an electrical signal was applied to the unit pixel structure designed with a light modulation area of 1㎛ x 4㎛, the optical properties of the phase-change material-based multilayer structure were successfully observed to change.

The research team plans to implement digital hologram images in the form of panels based on phase-transition materials within two years. Through this, we plan to apply it to video implementation and flexible hologram display panels.

ETRI's Hwang Chi-seon, head of the Immersive Display Research Group, said, "The current level of research results is at the level of implementing stationary holographic images using phase-transition materials, but through continued research, we plan to develop a next-generation optical modulation device capable of video playback."