Amorphous Silicon Optics Enter the Visible Range

Junsuk Rho, a professor in POSTECH’s mechanical and chemical engineering departments, has developed a transparent amorphous silicon capable of transmitting visible light. The silicon, with which the researchers were also able to effectively control light in the visible range of the spectrum, is a necessary component to the future development of paper-thin lenses for use in head-mounted displays for real-time virtual and augmented reality.

The team developed the visibly transparent amorphous silicon by improving the plasma-enhanced chemical vapor deposition (PECVD) method, which is widely used by display manufacturers, including those in Korea.

“The discovery of an optical element capable of controlling all visible light has revealed clues about the relationship between the atomic bonding structure and the visible light region, which has not been of interest until now,” Rho said. “As we can produce optical devices that can control all colors at low cost, we are now one step closer to commercializing virtual and augmented reality and hologram technologies only seen in movies.”

Because light bends more when passing through a material with a higher refractive index, such materials are favorable in the design of devices for virtual and augmented reality. Most materials that possess a high refractive index tend to absorb light, however, which is not conducive to use in devices that produce images by controlling light. Optical materials have high transmittance with a low refractive index, or, conversely, a high refractive index and low transmittance, which is limiting to the production of lightweight and efficient optical devices.

To overcome this, the team used the PECVD method, a common technique in the development of amorphous silicon, and precisely determined what effect each of those parameters had on the intermolecular bonds.

Through this process, the team members successfully increased the regularity between silicon atoms by inserting hydrogen atoms between strained silicon atomic bonds. They also identified the atomic structure of amorphous silicon possessing a high refractive index and high transmittance. Finally, the researchers demonstrated that they could steer red, green, and blue light in the desired direction, something that couldn’t previously be controlled with conventional silicon.

With these properties, the material has significant potential for producing hologram devices or ultrathin lenses that are one-thousandth of the thickness of conventional lenses at a fraction of the cost. The research expands the relevance of silicon into the visible spectrum, having previously been relegated to infrared cameras.

The research was published in Advanced Materials (www.doi.org/10.1002/adma.202005893).